Retinoblastoma protein acts as Pax 8 transcriptional coactivator

Pax-8: Molecular biology, pathophysiology, and potential pathogenesis

Transcription factors, as the convergence points of multiple signaling pathways in eukaryotic cells, are closely involved in disease development. Pax-8, an important transcription factor belonging to the Pax family, exerts a crucial influence on the regulation of gene expression required for both physiological conditions and pathological processes. Pax-8 contributes to the pathogenesis of many human diseases, ranging from cardiovascular disease to many cancers, and therefore, it can be imagined that Pax-8 holds great therapeutic potential. In this review, we summarize the structure, distribution, function, and regulatory mechanisms of Pax-8 to provide a new research direction for Pax-8.

Mechanistic Insights of Thyroid Cancer Progression

Differentiated thyroid cancers (DTCs) are primarily initiated by mutations that activate the MAPK signaling cascade, typically at BRAF or RAS oncoproteins. DTCs can evolve to more aggressive forms, specifically, poorly differentiated (PDTC) and anaplastic thyroid cancers (ATC), by acquiring additional genetic alterations which deregulate key pathways. In this review, we focused on bona fide mutations involved in thyroid cancer progression for which consistent mechanistic data exist. Here we summarized the relevant literature, spanning approximately 2 decades, highlighting genetic alterations that are unquestionably enriched in PDTC/ATC. We describe the relevant functional data obtained in multiple in vitro and in vivo thyroid cancer models employed to study genetic alterations in the following genes and functional groups: TP53, effectors of the PI3K/AKT pathway, TERT promoter, members of the SWI/SNF chromatin remodeling complex, NF2, and EIF1AX. In addition, we briefly discuss other genetic alterations that are selected in aggressive thyroid tumors but for which mechanistic data is still either limited or nonexistent. Overall, we argue for the importance conveyed by preclinical studies for the clinical translation of genomic knowledge of thyroid cancers.

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.

PAX8, an Emerging Player in Ovarian Cancer

Ovarian Cancer is one of the most lethal and widespread gynecological malignancies. It is the seventh leading cause of all cancer deaths worldwide. High-Grade Serous Cancer (HGSC), the most commonly occurring subtype, alone contributes to 70% of all ovarian cancer deaths. This is mainly attributed to the complete lack of symptoms during the early stages of the disease and absence of an early diagnostic marker.PAX8 is emerging as an important histological marker for most of the epithelial ovarian cancers, as it is expressed in about 90% of malignant ovarian cancers, specifically in HGSC. PAX8 is a member of the Paired-Box gene family (PAX1-9) of transcription factors whose expression is tightly controlled temporally and spatially. The PAX genes are well known for their role in embryonic development and their expression continues to persist in some adult tissues. PAX8 is required for the normal development of Müllerian duct that includes Fallopian tube, uterus, cervix, and upper part of vagina. In adults, it is expressed in the Fallopian tube and uterine epithelium and not in the ovarian epithelium. Considering the recent studies that predict the events preceding the tumorigenesis of HGSC from the Fallopian tube, PAX8 appears to have an important role in the development of ovarian cancer.In this chapter, we review some of the published findings to highlight the significance of PAX8 as an important marker and an emerging player in the pathogenesis of ovarian cancer. We also discuss regarding the future perspectives of PAX8 wherein it could contribute to the betterment of ovarian cancer diagnosis and treatment.

Usefulness of PAX8 Immunohistochemistry in Adult Intraocular Tumor Diagnosis

PURPOSE

To evaluate the distribution of the PAX8 transcription factor protein in ocular tissues and to investigate if immunohistochemical stains for this biomarker are useful in the diagnosis of intraocular tumors.

DESIGN

Observational case series.

PARTICIPANTS

Excision and cytologic analysis specimens of 6 ciliary body epithelial neoplasms, 2 iris epithelial neoplasms, 3 retinal pigment epithelial neoplasms, 3 intraocular medulloepitheliomas, 15 uveal melanomas, and 5 uveal melanocytomas.

METHODS

Hematoxylin-eosin and PAX8 immunohistochemical stains were performed on all specimens. In appropriate cases, bleached preparations and other immunohistochemical stains, including AE1/AE3 cytokeratin, Lin28A, and CD45, were performed.

MAIN OUTCOME MEASURES

Distribution of PAX8 expression in normal and neoplastic tissue.

RESULTS

Strong nuclear PAX8 expression was observed in the normal corneal epithelium, iris sphincter pupillae muscle, iris pigment epithelium and dilator muscle complex, nonpigmented and pigmented epithelia of the ciliary body, lens epithelium, and a subset of retinal neurons. The normal retinal pigment epithelium and uveal melanocytes did not stain for PAX8. The ciliary body epithelial and neuroepithelial tumors (adenoma, adenocarcinoma, and medulloepithelioma) showed uniform strong nuclear PAX8 immunoreactivity. All melanocytic tumors (iris melanoma, ciliary-choroidal melanoma, and melanocytoma) and retinal pigment epithelial neoplasms showed negative results for PAX8. A subset of tumor-associated lymphocytes, most prominent in uveal melanoma, showed positive results for PAX8. The uniformity of the PAX8 staining was superior to the variable cytokeratin staining in the ciliary epithelial neoplasms and the variable Lin28A staining in malignant medulloepithelioma. The veracity of PAX8 staining was equally as robust on cytologic analysis and open-flap biopsy specimens of ciliary epithelial and iris epithelial neoplasms, melanocytoma, and melanoma.

CONCLUSIONS

PAX8 has proven to be a very useful diagnostic marker in a select group of adult intraocular tumors, and we highly recommend its inclusion in diagnostic antibody panels of morphologically challenging intraocular neoplasms.

Deacetylase activity-independent transcriptional activation by HDAC2 during TPA-induced HL-60 cell differentiation

The human myeloid leukemia cell line HL-60 differentiate into monocytes following treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA). However, the mechanism underlying the differentiation of these cells in response to TPA has not been fully elucidated. In this study, we performed ChIP-seq profiling of RNA Pol II, HDAC2, Acetyl H3 (AcH3), and H3K27me3 and analyzed differential chromatin state changes during TPA-induced differentiation of HL-60 cells. We focused on atypically active genes, which showed enhanced H3 acetylation despite increased HDAC2 recruitment. We found that HDAC2 positively regulates the expression of these genes in a histone deacetylase activity-independent manner. HDAC2 interacted with and recruited paired box 5 (PAX5) to the promoters of the target genes and regulated HL-60 cell differentiation by PAX5-mediated gene activation. Taken together, these data elucidated the specific-chromatin status during HL-60 cell differentiation following TPA exposure and suggested that HDAC2 can activate transcription of certain genes through interactions with PAX5 in a deacetylase activity-independent pathway.

Polymorphism in lncRNA AC016683.6 and its interaction with smoking exposure on the susceptibility of lung cancer

Background

Long non-coding RNAs play pivotal roles in the carcinogenesis of multiple types of cancers. This study is firstly to evaluate influence of rs4848320 and rs1110839 polymorphisms in long non-coding RNA AC016683.6 on the susceptibility of lung cancer.

Methods

The present study was a hospital-based case–control study with 434 lung cancer patients and 593 cancer-free controls. Genotyping of the two SNPs detected by Taqman^® allelic discrimination method.

Results

There were no statistically significant associations between rs4848320 and rs1110839 polymorphisms in AC016683.6 and risk of lung cancer in overall population. However, in the smoking population, rs4848320 and rs1110839 polymorphisms significantly increased the risk of lung cancer in dominant and homozygous models (Rs4848320: P = 0.029; Rs1110839: P = 0.034), respectively. In male population, rs1110839 genetic variant was related to the risk of lung cancer in all genetic models (GG vs. TT: P = 0.008; Dominant model: P = 0.029; Recessive model: P = 0.027) rather than heterozygous model. The crossover analyses provided rs4848320 and rs1110839 risk genotypes carriers combined with smoking exposure 2.218-fold, 1.755-fold increased risk of lung cancer (Rs4848320: P = 0.005; Rs1110839: P = 0.017). Additionally, there were significantly positive multiplicative interaction of rs4848320 polymorphism with smoking status, with adjusted OR of 2.244 (1.162–4.334), but rs1110839 polymorphism did not exist.

Conclusions

Rs4848320 and rs1110839 polymorphisms may be associated with lung cancer susceptibility. Interaction of rs4848320 risk genotypes with smoking exposure may strengthen the risk effect on lung cancer.

The RB tumor suppressor at the intersection of proliferation and immunity: relevance to disease immune evasion and immunotherapy

The retinoblastoma tumor suppressor (RB) was the first identified tumor suppressor based on germline predisposition to the pediatric eye tumor. Since these early studies, it has become apparent that the functional inactivation of RB is a common event in nearly all human malignancy. A great deal of research has gone into understanding how the loss of RB promotes tumor etiology and progression. Since malignant tumors are characterized by aberrant cell division, much of this research has focused upon the ability of RB to regulate the cell cycle by repression of proliferation-related genes. However, it is progressively understood that RB is an important mediator of multiple functions. One area that is gaining progressive interest is the emerging role for RB in regulating diverse features of immune function. These findings suggest that RB is more than simply a regulator of cellular proliferation; it is at the crossroads of proliferation and the immune response. Here we review the data related to the functional roles of RB on the immune system, relevance to immune evasion, and potential significance to the response to immune-therapy.

PAX8 is transcribed aberrantly in cervical tumors and derived cell lines due to complex gene rearrangements

The transcription factor PAX8, a member of the paired box-containing gene family with an important role in embryogenesis of the kidney, thyroid gland and nervous system, has been described as a biomarker in tumors of the thyroid, parathyroid, kidney and thymus. The PAX8 gene gives rise to four isoforms, through alternative mRNA splicing, but the splicing pattern in tumors is not yet established. Cervical cancer has a positive expression of PAX8; however, there is no available data determining which PAX8 isoform or isoforms are present in cervical cancer tissues as well as in cervical carcinoma-derived cell lines. Instead of a differential pattern of splicing isoforms, we found numerous previously unreported PAX8 aberrant transcripts ranging from 378 to 542 bases and present in both cervical carcinoma-derived cell lines and tumor samples. This is the first report of PAX8 aberrant transcript production in cervical cancer. Reported PAX8 isoforms possess differential transactivation properties; therefore, besides being a helpful marker for detection of cancer, PAX8 isoforms can plausibly exert differential regulation properties during carcinogenesis.

Pax factors in transcription and epigenetic remodelling

The nine Pax transcription factors that constitute the mammalian family of paired domain (PD) factors play key roles in many developmental processes. As DNA binding transcription factors, they exhibit tremendous variability and complexity in their DNA recognition patterns. This is ascribed to the presence of multiple DNA binding structural domains, namely helix-turn-helix (HTH) domains. The PD contains two HTH subdomains and four of the nine Pax factors have an additional HTH domain, the homeodomain (HD). We now review these diverse DNA binding modalities together with their properties as transcriptional activators and repressors. The action of Pax factors on gene expression is also exerted through recruitment of chromatin remodelling complexes that introduce either activating or repressive chromatin marks. Interestingly, the recent demonstration that Pax7 has pioneer activity, the unique property to "open" chromatin, further underlines the mechanistic versatility and the developmental importance of these factors.

Rb1 mRNA expression in developing mouse teeth

Rb1 is a tumor suppressor gene that regulates cell cycle progression through interactions with E2F transcription factors. In recent years, new roles for Rb1 in regulating cellular differentiation have also emerged. For example, it has been shown that Rb1 regulates osteoblast differentiation in a cell cycle independent manner, by binding to the transcription factor Runx2, and facilitating the up-regulation of late bone differentiation markers. Based on the facts that Runx2 also functions in tooth development, and that little is known about potential roles for Rb1 in mammalian tooth development, here we evaluated the expression of Rb1 mRNA in developmentally staged mouse teeth. Our data show that Rb1 mRNA is expressed in both dental epithelial and dental mesenchymal progenitor cells. In addition, Rb1 mRNA appears upregulated in differentiating ameloblasts and odontoblasts, suggesting roles for Rb1 in tooth differentiation.

PAX8 promotes tumor cell growth by transcriptionally regulating E2F1 and stabilizing RB protein

The retinoblastoma protein (RB)–E2F1 pathway has a central role in regulating the cell cycle. Several PAX proteins (tissue-specific developmental regulators), including PAX8, interact with the RB protein, and thus regulate the cell cycle directly or indirectly. Here, we report that PAX8 expression is frequent in renal cell carcinoma, bladder, ovarian and thyroid cancer cell lines, and that silencing of PAX8 in cancer cell lines leads to a striking reduction in the expression of E2F1 and its target genes, as well as a proteasome-dependent destabilization of RB protein, with the RB1 mRNA level remaining unaffected. Cancer cells expressing PAX8 undergo a G₁/S arrest and eventually senesce following PAX8 silencing. We demonstrate that PAX8 transcriptionally regulates the E2F1 promoter directly, and E2F1 transcription is enhanced after RB depletion. RB is recruited to the PAX8-binding site, and is involved in PAX8-mediated E2F1 transcription in cancer cells. Therefore, our results suggest that, in cancer, frequent and persistent expression of PAX8 is required for cell growth control through transcriptional activation of E2F1 expression and upregulation of the RB–E2F1 pathway.

RB1, development, and cancer

The RB1 gene is the first tumor suppressor gene identified whose mutational inactivation is the cause of a human cancer, the pediatric cancer retinoblastoma. The 25 years of research since its discovery has not only illuminated a general role for RB1 in human cancer, but also its critical importance in normal development. Understanding the molecular function of the RB1 encoded protein, pRb, is a long-standing goal that promises to inform our understanding of cancer, its relationship to normal development, and possible therapeutic strategies to combat this disease. Achieving this goal has been difficult, complicated by the complexity of pRb and related proteins. The goal of this review is to explore the hypothesis that, at its core, the molecular function of pRb is to dynamically regulate the location-specific assembly or disassembly of protein complexes on the DNA in response to the output of various signaling pathways. These protein complexes participate in a variety of molecular processes relevant to DNA including gene transcription, DNA replication, DNA repair, and mitosis. Through regulation of these processes, RB1 plays a uniquely prominent role in normal development and cancer.

Osteopontin is up-regulated and associated with neutrophil and macrophage infiltration in glioblastoma

Osteopontin (OPN) is a glycophosphoprotein with multiple intracellular and extracellular functions. In vitro, OPN enhances migration of mouse neutrophils and macrophages. In cancer, extracellular OPN facilitates migration of cancer cells via its RGD sequence. The present study was designed to investigate whether osteopontin is responsible for neutrophil and macrophage infiltration in human cancer and in particular in glioblastoma. We found that in vitro mouse neutrophil migration was RGD-dependent. In silico, we found that the OPN gene was one of the 5% most highly expressed genes in 20 out of 35 cancer microarray data sets in comparison with normal tissue in at least 30% of cancer patients. In some types of cancer, such as ovarian cancer, lung cancer and melanoma, the OPN gene was one of those with the highest expression levels in at least 90% of cancer patients. In glioblastoma, the most invasive type of brain tumours/glioma, but not in lower grades of glioma it was one of the 5% highest expressed genes in 90% of patients. In situ, we found increased protein levels of OPN in human glioblastoma versus normal human brain confirming in silico results. OPN protein expression was co-localized with neutrophils and macrophages. In conclusion, OPN in tumours not only induces migration of cancer cells but also of leucocytes.

Signal transduction in the human thyrocyte and its perversion in thyroid tumors

The study of normal signal transduction pathways regulating the proliferation and differentiation of a cell type allows to predict and to understand the perversions of these pathways which lead to tumorigenesis. In the case of the human thyroid cell, three cascades are mostly involved in tumorigenesis: The pathways and genetic events affecting them are described. Caveats in the use of models and the interpretation of results are formulated and the still pending questions are outlined.

PAX5 is expressed in small-cell lung cancer and positively regulates c-Met transcription

PAX5 is a nuclear transcription factor required for B cell development, and its expression was evaluated in upper aerodigestive malignancies and pancreatic cancer by immunoblotting. The PAX5 protein expression was relatively strong in small-cell lung cancer (SCLC, 11/12); however, its expression was not detected in non-SCLC (NSCLC, n=13), mesothelioma (n=7), pancreatic (n=6), esophageal (n=6) and head and neck cancer cell lines (n=12). In comparison, PAX8 and PAX3 expressions were absent or non-detectable in SCLC cell lines; however, PAX8 was expressed in most of the tested NSCLC cell lines (13/13) and also frequently in all the other cell lines. We also detected frequent expressions of PAX2 and PAX9 protein in the various cell lines. Utilizing neuroendocrine tumor samples, we found that the frequency as well as the average intensity of the expression of PAX5 increased from pulmonary carcinoid (9%, moderate and strong PAX5 expression, n=44), to large-cell neuroendocrine carcinoma (LCNC, 27%, n=11) to SCLC (33%, n=76). FISH analysis revealed no translocations of the PAX5 gene, but polyploidy in some SCLC tumor tissues (6/37). We determined that PAX5 could regulate the transcription of c-Met using luciferase-coupled reporter and chromatin immunoprecipitation analysis. In addition, the phospho-c-Met (active form) and PAX5 were both localized to the same intra-nuclear compartment in hepatocyte growth factor treated SCLC cells and interacted with each other. Finally, we determined the therapeutic translational potential of PAX5 using PAX5 knockdown SCLC cells in conjunction with Topoisomerase 1 (SN38) and c-Met (SU11274) inhibitors. Loss of endogenous PAX5 significantly decreased the viability of SCLC cells, especially when combined with SN38 or SU11274, and maximum effect was seen when both inhibitors were used. Therefore, we propose that PAX5 could be an important regulator of c-Met transcription and a potential target for therapy in SCLC.

Pigmentation PAX-ways: the role of Pax3 in melanogenesis, melanocyte stem cell maintenance, and disease

Transcription factors initiate programs of gene expression and are catalysts in downstream molecular cascades that modulate a variety of cellular processes. Pax3 is a transcription factor that is important in the melanocyte and influences melanocytic proliferation, resistance to apoptosis, migration, lineage specificity and differentiation. In this review, we focus on Pax3 and the molecular pathways that Pax3 is a part of during melanogenesis and in the melanocyte stem cell. These roles of Pax3 are emphasized during the development of diseases and syndromes resulting from either too much or too little Pax3 function. Due to its key task in melanocyte stem cells and tumors, the Pax3 pathway may provide an ideal target for either stem cell or cancer therapies.

Thyroid-stimulating hormone/cAMP-mediated proliferation in thyrocytes

Current research on thyrotropin-activated proliferation in the thyrocyte needs to be aimed at a better understanding of crosstalk and negative-feedback mechanisms with other proliferative pathways, especially the insulin/IGF-1-induced phosphoinositol-3 kinase pathway and the serum-induced MAPK or Wnt pathways. Convergence of proliferative pathways in mTOR is a hotspot of current research, and combined treatment using double class inhibitors for thyroid cancer may bring some success. New thyroid-stimulating hormone receptor (TSHR)-interacting proteins, a better picture of cAMP targets, a deeper knowledge of the action of the protein kinase A regulatory subunits, especially their interactions with the replication machinery, and a further understanding of mechanisms that lead to cell cycle progression through G₁/S and G₂/M checkpoints are areas that need further elucidation. Finally, massive information coming from microarray data analysis will prompt our understanding of thyroid-stimulating hormone-promoted thyrocyte proliferation in health and disease.

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.

The role of Pax genes in the development of tissues and organs: Pax3 and Pax7 regulate muscle progenitor cell functions

Pax genes play key roles in the formation of tissues and organs during embryogenesis. Pax3 and Pax7 mark myogenic progenitor cells and regulate their behavior and their entry into the program of skeletal muscle differentiation. Recent results have underlined the importance of the Pax3/7 population of cells for skeletal muscle development and regeneration. We present our current understanding of different aspects of Pax3/7 function in myogenesis, focusing on the mouse model. This is compared with that of other Pax proteins in the emergence of tissue specific lineages and their differentiation as well as in cell survival, proliferation, and migration. Finally, we consider the molecular mechanisms that underlie the function of Pax transcription factors, including the cofactors and regulatory networks with which they interact.

Rb induces a proliferative arrest and curtails Brn-2 expression in retinoblastoma cells

BACKGROUND

Retinoblastoma is caused by loss of the Rb protein in early retinal cells. Although numerous Rb functions have been identified, Rb effects that specifically relate to the suppression of retinoblastoma have not been defined.

RESULTS

In this study, we examined the effects of restoring Rb to Y79 retinoblastoma cells, using novel retroviral and lentiviral vectors that co-express green fluorescent protein (GFP). The lentiviral vector permitted transduction with sufficient efficiency to perform biochemical analyses. Wild type Rb (RbWT) and to a lesser extent the low penetrance mutant Rb661W induced a G0/G1 arrest associated with induction of p27KIP1 and repression of cyclin E1 and cyclin E2. Microarray analyses revealed that in addition to down-regulating E2F-responsive genes, Rb repressed expression of Brn-2 (POU3F2), which is implicated as an important transcriptional regulator in retinal progenitor cells and other neuroendocrine cell types. The repression of Brn-2 was a specific Rb effect, as ectopic p27 induced a G0/G1 block, but enhanced, rather than repressed, Brn-2 expression.

CONCLUSION

In addition to Rb effects that occur in many cell types, Rb regulates a gene that selectively governs the behavior of late retinal progenitors and related cells.

The retinoblastoma tumor-suppressor gene, the exception that proves the rule

The retinoblastoma tumor-suppressor gene (Rb1) is centrally important in cancer research. Mutational inactivation of Rb1 causes the pediatric cancer retinoblastoma, while deregulation of the pathway in which it functions is common in most types of human cancer. The Rb1-encoded protein (pRb) is well known as a general cell cycle regulator, and this activity is critical for pRb-mediated tumor suppression. The main focus of this review, however, is on more recent evidence demonstrating the existence of additional, cell type-specific pRb functions in cellular differentiation and survival. These additional functions are relevant to carcinogenesis suggesting that the net effect of Rb1 loss on the behavior of resulting tumors is highly dependent on biological context. The molecular mechanisms underlying pRb functions are based on the cellular proteins it interacts with and the functional consequences of those interactions. Better insight into pRb-mediated tumor suppression and clinical exploitation of pRb as a therapeutic target will require a global view of the complex, interdependent network of pocket protein complexes that function simultaneously within given tissues.

Regulation of cell lineage specification by the retinoblastoma tumor suppressor

Early studies of the retinoblastoma gene (RB) have uncovered its critical role as a regulator of the G(1)/S cell cycle phase progression. Surprisingly, genetic approaches in mammals and nematodes have also shown RB controls cell lineage specification and aspects of differentiation. The RB gene product accomplishes this by diverse mechanisms such as by interacting with tissue-specific transcription factors, enhancing RNA interference, and modifying chromatin structure. We review recent studies uncovering novel mechanisms by which RB works in several cell lineages and we provide perspectives on how these new findings might relate to RB tumor suppression.

An E2F binding-deficient Rb1 protein partially rescues developmental defects associated with Rb1 nullizygosity

Rb1 is essential for normal embryonic development, as null mice die in midgestation with widespread unscheduled cell proliferation. Rb1 protein (pRb) mediates cell cycle control by binding E2F transcription factors and repressing expression from E2F-dependent promoters. An increasing amount of evidence suggests that pRb loss also compromises cellular differentiation. Since differentiation is often dependent on cell cycle exit, it is currently unclear whether the effects of pRb on differentiation are an indirect consequence of pRb/E2F-mediated cell cycle control or whether they reflect direct cell-type-specific pRb functions. We have mutated Rb1 in the mouse to express a protein (R654W) specifically deficient in binding E2F1, E2F2, and E2F3. R654W mutant embryos exhibit cell cycle defects the same as those of Rb1 null embryos, reinforcing the importance of the interactions of pRb with E2F1, E2F2, and E2F3 for cell cycle control. However, R654W embryos survive at least 2 days longer than Rb1 null embryos, and increased life span is associated with improved erythrocyte and fetal liver macrophage differentiation. In contrast, R654W pRb does not rescue differentiation defects associated with pRb-deficient retinae. These data indicate that Rb1 makes important cell-type-specific contributions to cellular differentiation that are genetically separable from its general ability to stably bind E2F1, E2F2, and E2F3 and regulate the cell cycle.

A PANorama of PAX genes in cancer and development

Populations of self-renewing cells that arise during normal embryonic development harbour the potential for rapid proliferation, migration or transdifferentiation and, therefore, tumour generation. So, control mechanisms are essential to prevent rapidly expanding populations from malignant growth. Transcription factors have crucial roles in ensuring establishment of such regulation, with the Pax gene family prominent amongst these. This review examines the role of Pax family members during embryogenesis, and their contribution to tumorigenesis when subverted.