Keeping an eye on retinoblastoma control of human embryonic stem cells

Cell cycle heterogeneity directs spontaneous 2C state entry and exit in mouse embryonic stem cells

Mouse embryonic stem cells (ESCs) show cell-to-cell heterogeneity. A small number of two-cell-like cells (2CLCs) marked by endogenous retrovirus activation emerge spontaneously. The 2CLCs are unstable and they are prone to transiting back to the pluripotent state without extrinsic stimulus. To understand how this bidirectional transition takes place, we performed single-cell RNA sequencing on isolated 2CLCs that underwent 2C-like state exit and re-entry, and revealed a step-by-step transitional process between 2C-like and pluripotent states. Mechanistically, we found that cell cycle played an important role in mediating these transitions by regulating assembly of the nucleolus and peri-nucleolar heterochromatin to influence 2C gene Dux expression. Collectively, our findings provide a roadmap of the 2C-like state entry and exit in ESCs and also a causal role of the cell cycle in promoting these transitions.

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The entry to and exit from the 2C-like state showed a step-by-step roadmap

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Cell cycle participates in mediating dynamic transitions between ESCs and 2CLCs

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G1/S phase arrest facilitates the Dux locus escape from heterochromatin

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Nucleolus-heterochromatin remodeling is involved in 2C activation

Rbbp4 Suppresses Premature Differentiation of Embryonic Stem Cells

Polycomb group (PcG) proteins exist in distinct multi-protein complexes and play a central role in silencing developmental genes, yet the underlying mechanisms remain elusive. Here, we show that deficiency of retinoblastoma binding protein 4 (RBBP4), a component of the Polycomb repressive complex 2 (PRC2), in embryonic stem cells (ESCs) leads to spontaneous differentiation into mesendodermal lineages. We further show that Rbbp4 and core PRC2 share an important number of common genomic targets, encoding regulators involved in early germ layer specification. Moreover, we find that Rbbp4 is absolutely essential for genomic targeting of PRC2 to a subset of developmental genes. Interestingly, we demonstrate that Rbbp4 is necessary for sustaining the expression of Oct4 and Sox2 and that the forced co-expression of Oct4 and Sox2 fully rescues the pluripotency of Rbbp4-null ESCs. Therefore, our study indicates that Rbbp4 links maintenance of the pluripotency regulatory network with repression of mesendoderm lineages.

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RBBP4 deficiency in ESCs leads to spontaneous differentiation into mesendodermal lineages

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Rbbp4 binding sites in ESCs substantially overlap with PRC2 binding

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Rbbp4 is absolutely essential for PRC2 chromatin occupancy

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Rbbp4 is necessary for sustaining the expression levels of Oct4 and Sox2

Brief Report: Cell Cycle Dynamics of Human Pluripotent Stem Cells Primed for Differentiation

Understanding the molecular properties of the cell cycle of human pluripotent stem cells (hPSCs) is critical for effectively promoting differentiation. Here, we use the Fluorescence Ubiquitin Cell Cycle Indicator system adapted into hPSCs and perform RNA sequencing on cell cycle sorted hPSCs primed and unprimed for differentiation. Gene expression patterns of signaling factors and developmental regulators change in a cell cycle‐specific manner in cells primed for differentiation without altering genes associated with pluripotency. Furthermore, we identify an important role for PI3K signaling in regulating the early transitory states of hPSCs toward differentiation. stem cells2019;37:1151–1157

A transient DMSO treatment increases the differentiation potential of human pluripotent stem cells through the Rb family

The propensity for differentiation varies substantially across human pluripotent stem cell (hPSC) lines, greatly restricting the use of hPSCs for cell replacement therapy or disease modeling. Here, we investigate the underlying mechanisms and demonstrate that activation of the retinoblastoma (Rb) pathway in a transient manner is important for differentiation. In prior work, we demonstrated that pre-treating hPSCs with dimethylsulfoxide (DMSO) before directed differentiation enhanced differentiation potential across all three germ layers. Here, we show that exposure to DMSO improves the efficiency of hPSC differentiation through Rb and by repressing downstream E2F-target genes. While transient inactivation of the Rb family members (including Rb, p107, and p130) suppresses DMSO’s capacity to enhance differentiation across all germ layers, transient expression of a constitutively active (non-phosphorylatable) form of Rb increases the differentiation efficiency similar to DMSO. Inhibition of downstream targets of Rb, such as E2F signaling, also promotes differentiation of hPSCs. More generally, we demonstrate that the duration of Rb activation plays an important role in regulating differentiation capacity.

Regulation of cyclin E1 expression in human pluripotent stem cells and derived neural progeny

Human pluripotent stem cells (hPSCs), including embryonic and induced pluripotent stem cells (hESCs and hiPSCs) show unique cell cycle characteristics, such as a short doubling time due to an abbreviated G1 phase. Whether or not the core cell cycle machinery directly regulates the stemness and/or the differentiation potential of hPSCs remains to be determined. To date, several scenarios describing the atypical cell cycle of hPSCs have been suggested, and therefore there is still controversy over how cyclins, master regulators of the cell cycle, are expressed and regulated. Furthermore, the cell cycle profile and the expression pattern of major cyclins in hESCs-derived neuroprogenitors (NP) have not been studied yet. Therefore, herein we characterized the expression pattern of major cyclins in hPSCs and NP. We determined that all studied cyclins mRNA expression levels fluctuate along cell cycle. Particularly, after a thorough analysis of synchronized cell populations, we observed that cyclin E1 mRNA levels increased sharply in G1/S concomitantly with cyclin E1 protein accumulation in hPSCs and NP. Additionally, we demonstrated that cyclin E1 mRNA expression levels involves the activation of MEK/ERK pathway and the transcription factors c-Myc and E2Fs in hPSCs. Lastly, our results reveal that proteasome mediates the marked down-regulation (degradation) of cyclin E1 protein observed in G2/M by a mechanism that requires a functional CDK2 but not GSK3β activity.

ABBREVIATIONS

hPSCs: human pluripotent stem cells; hESCs: human embryonic stem cells; hiPSCs: human induced pluripotent stem cells; NP: neuroprogenitors; HF: human foreskin fibroblasts; MEFs: mouse embryonic fibroblasts; iMEFs: irradiated mouse embryonic fibroblasts; CDKs: cyclindependent kinases; CKIs: CDK inhibitors; CNS: central nervous system; Oct-4: Octamer-4; EB: embryoid body; AFP: Alpha-fetoprotein; cTnT: Cardiac Troponin T; MAP-2: microtubule-associated protein; TUJ-1: neuron-specific class III β-tubulin; bFGF: basic fibroblastic growth factor; PI3K: Phosphoinositide 3-kinase; KSR: knock out serum replacement; CM: iMEF conditioned medium; E8: Essential E8 medium.

Cycling to Meet Fate: Connecting Pluripotency to the Cell Cycle

Pluripotent stem cells are characterized by their high proliferative rates, their ability to self-renew and their potential to differentiate to all the three germ layers. This rapid proliferation is brought about by a highly modified cell cycle that allows the cells to quickly shuttle from DNA synthesis to cell division, by reducing the time spent in the intervening gap phases. Many key regulators that define the somatic cell cycle are either absent or exhibit altered behavior, allowing the pluripotent cell to bypass cell cycle checkpoints typical of somatic cells. Experimental analysis of this modified stem cell cycle has been challenging due to the strong link between rapid proliferation and pluripotency, since perturbations to the cell cycle or pluripotency factors result in differentiation. Despite these hurdles, our understanding of this unique cell cycle has greatly improved over the past decade, in part because of the availability of new technologies that permit the analysis of single cells in heterogeneous populations. This review aims to highlight some of the recent discoveries in this area with a special emphasis on different states of pluripotency. We also discuss the highly interlinked network that connects pluripotency factors and key cell cycle genes and review evidence for how this interdependency may promote the rapid cell cycle. This issue gains translational importance since disruptions in stem cell proliferation and differentiation can impact disorders at opposite ends of a spectrum, from cancer to degenerative disease.

TRIM8: Making the Right Decision between the Oncogene and Tumour Suppressor Role

The TRIM8/GERP protein is a member of the TRIM family defined by the presence of a common domain structure composed of a tripartite motif including a RING-finger, one or two B-box domains, and a coiled-coil motif. The TRIM8 gene maps on chromosome 10 within a region frequently found deleted and rearranged in tumours and transcribes a 3.0-kB mRNA. Its expression is mostly ubiquitously in murine and human tissues, and in epithelial and lymphoid cells, it can be induced by IFNγ. The protein spans 551 aa and is highly conserved during evolution. TRIM8 plays divergent roles in many biological processes, including important functions in inflammation and cancer through regulating various signalling pathways. In regulating cell growth, TRIM8 exerts either a tumour suppressor action, playing a prominent role in regulating p53 tumour suppressor activity, or an oncogene function, through the positive regulation of the NF-κB pathway. The molecular mechanisms underlying this dual role in human cancer will be discussed in depth in this review, and it will highlight the challenge and importance of developing novel therapeutic strategies specifically aimed at blocking the pro-oncogenic arm of the TRIM8 signalling pathway without affecting its tumour suppressive effects.

G1 cyclins protect pluripotency

G1 cyclins are considered essential for DNA replication and cell division. A recent report now shows that some cells can cycle in the absence of G1 cyclins. In embryonic stem cells and cancer cells, G1 cyclins are required to activate cyclin-dependent kinases to phosphorylate core pluripotency factors and maintain pluripotency.

Cell cycle regulation of embryonic stem cells and mouse embryonic fibroblasts lacking functional Pax7

The transcription factor Pax7 plays a key role during embryonic myogenesis and in adult organisms in that it sustains the proper function of satellite cells, which serve as adult skeletal muscle stem cells. Recently we have shown that lack of Pax7 does not prevent the myogenic differentiation of pluripotent stem cells. In the current work we show that the absence of functional Pax7 in differentiating embryonic stem cells modulates cell cycle facilitating their proliferation. Surprisingly, deregulation of Pax7 function also positively impacts at the proliferation of mouse embryonic fibroblasts. Such phenotypes seem to be executed by modulating the expression of positive cell cycle regulators, such as cyclin E.

Retinoblastoma tumor suppressor functions shared by stem cell and cancer cell strategies

Carcinogenic transformation of somatic cells resembles nuclear reprogramming toward the generation of pluripotent stem cells. These events share eternal escape from cellular senescence, continuous self-renewal in limited but certain population of cells, and refractoriness to terminal differentiation while maintaining the potential to differentiate into cells of one or multiple lineages. As represented by several oncogenes those appeared to be first keys to pluripotency, carcinogenesis and nuclear reprogramming seem to share a number of core mechanisms. The retinoblastoma tumor suppressor product retinoblastoma (RB) seems to be critically involved in both events in highly complicated manners. However, disentangling such complicated interactions has enabled us to better understand how stem cell strategies are shared by cancer cells. This review covers recent findings on RB functions related to stem cells and stem cell-like behaviors of cancer cells.

Undifferentiated State Induced by Rb-p53 Double Inactivation in Mouse Thyroid Neuroendocrine Cells and Embryonic Fibroblasts

Retinoblastoma tumor suppressor protein (RB) is inactivated more frequently during tumor progression than during tumor initiation. However, its exact role in controlling the malignant features associated with tumor progression is poorly understood. We established in vivo and in vitro models to investigate the undifferentiated state induced by Rb inactivation. Rb heterozygous mice develop well-differentiated thyroid medullary carcinoma. We found that additional deletion of Trp53, without change in lineage, converted these Rb-deficient tumors to a poorly differentiated type associated with higher self-renewal activity. Freshly prepared mouse embryonic fibroblasts (MEFs) of Rb(-/-) ; Trp53(-/-) background formed stem cell-like spheres that expressed significant levels of embryonic genes despite of lacking the ability to form colonies on soft agar or tumors in immune-deficient mice. This suggested that Rb-p53 double inactivation resulted in an undifferentiated status but without carcinogenic conversion. We next established Rb(-/-) ; N-ras(-/-) MEFs that harbored a spontaneous carcinogenic mutation in Trp53. These cells (RN6), in an Rb-dependent manner, efficiently generated spheres that expressed very high levels of embryonic genes, and appeared to be carcinogenic. We then screened an FDA-approved drug library to search for agents that suppressed the spherogenic activity of RN6 cells. Data revealed that RN6 cells were sensitive to specific agents including ones those are effective against cancer stem cells. Taken together, all these findings suggest that the genetic interaction between Rb and p53 is a critical determinant of the undifferentiated state in normal and tumor cells.

Engineering dynamic cell cycle control with synthetic small molecule-responsive RNA devices

Background

The cell cycle plays a key role in human health and disease, including development and cancer. The ability to easily and reversibly control the mammalian cell cycle could mean improved cellular reprogramming, better tools for studying cancer, more efficient gene therapy, and improved heterologous protein production for medical or industrial applications.

Results

We engineered RNA-based control devices to provide specific and modular control of gene expression in response to exogenous inputs in living cells. Specifically, we identified key regulatory nodes that arrest U2-OS cells in the G0/1 or G2/M phases of the cycle. We then optimized the most promising key regulators and showed that, when these optimized regulators are placed under the control of a ribozyme switch, we can inducibly and reversibly arrest up to ~80 % of a cellular population in a chosen phase of the cell cycle. Characterization of the reliability of the final cell cycle controllers revealed that the G0/1 control device functions reproducibly over multiple experiments over several weeks.

Conclusions

To our knowledge, this is the first time synthetic RNA devices have been used to control the mammalian cell cycle. This RNA platform represents a general class of synthetic biology tools for modular, dynamic, and multi-output control over mammalian cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s13036-015-0019-7) contains supplementary material, which is available to authorized users.

The Rb-E2F transcriptional regulatory pathway in tumor angiogenesis and metastasis

The retinoblastoma tumor suppressor protein Rb plays a major role in regulating G1/S transition and is a critical regulator of cell proliferation. Rb protein exerts its growth regulatory properties mainly by physically interacting with the transcriptionally active members of the E2F transcription factor family, especially E2Fs 1, 2, and 3. Given its critical role in regulating cell proliferation, it is not surprising that Rb is inactivated in almost all tumors, either through the mutation of Rb gene itself or through the mutations of its upstream regulators including K-Ras and INK4. Recent studies have revealed a significant role for Rb and its downstream effectors, especially E2Fs, in regulating various aspects of tumor progression, angiogenesis, and metastasis. Thus, components of the Rb-E2F pathway have been shown to regulate the expression of genes involved in angiogenesis, including VEGF and VEGFR, genes involved in epithelial-mesenchymal transition including E-cadherin and ZEB proteins, and genes involved in invasion and migration like matrix metalloproteinases. Rb has also been shown to play a major role in the functioning of normal and cancer stem cells; further, Rb and E2F appear to play a regulatory role in the energy metabolism of cancer cells. These findings raise the possibility that mutational events that initiate tumorigenesis by inducing uncontrolled cell proliferation might also contribute to the progression and metastasis of cancers through the mediation of the Rb-E2F transcriptional regulatory pathway. This review highlights these recent studies on tumor promoting functions of the Rb-E2F pathway.

Inhibition of pluripotency networks by the Rb tumor suppressor restricts reprogramming and tumorigenesis

Mutations in the retinoblastoma tumor suppressor gene Rb are involved in many forms of human cancer. In this study, we investigated the early consequences of inactivating Rb in the context of cellular reprogramming. We found that Rb inactivation promotes the reprogramming of differentiated cells to a pluripotent state. Unexpectedly, this effect is cell cycle independent, and instead reflects direct binding of Rb to pluripotency genes, including Sox2 and Oct4, which leads to a repressed chromatin state. More broadly, this regulation of pluripotency networks and Sox2 in particular is critical for the initiation of tumors upon loss of Rb in mice. These studies therefore identify Rb as a global transcriptional repressor of pluripotency networks, providing a molecular basis for previous reports about its involvement in cell fate pliability, and implicate misregulation of pluripotency factors such as Sox2 in tumorigenesis related to loss of Rb function.

A nontranscriptional role for Oct4 in the regulation of mitotic entry

Rapid progression through the cell cycle and a very short G1 phase are defining characteristics of embryonic stem cells. This distinct cell cycle is driven by a positive feedback loop involving Rb inactivation and reduced oscillations of cyclins and cyclin-dependent kinase (Cdk) activity. In this setting, we inquired how ES cells avoid the potentially deleterious consequences of premature mitotic entry. We found that the pluripotency transcription factor Oct4 (octamer-binding transcription factor 4) plays an unappreciated role in the ES cell cycle by forming a complex with cyclin-Cdk1 and inhibiting Cdk1 activation. Ectopic expression of Oct4 or a mutant lacking transcriptional activity recapitulated delayed mitotic entry in HeLa cells. Reduction of Oct4 levels in ES cells accelerated G2 progression, which led to increased chromosomal missegregation and apoptosis. Our data demonstrate an unexpected nontranscriptional function of Oct4 in the regulation of mitotic entry.

Shared mechanisms in stemness and carcinogenesis: lessons from oncogenic viruses

A rise in technologies for epigenetic reprogramming of cells to pluripotency, highlights the potential of understanding and manipulating cellular plasticity in unprecedented ways. Increasing evidence points to shared mechanisms between cellular reprogramming and the carcinogenic process, with the emerging possibility to harness these parallels in future therapeutics. In this review, we present a synopsis of recent work from oncogenic viruses which contributes to this body of knowledge, establishing a nexus between infection, cancer, and stemness.

The RB family is required for the self-renewal and survival of human embryonic stem cells

The mechanisms ensuring the long-term self-renewal of human embryonic stem cells are still only partly understood, limiting their use in cellular therapies. Here we found that increased activity of the RB cell cycle inhibitor in human embryonic stem cells induces cell cycle arrest, differentiation and cell death. Conversely, inactivation of the entire RB family (RB, p107 and p130) in human embryonic stem cells triggers G2/M arrest and cell death through functional activation of the p53 pathway and the cell cycle inhibitor p21. Differences in E2F target gene activation upon loss of RB family function between human embryonic stem cells, mouse embryonic stem cells and human fibroblasts underscore key differences in the cell cycle regulatory networks of human embryonic stem cells. Finally, loss of RB family function promotes genomic instability in both human and mouse embryonic stem cells, uncoupling cell cycle defects from chromosomal instability. These experiments indicate that a homeostatic level of RB activity is essential for the self-renewal and the survival of human embryonic stem cells.

SATB1 collaborates with loss of p16 in cellular transformation

Tumor progression is associated with invasiveness and metastatic potential. The special AT-rich binding protein 1 (SATB1) has been identified as a key factor in the progression of breast cancer cells to a malignant phenotype and is associated with progression of human tumors. In normal development, SATB1 coordinates gene expression of progenitor cells by functioning as a genome organizer. In contrast to progenitor and tumor cells, SATB1 expression in nontransformed cells is not compatible with proliferation. Here we show that SATB1 expression in mouse embryonic fibroblasts induces cell cycle arrest and senescence that is associated with elevated p16 protein levels. Deletion of p16 overcomes the SATB1-induced senescence. We further provide evidence for an interaction of SATB1 with the retinoblastoma (RB)/E2F pathway downstream of p16. A combined deletion of the RB proteins, RB, p107 and p130 (triple-mutant; TM), prevents SATB1-induced G1 arrest, which is restored upon the reintroduction of RB into SATB1-expressing TM fibroblasts. SATB1 interacts with the E2F/RB complex and regulates the cyclin E promoter in an E2F-dependent manner. These findings demonstrate that p16 and the RB/E2F pathway are critical for SATB1-induced cell cycle arrest. In the absence of p16, SATB1 causes anchorage-independent growth and invasive phenotype in fibroblasts. Our data illustrate that p16 mutations collaborate with the oncogenic activity of SATB1. Consistent with our finding, a literature survey shows that deletion of p16 is generally associated with SATB1 expressing human cell lines and tumors.

A simple tool to improve pluripotent stem cell differentiation

We develop a method to overcome previously documented restrictions on the differentiation propensities of pluripotent stem cells. Culturing pluripotent stem cells in dimethylsulfoxide (DMSO) activates the retinoblastoma protein, increases the proportion of cells in the early G1 phase of the cell cycle, and subsequently improves their competency for directed differentiation into multiple lineages in more than 25 stem cell lines. DMSO treatment also promotes terminal differentiation into functional derivatives.

Human embryonic stem cells exhibit increased propensity to differentiate during the G1 phase prior to phosphorylation of retinoblastoma protein

While experimentally induced arrest of human embryonic stem cells (hESCs) in G1 has been shown to stimulate differentiation, it remains unclear whether the unperturbed G1 phase in hESCs is causally related to differentiation. Here, we use centrifugal elutriation to isolate and investigate differentiation propensities of hESCs in different phases of their cell cycle. We found that isolated G1 cells exhibit higher differentiation propensity compared with S and G2 cells, and they differentiate at low cell densities even under self-renewing conditions. This differentiation of G1 cells was partially prevented in dense cultures of these cells and completely abrogated in coculture with S and G2 cells. However, coculturing without cell-to-cell contact did not rescue the differentiation of G1 cells. Finally, we show that the subset of G1 hESCs with reduced phosphorylation of retinoblastoma has the highest propensity to differentiate and that the differentiation is preceded by cell cycle arrest. These results provide direct evidence for increased propensity of hESCs to differentiate in G1 and suggest a role for neighboring cells in preventing differentiation of hESCs as they pass through a differentiation sensitive, G1 phase.

RB116: an RB1+ retinoblastoma cell line expressing primitive markers

PURPOSE

Retinoblastoma (RB), an intraocular tumor of childhood, is commonly associated with mutations in the RB1 gene. RB116 is a novel, early passage RB cell line that has not been previously characterized. In this study, we examined RB116 for the expression of RB1 and tested the hypothesis that RB116 cells would express stem cell markers as well as retinal progenitor cell markers. We compared RB116 cells with other well known RB cell lines, including Y79 and WERI-RB27.

METHODS

We evaluated expression of RB1 in RB116 cells by sequencing, multiplex ligation-dependent probe amplification, quantitative reverse transcriptase polymerase chain reaction (qRT-PCR), western immunoblot, and immunocytochemistry. Next, RB116 cells, along with Y79 and WERI-RB27 cells, were examined for expression of stem cell markers (ABCG2, Nanog, Oct3/4, ALDH1A1) and retinal progenitor markers (PAX6, CHX10) by quantitative immunocytochemistry. Immunocytochemical findings were accompanied by PCR analysis.

RESULTS

RB116 cells expressed RB1 at the mRNA and protein levels, with no mutations detected by either sequencing analysis, or gene dosage abnormalities detected by multiplex ligation-dependent probe amplification. The RB1 protein was immunoreactive in RB116 cells with an atypical perinuclear localization. RB116 cells also expressed stem cell markers, with 3%-5% of cells immunopositive for ABCG2, Oct3/4 and ALDH1A1, with at least 18% of cells immunoreactive to Nanog. These findings were confirmed by RT-PCR. Small percentages of RB116 cells also exhibited immunoreactivity to retinal progenitor markers PAX6 (9.8%) and CHX10 (1.2%). Expression of mRNAs for these markers was confirmed by qRT-PCR.

CONCLUSIONS

RB116 cells demonstrate RB1 expression accompanied by atypical perinuclear localization. RB116 cells also express primitive stem cell and retinal progenitor cell markers. Further studies on the phenotypes of both RB1-positive and RB1-negative human RB cells may be important in assessing differentiation potential of these cells, as well as designing targeted differentiation therapies.

The retinoblastoma tumor suppressor and stem cell biology

Stem cells play a critical role during embryonic development and in the maintenance of homeostasis in adult individuals. A better understanding of stem cell biology, including embryonic and adult stem cells, will allow the scientific community to better comprehend a number of pathologies and possibly design novel approaches to treat patients with a variety of diseases. The retinoblastoma tumor suppressor RB controls the proliferation, differentiation, and survival of cells, and accumulating evidence points to a central role for RB activity in the biology of stem and progenitor cells. In some contexts, loss of RB function in stem or progenitor cells is a key event in the initiation of cancer and determines the subtype of cancer arising from these pluripotent cells by altering their fate. In other cases, RB inactivation is often not sufficient to initiate cancer but may still lead to some stem cell expansion, raising the possibility that strategies aimed at transiently inactivating RB might provide a novel way to expand functional stem cell populations. Future experiments dedicated to better understanding how RB and the RB pathway control a stem cell's decisions to divide, self-renew, or give rise to differentiated progeny may eventually increase our capacity to control these decisions to enhance regeneration or help prevent cancer development.

Twists in views on RB functions in cellular signaling, metabolism and stem cells

One-quarter of a century ago, identification of the human retinoblastoma gene (RB) loci proved Knudson's 'two-hit theory' that tumor suppressor genes exist. Since then, numerous works delineated crucial roles for the RB protein (pRB)-E2F transcription factor complex in G1-S phase transition. In addition, discovering the relationship between pRB and tissue-specific transcription factors enabled a better understanding of how cell cycle exit and terminal differentiation are coupled. Recent works provoked many exciting twists in views on pRB functions during cancer initiation and progression beyond its previously well-appreciated roles. Various mitogenic and cytostatic cellular signals appeared to modulate pRB functions and thus affect a wide variety of effector molecules. In addition, genetic studies in mice as well as other creatures incessantly force us to revise our views on pRB functions. This review will focus particularly on the roles of pRB in regulating intracellular signaling, cell metabolism, chromatin function, stem cells and cancer stem cells.

New insights into cell cycle regulation and DNA damage response in embryonic stem cells

Embryonic stem cells (ESCs) have unlimited proliferative potential, while retaining the ability to differentiate into descendants of all three embryonic layers. High proliferation rate of ESCs is accompanied by a shortening of the G(1) phase and the lack of G(1) checkpoint following DNA damage. The absence of G(1) arrest in ESCs after DNA damage is likely caused by a dysfunction of the p53-dependent p21Waf1 pathway that is a key event for the maintenance of pluripotency. There are controversial data on the functional status of p53, but it is well established that one of the key p53 target-p21Waf1-is expressed in ESCs at a very low level. Despite the lack of G(1) checkpoint, ESCs are capable to repair DNA defects; moreover the DNA damage response (DDR) signaling operates very effectively throughout the cell cycle. This review covers also the results obtained with the reprogramming of somatic cells into the induced pluripotent stem cells, for which have been shown that a partial dysfunction of the p53Waf1 pathway increases the frequency of generation of pluripotent cells. In summary, these results indicate that the G(1) checkpoint control and DDR are distinct from somatic cells and their status is tightly connected with maintaining of pluripotency and self-renewal.

Newly identified aspects of tumor suppression by RB

The retinoblastoma (RB) tumor suppressor belongs to a cellular pathway that plays a crucial role in restricting the G1-S transition of the cell cycle in response to a large number of extracellular and intracellular cues. Research in the last decade has highlighted the complexity of regulatory networks that ensure proper cell cycle progression, and has also identified multiple cellular functions beyond cell cycle regulation for RB and its two family members, p107 and p130. Here we review some of the recent evidence pointing to a role of RB as a molecular adaptor at the crossroads of multiple pathways, ensuring cellular homeostasis in different contexts. In particular, we discuss the pro- and anti-tumorigenic roles of RB during the early stages of cancer, as well as the importance of the RB pathway in stem cells and cell fate decisions.

MicroRNAs in embryonic stem cell function and fate

Since their discovery in the early 1990s, microRNAs (miRs) have gone from initially being considered an oddity to being recognized as a level of gene expression regulation that is integral to the normal function of cells and organisms. They are implicated in many if not all biological processes in animals, from apoptosis and cell signaling to organogenesis and development. Our understanding of cell regulatory states, as determined primarily by transcription factor (TF) profiles, is incomplete without consideration of the corresponding miR profile. The miR complement of a cell provides robust and redundant control over the output of hundreds of possible targets for each miR. miRs are common components of regulatory pathways, and in some cases can constitute on-off switches that regulate crucial fate decisions. In this review, we summarize our current knowledge about the biogenesis and regulation of miRs and describe their involvement in the pathways that regulate cell division, pluripotency, and reprogramming to the pluripotent state.

Cell cycle regulation during proliferation and differentiation of mammalian muscle precursor cells

Proliferation and differentiation of muscle precursor cells are intensively studied not only in the developing mouse embryo but also using models of skeletal muscle regeneration or analyzing in vitro cultured cells. These analyses allowed to show the universality of the cell cycle regulation and also uncovered tissue-specific interplay between major cell cycle regulators and factors crucial for the myogenic differentiation. Examination of the events accompanying proliferation and differentiation leading to the formation of functional skeletal muscle fibers allows understanding the molecular basis not only of myogenesis but also of skeletal muscle regeneration. This chapter presents the basis of the cell cycle regulation in proliferating and differentiating muscle precursor cells during development and after muscle injury. It focuses at major cell cycle regulators, myogenic factors, and extracellular environment impacting on the skeletal muscle.

Human embryonic stem cells are capable of executing G1/S checkpoint activation

Embryonic stem cells progress very rapidly through the cell cycle, allowing limited time for cell cycle regulatory circuits that typically function in somatic cells. Mechanisms that inhibit cell cycle progression upon DNA damage are of particular importance, as their malfunction may contribute to the genetic instability observed in human embryonic stem cells (hESCs). In this study, we exposed undifferentiated hESCs to DNA-damaging ultraviolet radiation-C range (UVC) light and examined their progression through the G1/S transition. We show that hESCs irradiated in G1 phase undergo cell cycle arrest before DNA synthesis and exhibit decreased cyclin-dependent kinase two (CDK2) activity. We also show that the phosphatase Cdc25A, which directly activates CDK2, is downregulated in irradiated hESCs through the action of the checkpoint kinases Chk1 and/or Chk2. Importantly, the classical effector of the p53-mediated pathway, protein p21, is not a regulator of G1/S progression in hESCs. Taken together, our data demonstrate that cultured undifferentiated hESCs are capable of preventing entry into S-phase by activating the G1/S checkpoint upon damage to their genetic complement.

G1 arrest and differentiation can occur independently of Rb family function

Repression of E2F target genes is required for cell cycle arrest in Rb family (Rb, p107, and p130)-deficient cells.

The ability of progenitor cells to exit the cell cycle is essential for proper embryonic development and homeostasis, but the mechanisms governing cell cycle exit are still not fully understood. Here, we tested the requirement for the retinoblastoma (Rb) protein and its family members p107 and p130 in G0/G1 arrest and differentiation in mammalian cells. We found that Rb family triple knockout (TKO) mouse embryos survive until days 9–11 of gestation. Strikingly, some TKO cells, including in epithelial and neural lineages, are able to exit the cell cycle in G0/G1 and differentiate in teratomas and in culture. This ability of TKO cells to arrest in G0/G1 is associated with the repression of key E2F target genes. Thus, G1 arrest is not always dependent on Rb family members, which illustrates the robustness of cell cycle regulatory networks during differentiation and allows for the identification of candidate pathways to inhibit the expansion of cancer cells with mutations in the Rb pathway.

Tandem E2F binding sites in the promoter of the p107 cell cycle regulator control p107 expression and its cellular functions

The retinoblastoma tumor suppressor (Rb) is a potent and ubiquitously expressed cell cycle regulator, but patients with a germline Rb mutation develop a very specific tumor spectrum. This surprising observation raises the possibility that mechanisms that compensate for loss of Rb function are present or activated in many cell types. In particular, p107, a protein related to Rb, has been shown to functionally overlap for loss of Rb in several cellular contexts. To investigate the mechanisms underlying this functional redundancy between Rb and p107 in vivo, we used gene targeting in embryonic stem cells to engineer point mutations in two consensus E2F binding sites in the endogenous p107 promoter. Analysis of normal and mutant cells by gene expression and chromatin immunoprecipitation assays showed that members of the Rb and E2F families directly bound these two sites. Furthermore, we found that these two E2F sites controlled both the repression of p107 in quiescent cells and also its activation in cycling cells, as well as in Rb mutant cells. Cell cycle assays further indicated that activation of p107 transcription during S phase through the two E2F binding sites was critical for controlled cell cycle progression, uncovering a specific role for p107 to slow proliferation in mammalian cells. Direct transcriptional repression of p107 by Rb and E2F family members provides a molecular mechanism for a critical negative feedback loop during cell cycle progression and tumorigenesis. These experiments also suggest novel therapeutic strategies to increase the p107 levels in tumor cells.

The retinoblastoma tumor suppressor Rb belongs to a family of cell cycle inhibitors along with the related proteins p107 and p130. Strong evidence indicates that the three family members have both specific and overlapping functions and expression patterns in mammalian cells, including in cancer cells. However, the molecular mechanisms underlying the functional differences and similarities among Rb, p107, and p130 are still poorly understood. One proposed mechanism of compensation is a negative feedback loop involving increased p107 transcription in Rb-deficient cells. To dissect the mechanisms controlling p107 expression in both wild-type and Rb-deficient cells, we have engineered inactivating point mutations into the E2F binding sites in the endogenous p107 promoter using gene targeting in mouse embryonic stem cells. Gene expression and DNA binding assays revealed that these two sites are essential for the control of p107 transcription in wild-type and Rb mutant cells, and cell cycle assays showed their importance for normal functions of p107. These experiments identify a key node in cell cycle regulatory networks.

RB's original CIN?

The retinoblastoma tumor suppressor RB is the downstream mediator of a cellular pathway that is thought to prevent cancer by controlling the ability of cells to enter or exit the cell cycle in G0/G1. Recently, however, accumulating evidence has suggested that RB, its family members p107 and p130, and their partners, the E2F family of transcription factors, may have important cellular functions beyond the G1/S transition of the cell cycle, including during DNA replication and at the transition into mitosis. In this issue of Genes & Development, three studies demonstrate a critical role for RB in proper chromosome condensation, centromeric function, and chromosome stability in mammalian cells, and link these cellular functions of RB to tumor suppression in mice. Here we discuss how transcriptional and post-transcriptional mechanisms under the control of the RB pathway ensure accurate progression through mitosis, thereby preventing cancer development.