Mitogen requirement for cell cycle progression in the absence of pocket protein activity

Transient genomic instability drives tumorigenesis through accelerated clonal evolution

In this study, Shoshani et al. tested the role of aneuploidy in tumor initiation and progression, and generated mice with random aneuploidies by transient induction of polo-like kinase 4 (Plk4), a master regulator of centrosome number. Their findings show how transient CIN generates cells with random aneuploidies from which ones that acquire a karyotype with specific chromosome gains are sufficient to drive cancer formation, and that distinct CIN mechanisms can lead to similar karyotypic cancer-causing outcomes.

Abnormal numerical and structural chromosome content is frequently found in human cancer. To test the role of aneuploidy in tumor initiation and progression, we generated mice with random aneuploidies by transient induction of polo-like kinase 4 (Plk4), a master regulator of centrosome number. Short-term chromosome instability (CIN) from transient Plk4 induction resulted in formation of aggressive T-cell lymphomas in mice with heterozygous inactivation of one p53 allele and accelerated tumor development in the absence of p53. Transient CIN increased the frequency of lymphoma-initiating cells with a specific karyotype profile, including trisomy of chromosomes 4, 5, 14, and 15 occurring early in tumorigenesis. Tumor development in mice with chronic CIN induced by an independent mechanism (through inactivation of the spindle assembly checkpoint) gradually trended toward a similar karyotypic profile, as determined by single-cell whole-genome DNA sequencing. Overall, we show how transient CIN generates cells with random aneuploidies from which ones that acquire a karyotype with specific chromosome gains are sufficient to drive cancer formation, and that distinct CIN mechanisms can lead to similar karyotypic cancer-causing outcomes.

The Krüppel-like factor Cabut has cell cycle regulatory properties similar to E2F1

Using a gain-of-function screen in Drosophila, we identified the Krüppel-like factor Cabut (Cbt) as a positive regulator of cell cycle gene expression and cell proliferation. Enforced cbt expression is sufficient to induce an extra cell division in the differentiating fly wing or eye, and also promotes intestinal stem cell divisions in the adult gut. Although inappropriate cell proliferation also results from forced expression of the E2f1 transcription factor or its target, Cyclin E, Cbt does not increase E2F1 or Cyclin E activity. Instead, Cbt regulates a large set of E2F1 target genes independently of E2F1, and our data suggest that Cbt acts via distinct binding sites in target gene promoters. Although Cbt was not required for cell proliferation during wing or eye development, Cbt is required for normal intestinal stem cell divisions in the midgut, which expresses E2F1 at relatively low levels. The E2F1-like functions of Cbt identify a distinct mechanism for cell cycle regulation that may be important in certain normal cell cycles, or in cells that cycle inappropriately, such as cancer cells.

The RECQL helicase prevents replication fork collapse during replication stress

Most tumors lack the G1/S phase checkpoint and are insensitive to antigrowth signals. Loss of G1/S control can severely perturb DNA replication as revealed by slow replication fork progression and frequent replication fork stalling. Cancer cells may thus rely on specific pathways that mitigate the deleterious consequences of replication stress. To identify vulnerabilities of cells suffering from replication stress, we performed an shRNA-based genetic screen. We report that the RECQL helicase is specifically essential in replication stress conditions and protects stalled replication forks against MRE11-dependent double strand break (DSB) formation. In line with these findings, knockdown of RECQL in different cancer cells increased the level of DNA DSBs. Thus, RECQL plays a critical role in sustaining DNA synthesis under conditions of replication stress and as such may represent a target for cancer therapy.

Neferine induces apoptosis by modulating the ROS‑mediated JNK pathway in esophageal squamous cell carcinoma

Current treatments for esophageal squamous cell carcinoma (ESCC) have limited efficacy. Therefore, the development of novel therapeutic targets to effectively manage the disease and boost survival rates is imperative Neferine, a natural product extracted from Nelumbo nucifera (lotus) leaves, has been revealed to inhibit the growth of hepatocarcinoma, breast cancer and lung cancer cells. However, its effect on ESCC is unknown. In the present study, it was revealed that neferine exerted anti‑proliferative effects in ESCC. It was also revealed that it triggered arrest of the G2/M phase and enhanced apoptosis of ESCC cell lines. Moreover, its ability to trigger accumulation of reactive oxygen species (ROS) and activate the c‑Jun N‑terminal kinase (JNK) pathway was demonstrated. Further study revealed how N‑acetyl cysteine (NAC), a ROS inhibitor, attenuated these effects, demonstrating that ROS and JNK inhibitors mediated a marked reversal of neferine‑triggered cell cycle arrest and apoptosis in ESCC cells. Finally, it was revealed that neferine was involved in the inhibition of Nrf2, an antioxidant factor. Collectively, these findings demonstrated the antitumor effect of neferine in ESCC, through the ROS‑mediated JNK pathway and inhibition of Nrf2, indicating its potential as a target for development of novel and effective therapeutic agents against ESCC.

WAPL-Dependent Repair of Damaged DNA Replication Forks Underlies Oncogene-Induced Loss of Sister Chromatid Cohesion

Premature loss of sister chromatid cohesion at metaphase is a diagnostic marker for different cohesinopathies. Here, we report that metaphase spreads of many cancer cell lines also show premature loss of sister chromatid cohesion. Cohesion loss occurs independently of mutations in cohesion factors including SA2, a cohesin subunit frequently inactivated in cancer. In untransformed cells, induction of DNA replication stress by activation of oncogenes or inhibition of DNA replication is sufficient to trigger sister chromatid cohesion loss. Importantly, cell growth under conditions of replication stress requires the cohesin remover WAPL. WAPL promotes rapid RAD51-dependent repair and restart of broken replication forks. We propose that active removal of cohesin allows cancer cells to overcome DNA replication stress. This leads to oncogene-induced cohesion loss from newly synthesized sister chromatids that may contribute to genomic instability and likely represents a targetable cancer cell vulnerability.

UBE2C overexpression in melanoma and its essential role in G2/M transition

Ubiquitin‑conjugating enzyme E2C (UBE2C) is a key regulator of cell cycle progression, and its aberrant expression has been implicated in various malignancies. However, its clinical and biological roles in malignant melanoma is still unclear. In this study, we found a significant high expression level of UBE2C in melanoma by an in silico analysis of The Cancer Genome Atlas (TCGA) database, which was further validated using fresh melanoma samples. The KM plotter showed that UBE2C level was statistically related to the overall survival (OS) of melanoma patients (p<0.01). RNA interference of UBE2C inhibited the growth of melanoma cells via deactivating ERK/Akt signaling pathways, and blocked the G2/M transition through downregulation of both the level and the activity of mitosis promoting factor (MPF), triggering the apoptosis of melanoma cells. Further, silencing of UBE2C significantly inhibited the xenografted tumor growth on nude mice, indicating an important role of UBE2C in melanoma growth in vivo. Together, our results show that UBE2C may serve as a novel prognostic biomarker as well as a potential therapeutic target for melanoma.

Boolean model of growth signaling, cell cycle and apoptosis predicts the molecular mechanism of aberrant cell cycle progression driven by hyperactive PI3K

The PI3K/AKT signaling pathway plays a role in most cellular functions linked to cancer progression, including cell growth, proliferation, cell survival, tissue invasion and angiogenesis. It is generally recognized that hyperactive PI3K/AKT1 are oncogenic due to their boost to cell survival, cell cycle entry and growth-promoting metabolism. That said, the dynamics of PI3K and AKT1 during cell cycle progression are highly nonlinear. In addition to negative feedback that curtails their activity, protein expression of PI3K subunits has been shown to oscillate in dividing cells. The low-PI3K/low-AKT1 phase of these oscillations is required for cytokinesis, indicating that oncogenic PI3K may directly contribute to genome duplication. To explore this, we construct a Boolean model of growth factor signaling that can reproduce PI3K oscillations and link them to cell cycle progression and apoptosis. The resulting modular model reproduces hyperactive PI3K-driven cytokinesis failure and genome duplication and predicts the molecular drivers responsible for these failures by linking hyperactive PI3K to mis-regulation of Polo-like kinase 1 (Plk1) expression late in G2. To do this, our model captures the role of Plk1 in cell cycle progression and accurately reproduces multiple effects of its loss: G2 arrest, mitotic catastrophe, chromosome mis-segregation / aneuploidy due to premature anaphase, and cytokinesis failure leading to genome duplication, depending on the timing of Plk1 inhibition along the cell cycle. Finally, we offer testable predictions on the molecular drivers of PI3K oscillations, the timing of these oscillations with respect to division, and the role of altered Plk1 and FoxO activity in genome-level defects caused by hyperactive PI3K. Our model is an important starting point for the predictive modeling of cell fate decisions that include AKT1-driven senescence, as well as the non-intuitive effects of drugs that interfere with mitosis.

Loss of p53 suppresses replication-stress-induced DNA breakage in G1/S checkpoint deficient cells

In cancer cells, loss of G1/S control is often accompanied by p53 pathway inactivation, the latter usually rationalized as a necessity for suppressing cell cycle arrest and apoptosis. However, we found an unanticipated effect of p53 loss in mouse and human G1-checkpoint-deficient cells: reduction of DNA damage. We show that abrogation of the G1/S-checkpoint allowed cells to enter S-phase under growth-restricting conditions at the expense of severe replication stress manifesting as decelerated DNA replication, reduced origin firing and accumulation of DNA double-strand breaks. In this system, loss of p53 allowed mitogen-independent proliferation, not by suppressing apoptosis, but rather by restoring origin firing and reducing DNA breakage. Loss of G1/S control also caused DNA damage and activation of p53 in an in vivo retinoblastoma model. Moreover, in a teratoma model, loss of p53 reduced DNA breakage. Thus, loss of p53 may promote growth of incipient cancer cells by reducing replication-stress-induced DNA damage.

Deletion of the MAD2L1 spindle assembly checkpoint gene is tolerated in mouse models of acute T-cell lymphoma and hepatocellular carcinoma

Chromosome instability (CIN) is deleterious to normal cells because of the burden of aneuploidy. However, most human solid tumors have an abnormal karyotype implying that gain and loss of chromosomes by cancer cells confers a selective advantage. CIN can be induced in the mouse by inactivating the spindle assembly checkpoint. This is lethal in the germline but we show here that adult T cells and hepatocytes can survive conditional inactivation of the Mad2l1 SAC gene and resulting CIN. This causes rapid onset of acute lymphoblastic leukemia (T-ALL) and progressive development of hepatocellular carcinoma (HCC), both lethal diseases. The resulting DNA copy number variation and patterns of chromosome loss and gain are tumor-type specific, suggesting differential selective pressures on the two tumor cell types.

Cell cycle proliferation decisions: the impact of single cell analyses

Cell proliferation is a fundamental requirement for organismal development and homeostasis. The mammalian cell division cycle is tightly controlled to ensure complete and precise genome duplication and segregation of replicated chromosomes to daughter cells. The onset of DNA replication marks an irreversible commitment to cell division, and the accumulated efforts of many decades of molecular and cellular studies have probed this cellular decision, commonly called the restriction point. Despite a long-standing conceptual framework of the restriction point for progression through G1 phase into S phase or exit from G1 phase to quiescence (G0), recent technical advances in quantitative single cell analysis of mammalian cells have provided new insights. Significant intercellular heterogeneity revealed by single cell studies and the discovery of discrete subpopulations in proliferating cultures suggests the need for an even more nuanced understanding of cell proliferation decisions. In this review, we describe some of the recent developments in the cell cycle field made possible by quantitative single cell experimental approaches.

Senescence from G2 arrest, revisited

Senescence was classically defined as an irreversible cell cycle arrest in G1 phase (G1 exit) triggered by eroded telomeres in aged primary cells. The molecular basis of this G1 arrest is thought to be due to a DNA damage response, resulting in accumulation of the cyclin dependent kinase (Cdk) inhibitors p21 and p16 that block the inactivating phosphorylation of the retinoblastoma tumor suppressor pRb, thereby preventing DNA replication. More than a decade ago, several studies showed that p21 also mediates permanent DNA damage-induced cell cycle arrest in G2 (G2 exit) by inhibiting mitotic Cdk complexes and pRb phosphorylation. The idea that the senescence program can also be launched after G2 arrest has gained support from several recent publications, including evidence for its existence in vivo.

Chromosome instability induced by Mps1 and p53 mutation generates aggressive lymphomas exhibiting aneuploidy-induced stress

Aneuploidy is a hallmark of human solid cancers that arises from errors in mitosis and results in gain and loss of oncogenes and tumor suppressors. Aneuploidy poses a growth disadvantage for cells grown in vitro, suggesting that cancer cells adapt to this burden. To understand better the consequences of aneuploidy in a rapidly proliferating adult tissue, we engineered a mouse in which chromosome instability was selectively induced in T cells. A flanked by Lox mutation was introduced into the monopolar spindle 1 (Mps1) spindle-assembly checkpoint gene so that Cre-mediated recombination would create a truncated protein (Mps1(DK)) that retained the kinase domain but lacked the kinetochore-binding domain and thereby weakened the checkpoint. In a sensitized p53(+/-) background we observed that Mps1(DK/DK) mice suffered from rapid-onset acute lymphoblastic lymphoma. The tumors were highly aneuploid and exhibited a metabolic burden similar to that previously characterized in aneuploid yeast and cultured cells. The tumors nonetheless grew rapidly and were lethal within 3-4 mo after birth.

Sequential inductions of the ZEB1 transcription factor caused by mutation of Rb and then Ras proteins are required for tumor initiation and progression

Rb1 restricts cell cycle progression, and it imposes cell contact inhibition to suppress tumor outgrowth. It also triggers oncogene-induced senescence to block Ras mutation. Loss of the Rb1 pathway, which is a hallmark of cancer cells, then provides a permissive environment for Ras mutation, and Ras is sufficient for invasive tumor formation in Rb1 family mutant mouse embryo fibroblasts (MEFs). These results demonstrate that sequential mutation of the Rb1 and Ras pathways comprises a tumor initiation axis. Both Rb1 and Ras regulate expression of the transcription factor ZEB1, thereby linking tumor initiation to the subsequent invasion and metastasis, which is induced by ZEB1. ZEB1 acts in a negative feedback loop to block expression of miR-200, which is thought to facilitate tumor invasion and metastasis. However, ZEB1 also represses cyclin-dependent kinase (cdk) inhibitors to control the cell cycle; its mutation in MEFs leads to induction of these inhibitors and premature senescence. Here, we provide evidence for two sequential inductions of ZEB1 during Ras transformation of MEFs. Rb1 constitutively represses cdk inhibitors, and induction of ZEB1 when the Rb1 pathway is lost is required to maintain this repression, allowing for the classic immortalization and loss of cell contact inhibition seen when the Rb1 pathway is lost. In vivo, we show that this induction of ZEB1 is required for Ras-initiated tumor formation. ZEB1 is then further induced by Ras, beyond the level seen with Rb1 mutation, and this Ras superinduction is required to reach a threshold of ZEB1 sufficient for repression of miR-200 and tumor invasion.

Retinoblastoma gene-independent G1 phase arrest by flavone, phosphatidylinositol 3-kinase inhibitor, and histone deacetylase inhibitor

In most human malignant tumors, retinoblastoma tumor-suppressor gene (RB) product is inactivated by phosphorylation. Therefore, cancer preventive agents or molecular-targeting agents can inhibit the tumor growth at G(1) phase through RB reactivation. However, little is known about the effectiveness of RB reactivating agents against malignancies with mutated RB. We report here that chemopreventive agent flavone, phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002, and histone deacetylase (HDAC) inhibitor trichostatin A (TSA) also induce G(1) phase arrest in malignant tumor cells with mutated RB. In human prostate cancer DU145 cells with mutated RB, flavone increased cyclin-dependent kinase (CDK) inhibitors p21 and p27, and reduced cdk4 and cdk6, resulting in decrement of phosphorylated RB family proteins p130 and p107. LY294002 also dephosphorylated p107 and p130 proteins, whereas TSA dephosphorylated p130, but not p107. Furthermore, flavone induced G(1) phase arrest in both mouse embryo fibroblast (MEF) wild-type and MEF RB(-/-) cells, but did not do so in RB, p107, and p130 triple-knockout MEF cells. These results suggested that p130 and p107 contributed to G(1) phase arrest by flavone in RB-mutated cells. However, flavone induced tumor suppressor microRNA miR-34a with reduction of E2F1 and E2F3, known to be downregulated by miR-34a, raising the possibility that miR-34a might partially contribute to G(1) arrest by flavone. These results raise the possibility that RB reactivating chemopreventive agents or molecular targeting agents might also be effective against a variety of malignant tumor cells with mutant RB.

Conserved RB functions in development and tumor suppression

The variety of human cancers in which the retinoblastoma protein pRb is inactivated reflects both its broad importance for tumor suppression and its multitude of cellular functions. Accumulating evidence indicates that pRb contributes to a diversity of cellular functions, including cell proliferation, differentiation, cell death, and genome stability. pRb performs these diverse functions through the formation of large complexes that include E2F transcription factors and chromatin regulators. In this review we will discuss some of the recent advances made in understanding the structure and function of pRb as they relate to tumor suppression, and highlight research using Drosophila melanogaster that reveals important, evolutionarily conserved functions of the RB family.

pRB, a tumor suppressor with a stabilizing presence

The product of the retinoblastoma tumor-susceptibility gene (RB1) is a key regulator of cell proliferation and this function is thought to be central to its tumor suppressive activity. Several studies have demonstrated that inactivation of pRB not only allows inappropriate proliferation but also undermines mitotic fidelity, leading to genome instability and ploidy changes. Such properties promote tumor evolution and correlate with increased resistance to therapeutics and tumor relapse. These observations suggest that inactivation of pRB could contribute to both tumor initiation and progression. Further characterization of the role of pRB in chromosome segregation will provide insight into processes that are misregulated in human tumors and could reveal new therapeutic targets to kill or stall these chromosomally unstable lesions. We review the evidence that pRB promotes genome stability and discuss the mechanisms that probably contribute to this effect.

CINister thoughts

Chromosome instability (CIN) is the process that leads to aneuploidy, a known hallmark of human tumours for over a century. Nowadays, it is believed that CIN promotes tumorigenesis by shuffling the genome into a malignant order through translocations, amplifications, deletions (structural CIN), and gains and losses of whole chromosomes (numerical CIN or nCIN). The present review focuses on the causes and consequences of nCIN. Several roads can lead to nCIN, including a compromised spindle assembly checkpoint, cohesion defects, p53 deficiency and flawed microtubule-kinetochore attachments. Whereas the link between nCIN and tumorigenesis is becoming more evident, indications have emerged recently that nCIN can suppress tumour formation as well. To understand these paradoxical findings, novel reagents and more sophisticated mouse models are needed. This will provide us with a better understanding of nCIN and eventually with therapies that exploit this characteristic of human tumours.

Loss of Rb proteins causes genomic instability in the absence of mitogenic signaling

Loss of G1/S control is a hallmark of cancer, and is often caused by inactivation of the retinoblastoma pathway. However, mouse embryonic fibroblasts lacking the retinoblastoma genes RB1, p107, and p130 (TKO MEFs) are still subject to cell cycle control: Upon mitogen deprivation, they enter and complete S phase, but then firmly arrest in G2. We now show that G2-arrested TKO MEFs have accumulated DNA damage. Upon mitogen readdition, cells resume proliferation, although only part of the damage is repaired. As a result, mitotic cells show chromatid breaks and chromatid cohesion defects. These aberrations lead to aneuploidy in the descendent cell population. Thus, our results demonstrate that unfavorable growth conditions can cause genomic instability in cells lacking G1/S control. This mechanism may allow premalignant tumor cells to acquire additional genetic alterations that promote tumorigenesis.

Overlapping functions of Hdac1 and Hdac2 in cell cycle regulation and haematopoiesis

Histone deacetylases (HDACs) counterbalance acetylation of lysine residues, a protein modification involved in numerous biological processes. Here, Hdac1 and Hdac2 conditional knock-out alleles were used to study the function of class I Hdac1 and Hdac2 in cell cycle progression and haematopoietic differentiation. Combined deletion of Hdac1 and Hdac2, or inactivation of their deacetylase activity in primary or oncogenic-transformed fibroblasts, results in a senescence-like G(1) cell cycle arrest, accompanied by up-regulation of the cyclin-dependent kinase inhibitor p21(Cip). Notably, concomitant genetic inactivation of p53 or p21(Cip) indicates that Hdac1 and Hdac2 regulate p53-p21(Cip)-independent pathways critical for maintaining cell cycle progression. In vivo, we show that Hdac1 and Hdac2 are not essential for liver homeostasis. In contrast, total levels of Hdac1 and Hdac2 in the haematopoietic system are critical for erythrocyte-megakaryocyte differentiation. Dual inactivation of Hdac1 and Hdac2 results in apoptosis of megakaryocytes and thrombocytopenia. Together, these data indicate that Hdac1 and Hdac2 have overlapping functions in cell cycle regulation and haematopoiesis. In addition, this work provides insights into mechanism-based toxicities observed in patients treated with HDAC inhibitors.

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.

Elevated poly-(ADP-ribose)-polymerase activity sensitizes retinoblastoma-deficient cells to DNA damage-induced necrosis

The retinoblastoma (Rb) tumor suppressor is a key regulator of cell cycle checkpoints but also protects against cell death induced by stresses such as DNA damage and death receptor ligation. We report here that cell death of Rb-deficient cells exposed to key genotoxic agents was associated with increased expression of S phase-specific E2F target genes and cell death consistently occurred in the S phase of the cell cycle. Cell cycle arrest induced by serum starvation prevented S phase entry, attenuated DNA damage, and promoted survival, suggesting that Rb-null cells die due to a failure to prevent S phase entry. DNA damage-induced death of Rb-null cells was associated with nucleotide depletion, higher activity of poly-ADP-ribose-polymerase (Parp), and cell death that was primarily necrotic. Knockdown of Parp-1 or chemical inhibition of Parp activity prevented nucleotide depletion and restored the viability of Rb-deficient cells to wild-type levels. Furthermore, chemical inhibition of Parp activity in vivo attenuated the cytotoxic effects of cisplatin against Rb-deficient tumors, arguing that Parp inhibitors should not be used therapeutically in combination with genotoxic drugs against tumors that are inactivated for the Rb tumor suppressor.

Rb/Cdk2/Cdk4 triple mutant mice elicit an alternative mechanism for regulation of the G1/S transition

The G(1)/S-phase transition is a well-toned switch in the mammalian cell cycle. Cdk2, Cdk4, and the rate-limiting tumor suppressor retinoblastoma protein (Rb) have been studied in separate animal models, but interactions between the kinases and Rb in vivo have yet to be investigated. To further dissect the regulation of the G(1) to S-phase progression, we generated Cdk2(-/-)Cdk4(-/-)Rb(-/-) (TKO) mutant mice. TKO mice died at midgestation with major defects in the circulatory systems and displayed combined phenotypes of Rb(-/-) and Cdk2(-/-)Cdk4(-/-) mutants. However, TKO mouse embryonic fibroblasts were not only resistant to senescence and became immortal but displayed enhanced S-phase entry and proliferation rates similar to wild type. These effects were more remarkable in hypoxic compared with normoxic conditions. Interestingly, depletion of the pocket proteins by HPV-E7 or p107/p130 shRNA in the absence of Cdk2/Cdk4 elicited a mechanism for the G(1)/S regulation with increased levels of p27(Kip1) binding to Cdk1/cyclin E complexes. Our work indicates that the G(1)/S transition can be controlled in different ways depending on the situation, resembling a regulatory network.

Anchorage-independent growth of pocket protein-deficient murine fibroblasts requires bypass of G2 arrest and can be accomplished by expression of TBX2

Mouse embryonic fibroblasts (MEFs) deficient for pocket proteins (i.e., pRB/p107-, pRB/p130-, or pRB/p107/p130-deficient MEFs) have lost proper G(1) control and are refractory to Ras(V12)-induced senescence. However, pocket protein-deficient MEFs expressing Ras(V12) were unable to exhibit anchorage-independent growth or to form tumors in nude mice. We show that depending on the level of pocket proteins, loss of adhesion induces G(1) and G(2) arrest, which could be alleviated by overexpression of the TBX2 oncogene. TBX2-induced transformation occurred only in the absence of pocket proteins and could be attributed to downregulation of the p53/p21(CIP1) pathway. Our results show that a balance between the pocket protein and p53 pathways determines the level of transformation of MEFs by regulating cyclin-dependent kinase activities. Since transformation of human fibroblasts also requires ablation of both pathways, our results imply that the mechanisms underlying transformation of human and mouse cells are not as different as previously claimed.

Cellular mechanisms of tumour suppression by the retinoblastoma gene

The retinoblastoma (RB) tumour suppressor gene is functionally inactivated in a broad range of paediatric and adult cancers, and a plethora of cellular functions and partners have been identified for the RB protein. Data from human tumours and studies from mouse models indicate that loss of RB function contributes to both cancer initiation and progression. However, we still do not know the identity of the cell types in which RB normally prevents cancer initiation in vivo, and the specific functions of RB that suppress distinct aspects of the tumorigenic process are poorly understood.

Retinoblastoma teaches a new lesson

In this issue, Ajioka et al. (2007) report a new mouse model of retinoblastoma. They show that retinoblastoma is not driven by uncontrolled expansion of retinal progenitor cells, but rather is the result of cell cycle re-entry and expansion of differentiated horizontal interneurons in the retina.

Mechanisms controlling cell cycle exit upon terminal differentiation

Coordinating terminal differentiation with permanent exit from the cell cycle is crucial for proper organogenesis, yet how the cell cycle is blocked in differentiated tissues remains unclear. Important roles for retinoblastoma family proteins and Cyclin-dependent kinase inhibitors have been delineated, but in many cases it remains unclear what triggers cell cycle exit. This review focuses on describing recent advances in deciphering how terminal differentiation and exit from the cell cycle are coordinated.

Further studies on the arrest of cell proliferation in tumor cells at the invading front of colonic adenocarcinoma

BACKGROUND AND AIM

The author has previously reported that neoplastic glands at the leading invading edge of colorectal carcinomas often display flat tumor cells and cellular gaps called glandular pores. The aim of this study was to audit cell proliferation and p53 mutation in flat tumor cells and in tumor cells at the tip of glandular pores at the leading invading front of colonic carcinomas.

METHODS

Sections from 40 colonic adenocarcinomas were immunostained with the proliferation marker Ki67 and with p53 protein. Expression was assessed at the leading invading front in consecutive neoplastic glands having flat tumor cells and epithelial pores and in neoplastic glands showing neither flat tumor cells nor glandular pores.

RESULTS

Flat tumor cells in neoplastic glands usually showed no Ki67 expression but overexpressed p53 mutation. In neoplastic glands with pores 40% of the tumor cells at the tip of the pores showed no Ki67, but they overexpressed p53 mutation.

CONCLUSIONS

The results showed, for the first time, that p53-positive flat neoplastic colonic cells arrest their proliferation at the invading front. It is possible that these p53-positive/Ki67-negative neoplastic cells were temporarily removed from the cell cycle (G0). This paradoxical biological behavior of tumor cells might be connected with the formation of glandular pores and appears to indicate that arrest of cell proliferation at the advancing tumor front in colonic carcinomas occurs independently of p53 mutation. The possible existence of two independent molecular systems at the advancing tumor edge of colonic carcinomas, one supervising cell proliferation and the other zealously transferring the p53 mutation to daughter cells, is suggested.

In vivo significance of the G2 restriction point

Loss of activity of the retinoblastoma pathway is a common event in human cancer. Mouse models have revealed that tumorigenesis by loss of Rb was accelerated by concomitant loss of the cell cycle inhibitor p27KIP1. This has been attributed to reduced apoptosis and weakening of the G1 checkpoint. However, the role of p27KIP1 in a recently identified G2 restriction point may offer an alternative explanation for this synergy. Here, we have investigated the significance of the G2 restriction point in Rb-deficient pituitaries. We show that Rb loss in the pituitary gland activated the G2 restriction point, as evidenced by the appearance of cyclin B1-p27KIP1 complexes. Somewhat unexpectedly, these complexes remained present in Rb-deficient tumors. These results indicate that the G2 restriction point does operate in vivo. However, in the pituitary gland, this mechanism seems to retard rather than to prevent tumor growth.

Oncogenic pathways impinging on the G2-restriction point

In the absence of mitogenic stimuli, cells normally arrest in G(1/0), because they fail to pass the G1-restriction point. However, abrogation of the G1-restriction point (by loss of the retinoblastoma gene family) reveals a second-restriction point that arrests cells in G2. Serum-starvation-induced G2 arrest is effectuated through inhibitory interactions of p27(KIP1) and p21(CIP1) with cyclins A and B1 and can be reversed through mitogen re-addition. In this study, we have investigated the pathways that allow cell cycle re-entry from this G2 arrest. We provide evidence that recovery from G2 arrest depends on the rat sarcoma viral oncogene (RAS) and phosphatidylinositol-3 kinase pathways and show that oncogenic hits, such as overexpression of c-MYC or mutational activation of RAS can abrogate the G2-restriction point. Together, our results provide new mechanistic insight into multistep carcinogenesis.

p21 loss cooperates with INK4 inactivation facilitating immortalization and Bcl-2-mediated anchorage-independent growth of oncogene-transduced primary mouse fibroblasts

The INK4 and CIP cyclin-dependent kinase (Cdk) inhibitors (CKI) activate pocket protein function by suppressing Cdk4 and Cdk2, respectively. Although these inhibitors are lost in tumors, deletion of individual CKIs results in modest proliferation defects in murine models. We have evaluated cooperativity between loss of all INK4 family members (using cdk4r24c mutant alleles that confer resistant to INK4 inhibitors) and p21(Waf1/Cip1) in senescence and transformation of mouse embryo fibroblasts (MEF). We show that mutant cdk4r24c and p21 loss cooperate in pRb inactivation and MEF immortalization. Our studies suggest that cdk4r24c mediates resistance to p15(INK4B)/p16(INK4A) that accumulates over passage, whereas loss of p21 suppresses hyperoxia-induced Cdk2 inhibition and pRb dephosphorylation on MEF explantation in culture. Although cdk4r24c and p21 loss cooperate in H-ras(V12)/c-myc-induced foci formation, they are insufficient for oncogene-induced anchorage-independent growth. Interestingly, p21(-/-); cdk4r24c MEFs expressing H-ras(V12) and c-myc display detachment-induced apoptosis and are transformed by c-myc, H-ras(V12), and Bcl-2. We conclude that the INK4 family and p21 loss cooperate in promoting pRb inactivation, cell immortalization, and H-ras(V12)/c-myc-induced loss of contact inhibition. In addition, absence of pRb function renders H-ras(V12) + c-myc-transduced fibroblasts prone to apoptosis when deprived of the extracellular matrix, and oncogene-induced anchorage-independent growth of pocket protein-deficient cells requires apoptotic suppression.

A double-assurance mechanism controls cell cycle exit upon terminal differentiation in Drosophila

Terminal differentiation is often coupled with permanent exit from the cell cycle, yet it is unclear how cell proliferation is blocked in differentiated tissues. We examined the process of cell cycle exit in Drosophila wings and eyes and discovered that cell cycle exit can be prevented or even reversed in terminally differentiating cells by the simultaneous activation of E2F1 and either Cyclin E/Cdk2 or Cyclin D/Cdk4. Enforcing both E2F and Cyclin/Cdk activities is required to bypass exit because feedback between E2F and Cyclin E/Cdk2 is inhibited after cells differentiate, ensuring that cell cycle exit is robust. In some differentiating cell types (e.g., neurons), known inhibitors including the retinoblastoma homolog Rbf and the p27 homolog Dacapo contribute to parallel repression of E2F and Cyclin E/Cdk2. In other cell types, however (e.g., wing epithelial cells), unknown mechanisms inhibit E2F and Cyclin/Cdk activity in parallel to enforce permanent cell cycle exit upon terminal differentiation.

E2F-1 regulates expression of FOXO1 and FOXO3a

E2F and FOXO transcription factors both play a role in neuronal apoptosis. In addition, both E2F-induced apoptosis and FOXO function are inhibited by the kinase Akt. We therefore tested whether FOXO is downstream of E2F-1 during neuronal apoptosis. We found that expression of endogenous FOXO1 and FOXO3a is induced by E2F-1. The presence of putative E2F binding sites in the promoters of both genes suggested that FOXO genes are direct targets of E2F-1. Indeed, a 4-hydroxytamoxifen activated E2F-1-ER fusion protein induced FOXO expression in the presence of cycloheximide. Moreover, E2F-1 activated the FOXO1 promoter in transient reporter assays, and E2F-1-ER as well as endogenous E2F bound to the FOXO1 promoter in vivo. Yet, E2F-1-mediated apoptosis of differentiated PC12 cells after withdrawal of NGF was not accompanied by changes in FOXO expression, indicating that no transcriptional induction of FOXO occurs during E2F-1-dependent neuronal apoptosis. In summary, our data identify E2F-1 as a first transcription factor regulating FOXO expression, providing a link between E2F and FOXO proteins in the control of cell fate.

NEDD4-1 is a proto-oncogenic ubiquitin ligase for PTEN

The tumor suppressor PTEN, a critical regulator for multiple cellular processes, is mutated or deleted frequently in various human cancers. Subtle reductions in PTEN expression levels have profound impacts on carcinogenesis. Here we show that PTEN level is regulated by ubiquitin-mediated proteasomal degradation, and purified its ubiquitin ligase as HECT-domain protein NEDD4-1. In cells NEDD4-1 negatively regulates PTEN stability by catalyzing PTEN polyubiquitination. Consistent with the tumor-suppressive role of PTEN, overexpression of NEDD4-1 potentiated cellular transformation. Strikingly, in a mouse cancer model and multiple human cancer samples where the genetic background of PTEN was normal but its protein levels were low, NEDD4-1 was highly expressed, suggesting that aberrant upregulation of NEDD4-1 can posttranslationally suppress PTEN in cancers. Elimination of NEDD4-1 expression inhibited xenotransplanted tumor growth in a PTEN-dependent manner. Therefore, NEDD4-1 is a potential proto-oncogene that negatively regulates PTEN via ubiquitination, a paradigm analogous to that of Mdm2 and p53.

Pocket protein function in melanocyte homeostasis and neoplasia

Melanoma is the most lethal of human skin cancers and its incidence is increasing worldwide [L.K. Dennis (1999). Arch. Dermatol. 135, 275; C. Garbe et al. (2000). Cancer 89, 1269]. Melanomas often metastasize early during the course of the disease and are then highly intractable to current therapeutic regimens [M.F. Demierre and G. Merlino (2004). Curr. Oncol. Rep. 6, 406]. Consequently, understanding the factors that maintain melanocyte homeostasis and prevent their neoplastic transformation into melanoma is of utmost interest from the perspective of therapeutic interdiction. This review will focus on the role of the pocket proteins (PPs), Rb1 (retinoblastoma protein), retinoblastoma-like 1 (Rbl1 also known as p107) and retinoblastoma-like 2 (Rbl2 also known as p130), in melanocyte homeostasis, with particular emphasis on their functions in the cell cycle and the DNA damage repair response. The potential mechanisms of PP deregulation in melanoma and the possibility of PP-independent pathways to melanoma development will also be considered. Finally, the role of the PP family in ultraviolet radiation (UVR)-induced melanoma and the precise contribution that each PP family member makes to melanocyte homeostasis will be discussed in the context of a number of genetically engineered mouse models.

Restriction beyond the restriction point: mitogen requirement for G2 passage

Cell proliferation is dependent on mitogenic signalling. When absent, normal cells cannot pass the G₁ restriction point, resulting in cell cycle arrest. Passage through the G₁ restriction point involves inactivation of the retinoblastoma protein family. Consequently, loss of the retinoblastoma protein family leads to loss of the G₁ restriction point. Recent work in our lab has revealed that cells possess yet another mechanism that restricts proliferation in the absence of mitogens: arrest in the G₂ phase of the cell cycle. Here, we discuss the similarities and differences between these restriction points and the roles of cyclin-dependent kinase inhibitors (CKIs) herein.