Mice deficient for Rb are nonviable and show defects in neurogenesis and haematopoiesis

Inhibition of cyclin-dependent kinase activity triggers neuronal differentiation of mouse neuroblastoma cells

Studies on the molecular mechanisms underlying neuronal differentiation are frequently performed using cell lines established from neuroblastomas. In this study we have used mouse N1E-115 neuroblastoma cells that undergo neuronal differentiation in response to DMSO. During differentiation, cyclin-dependent kinase (cdk) activities decline and phosphorylation of the retinoblastoma gene product (pRb) is lost, leading to the appearance of a pRb-containing E2F DNA-binding complex. The loss of cdk2 activity is due to a decrease in cdk2 abundance whereas loss of cdk4 activity is caused by strong association with the cdk inhibitor (CKI) p27KIP1 and concurrent loss of cdk4 phosphorylation. Moreover, neuronal differentiation can be induced by overexpression of p27KIP1 or pRb, suggesting that inhibition of cdk activity leading to loss of pRb phosphorylation, is the major determinant for neuronal differentiation.

Role of pRb-related proteins in simian virus 40 large-T-antigen-mediated transformation

Simian virus 40 large T-antigen (TAg) transformation is thought to be mediated, at least in part, by binding to and modulating the function of certain cellular proteins, including the retinoblastoma tumor suppressor gene product, pRb. TAg can disrupt the inhibitory complexes formed by pRb with the oncogenic transcription factor E2F, and this mechanism has been suggested to be important for TAg-mediated transformation. Residues 102 to 114 of TAg (including the LXCXE motif) are required for binding to pRb. Mutations within this LXCXE motif abolish the ability of TAg to bind to pRb as well as to transform certain cell types. TAg can also bind to at least two other cellular proteins, p107 and p130, that are related to pRb by sequence homology and share the ability to bind E2F. However, whether p107 and p130 are also targets in TAg-mediated transformation is less clear. To assess the relative contribution of the inactivation of pRb, p107, and p130 to transformation by TAg, fibroblasts were prepared from embryos derived from matings of mice heterozygous for an Rb knockout allele. The ability of TAg to transform fibroblasts homozygous for either wild-type or knockout Rb alleles was evaluated. It is demonstrated that the integrity of the LXCXE motif provides a growth advantage in Rb+/+ and Rb-/- cells. Furthermore, wild-type TAg, but not the LXCXE mutants, could bind to p107 and p130 and disrupt p107-E2F and p130-E2F binding complexes. These results suggest that p107 and p130 participate in TAg-mediated transformation and that they may behave as tumor suppressors.

Abrogation of retinoblastoma protein function by c-Abl through tyrosine kinase-dependent and -independent mechanisms

The decision to enter the cell division cycle is governed by the interplay between growth activators and growth inhibitors. The retinoblastoma protein (RB) is an example of a growth inhibitor whose main function appears to be the binding and inactivation of key cell cycle activators. One target of RB is a proto-oncoprotein, the c-Abl tyrosine kinase. RB binds to the ATP-binding lobe in the kinase domain and inhibits the nuclear pool of c-Abl in quiescent and G1 cells. Phosphorylation of RB at G1/S releases c-Abl, leading to the activation of this nuclear tyrosine kinase. In this report, we describe the construction of a mutant Abl, replacing the ATP-binding lobe of c-Abl with that of c-Src. The mutant protein AS2 is active as a tyrosine kinase and can phosphorylate Abl substrates, such as the C-terminal repeated domain of RNA polymerase II. AS2, however, does not bind to RB, and its activity is not inhibited by RB. As a result, the nuclear pool of AS2 is no longer cell cycle regulated. Excess AS2, but not its kinase-defective counterpart, can overcome RB-induced growth arrest in Saos-2 cells. Interestingly, wild-type c-Abl, in both its kinase-active and -inactive forms, can also overcome RB. Furthermore, overexpression of a kinase-defective c-Abl in rodent fibroblasts accelerates the transition from quiescence to S phase and cooperates with c-Myc to induce transformation. These effects, however, do not occur with the kinase-defective form of AS2. Thus, the growth-stimulating function of the kinase-defective c-Abl is dependent on the binding and the abrogation of RB function. That RB function can be abolished by the overproduction of one of its binding proteins is consistent with the hypothesis that RB induces cell cycle arrest by acting as a "molecular matchmaker" to assemble protein complexes. Exclusive engagement of RB by one of its many targets is incompatible with the biological function of this growth suppressor protein.

Altered patterns of retinoblastoma gene product expression in adult soft-tissue sarcomas

Altered expression of the retinoblastoma (RB) tumour-suppressor gene product (pRB) has been detected in sporadic bone and soft-tissue sarcomas. Earlier studies, analysing small cohorts of sarcoma patients, have suggested that these alterations are more commonly associated with high-grade tumours, metastatic lesions and poorer survival. This study was designed to re-examine the prevalence and clinical significance of altered pRB expression in a large and selected group of soft-tissue sarcomas from 174 adult patients. Representative tissue sections from these sarcomas were analysed by immunohistochemistry using a well-characterised anti-pRB monoclonal antibody. Tumours were considered to have a positive pRB phenotype only when pure nuclear staining was demonstrated, and cases were segregated into one of three groups. Group 1 (n = 36) were patients whose tumours have minimal or undetectable pRB nuclear staining (< 20% of tumour cells) and were considered pRB negative. Patients with tumours staining in a heterogeneous pattern (20-79% of tumour cells) were classified as group 2 (n = 99). The staining of group 3 (n = 39) was strongly positive with a homogeneous pRB nuclear immunoreactivity (80-100% of tumour cells). pRB alterations were frequently observed in both low- and high-grade lesions. Altered pRB expression did not correlate with known predictors of survival and was not itself an independent predictor of outcome in the long-term follow-up. These findings support earlier observations that alterations of pRB expression are common events in soft-tissue sarcomas; nevertheless, long-term follow-up results indicate that altered patterns of pRB expression do not influence clinical outcome of patients affected with soft-tissue sarcomas. It is postulated that RB alterations are primary events in human sarcomas and may be involved in tumorigenesis or early phases of tumour progression in these neoplasias.

Constitutive overexpression of CDK2 inhibits neuronal differentiation of rat pheochromocytoma PC12 cells

Changes in the levels of cyclins A, D, and E, p21, and cyclin-dependent kinase 2 (CDK2) were examined in rat pheochromocytoma PC12 cells during neuronal differentiation induced by nerve growth factor (NGF). Expression of cyclin A decreased to an undetectable level after 5 days of exposure to NGF, while expression of CDK2 decreased gradually after day 3. In contrast, the levels of cyclins D1 and E increased gradually through day 10, yet the amount of cyclin E associated with CDK2 decreased concomitant with a decrease in the CDK2 protein level. p21 expression increased gradually after day 7, while the level of CDK2-associated p21 remained unchanged. When human cDNAs encoding cyclins and CDK2 were introduced into PC12 cells, only CDK2 overexpression inhibited NGF-induced differentiation. The cell lines overexpressing CDK2 showed stable and high levels of CDK2 kinase activity during differentiation, whereas parental and vector-transfected cell lines displayed a marked decline in CDK2 kinase activity 1 day after NGF treatment. In cell lines overexpressing cyclins A, D, and E, this reduction of the kinase activity was not apparent until day 3. These results suggest that down-regulation of CDK2 activity is a crucial event for the neuronal differentiation of PC12 cells.

Loss of neurofibromin results in neurotrophin-independent survival of embryonic sensory and sympathetic neurons

Mutations at the neurofibromatosis 1 (NF1) locus in humans and mice result in abnormal growth of neural crest-derived cells, including melanocytes and Schwann cells. We have exploited a targeted disruption of the NF1 gene in mice to examine the role of neurofibromin in the acquisition of neurotrophin dependence in embryonic neurons. We show that both neural crest- and placode-derived sensory neurons isolated from NF1(-/-) embryos develop, extend neurites, and survive in the absence of neurotrophins, whereas their wild-type counterparts die rapidly unless nerve growth factor (NGF) or brain-derived neurotrophic factor (BDNF) is added to the culture medium. Moreover, NF1 (-/-) sympathetic neurons survive for extended periods and acquire mature morphology in the presence of NGF-blocking antibodies. Our results are consistent with a model wherein neurofibromin acts as a negative regulator of neurotrophin-mediated signaling for survival of embryonic peripheral neurons.

Transgenic mouse models for tumour-suppressor genes

Tumour-suppressor genes are negative regulators of cell division and growth. Over the past decade, multiple, distinct tumour-suppressor genes have been identified and cloned. In recent years, the ability to specifically manipulate the mouse genome via overexpression, underexpression or deletion of genes using transgenic expression systems and embryonic stem cell (ES) technology has led to the identification and definition of the precise function of several tumour suppressor genes in vivo. Included in this group are mice with mutations in the p53 and retinoblastoma (Rb) genes. p53 Mutant mice are highly susceptible to tumour development and will serve as excellent models to understand the aetiology and pathology of several human cancers. In contrast to the role of the Rb gene in human retinoblastomas, mice heterozygous for a mutant Rb allele do not develop retinoblastoma, but develop pituitary tumours instead. Similar ES cell technology has been used to generate alpha-inhibin deficient mice. Inhibin-deficient mice develop gonadal and adrenal tumours with nearly 100% penetrance. These studies have identified inhibin as a novel secreted tumour suppressor. In the future, many of the unidentified functions of tumour-suppressor genes can be tested using this powerful in vivo assay system.

Characterization of a novel 350-kilodalton nuclear phosphoprotein that is specifically involved in mitotic-phase progression

A gene assigned to human chromosome 1q32-41 encodes a novel protein of 3,113 amino acids containing an internal tandem repeat of 177 amino acids. The protein, which we have named "mitosin," was identified by direct binding to purified retinoblastoma protein in vitro with a region distantly related to the retinoblastoma protein-binding site of E2F-1. Mitosin is expressed throughout S, G2, and M phases of the cell cycle but is absent in G1. Its localization is dramatically reorganized from a rather homogeneous nuclear distribution in S phase to paired dots at the kinetochore/centromere region, to the spindle apparatus, and then to the midbody during M-phase progression. This spatial reorganization coincides closely with the temporal phosphorylation patterns of mitosin. Overexpression of N-terminally truncated mutants blocks cell cycle progression mainly at G2/M. These results suggest that mitosin may play an important role in mitotic-phase progression.

Temporally distinct patterns of p53-dependent and p53-independent apoptosis during mouse lens development

Programmed cell death, or apoptosis, is a critical event in the development of multicellular organisms, and its perturbation is implicated in many diseases including cancer. The tumor suppressor protein p53 is known to mediate apoptosis induced by the DNA tumor virus oncoproteins, adenovirus E1A (AdE1A) and SV40 T antigen (SV40 Tag). We have recently demonstrated that the E6 and E7 oncoproteins of human papillomavirus type 16 (HPV-16) modulate apoptosis when expressed in the lens of transgenic mice. In this study we have identified the pathways that mediate E7 induction and E6 inhibition of apoptosis during different stages in the development of the lens. E7 transgenic mice made p53-null were only partially rescued in their apoptotic phenotype, indicating that both p53-dependent and -independent pathways mediate E7-induced apoptosis in the lens. The E6 transgene and p53-null genotype acted additively to reduce levels of apoptosis induced by E7 in neonatal lenses, indicating that E6 modulates apoptosis at least in part through p53-independent mechanisms. The partial reduction in E7-induced apoptosis by the p53-null genotype correlated with an increased incidence of lens tumors in adult E7 transgenic mice. Analyses of embryonic lenses at E13.5, E15.5, and E17.5 revealed a temporally distinct activation of p53-dependent and -independent apoptosis in the E7 lens. During the early stages of lens development, apoptosis was highly p53-dependent, whereas at later stages, apoptosis occurred through both p53-independent and -dependent pathways. This later time correlates temporally with the time of normal fiber cell denucleation, which can be inhibited by E6 through a p53-independent mechanism. These data suggest a similarity between the mechanism regulating E7-induced, p53-independent apoptosis and the apoptotic-like developmental process of fiber cell denucleation, and the mechanisms through which E6 suppresses both processes.

Cyclin D1 provides a link between development and oncogenesis in the retina and breast

Mice lacking cyclin D1 have been generated by gene targeting in embryonic stem cells. Cyclin D1-deficient animals develop to term but show reduced body size, reduced viability, and symptoms of neurological impairment. Their retinas display a striking reduction in cell number due to proliferative failure during embryonic development. In situ hybridization studies of normal mouse embryos revealed an extremely high level of cyclin D1 in the retina, suggesting a special dependence of this tissue on cyclin D1. In adult mutant females, the breast epithelial compartment fails to undergo the massive proliferative changes associated with pregnancy despite normal levels of ovarian steroid hormones. Thus, steroid-induced proliferation of mammary epithelium during pregnancy may be driven through cyclin D1.

WW6: an embryonic stem cell line with an inert genetic marker that can be traced in chimeras

Mutant mice produced by gene targeting in embryonic stem (ES) cells often have a complex or embryonic lethal phenotype. In these cases, it would be helpful to identify tissues and cell types first affected in mutant embryos by following the contribution to chimeras of ES cells homozygous for the mutant allele. Although a number of strategies for following ES cell development in vivo have been reported, each has limitations that preclude its general application. In this paper, we describe ES cell lines that can be tracked to every nucleated cell type in chimeras at all developmental stages. These lines were derived from blastocysts of mice that carry an 11-Mb beta-globin transgene on chromosome 3. The transgene is readily detected by DNA in situ hybridization, providing an inert, nuclear-localized marker whose presence is not affected by transcriptional or translational controls. The "WW" series of ES lines possess the essential features of previously described ES lines, including giving rise to a preponderance of male chimeras, all of which have to date exhibited germ-line transmission. In addition, clones selected for single or double targeting events form strong chimeras, demonstrating the feasibility of using WW6 cells to identify phenotypes associated with the creation of a null mutant.

The induction of multiple cell cycle events precedes target-related neuronal death

Unexpected nerve cell death has been reported in several experimental situations where neurons have been forced to re-enter the cell cycle after leaving the ventricular zone and entering the G0, non-mitotic stage. To determine whether an association between cell death and unscheduled cell cycling might be found in conjunction with any naturally occurring developmental events, we have examined target-related cell death in two neuronal populations, the granule cells of the cerebellar cortex and the neurons of the inferior olive. Both of these cell populations have a demonstrated developmental dependency on their synaptic target, the cerebellar Purkinje cell. Two mouse neurological mutants, staggerer (sg/sg) and lurcher (+/Lc), are characterized by intrinsic Purkinje cell deficiencies and, in both mutants, substantial numbers of cerebellar granule cells and inferior olive neurons die due to the absence of trophic support from their main postsynaptic target. We report here that the levels of three independent cell cycle markers--cyclin D, proliferating cell nuclear antigen and bromodeoxyuridine incorporation--are elevated in the granule cells before they die. Although lurcher Purkinje cells die during a similar developmental period, no compelling evidence for any cell cycle involvement in this instance of pre-programmed cell death could be found. While application of the TUNEL technique (in situ terminal transferase end-labeling of fragmented DNA) failed to label dying granule cells in either mutant, light and electron microscopic observations are consistent with the interpretation that the death of these cells is apoptotic in nature. Together, the data indicate that target-related cell death in the developing central nervous system is associated with a mechanism of cell death that involves an apparent loss of cell cycle control.

Homozygotes for CDKN2 (p16) germline mutation in Dutch familial melanoma kindreds

The p16 gene (CDKN2) which is localized on chromosome 9p21, is deleted in a significant number of sporadic cancers. Moreover, germline mutations identified in some melanoma-prone kindreds last year suggested that CDKN2 is identical to the 9p21-linked melanoma susceptibility gene (MLM); however, failure to identify p16 mutations in all melanoma kindreds putatively linked to 9p21 left some doubts. We have analysed CDKN2 coding sequences in 15 Dutch familial atypical multiple mole-melanoma (FAMMM) syndrome pedigrees, and identified a 19 basepair (bp) germline deletion in 13 of them. All 13 families originate from an endogamous population. The deletion causes a reading frame shift, predicted to result in a severely truncated p16 protein. Interestingly, two family members are homozygous for the deletion, one of whom shows no obvious signs of disease. This surprising finding demonstrates that homozygotes for this CDKN2 mutation are viable, and suggests the presence of a genetic mechanism that can compensate for the functional loss of p16. Our results also greatly strengthen the notion that p16 is indeed MLM.

Cyclin-dependent kinases and pRb: regulators of the proliferation-differentiation switch

The retinoblastoma susceptibility gene (RB1) is essential for normal embryonic development. Loss of RB1 leads to uncontrolled proliferation of a number of cell types but may also prevent proper terminal differentiation. The growth-suppressive and differentiation-inducing properties of pRb are impaired by cyclin-dependent kinase (cdk)-mediated phosphorylation. Hence, inhibition of cdk activity is probably a prerequisite for terminal differentiation. Indeed, forced cyclin or cdk expression can prevent terminal differentiation in various cell types, probably through inhibition of pRb and, possibly, differentiation-specific transcription factors.

Deficiency of retinoblastoma protein leads to inappropriate S-phase entry, activation of E2F-responsive genes, and apoptosis

The retinoblastoma susceptibility gene (Rb) participates in controlling the G1/S-phase transition, presumably by binding and inactivating E2F transcription activator family members. Mouse embryonic fibroblasts (MEFs) with no, one, or two inactivated Rb genes were used to determine the specific contributions of Rb protein to cell cycle progression and gene expression. MEFs lacking both Rb alleles (Rb-/-) entered S phase in the presence of the dihydrofolate reductase inhibitor methotrexate. Two E2F target genes, dihydrofolate reductase and thymidylate synthase, displayed elevated mRNA and protein levels in Rb- MEFs. Since absence of functional Rb protein in MEFs is sufficient for S-phase entry under growth-limiting conditions, these data indicate that the E2F complexes containing Rb protein, and not the Rb-related proteins p107 and p130, may be rate limiting for the G1/S transition. Antineoplastic drugs caused accumulation of p53 in the nuclei of both Rb+/+ and Rb-/- MEFs. While p53 induction led to apoptosis in Rb-/- MEFs, Rb+/- and Rb+/+ MEFs underwent cell cycle arrest without apoptosis. These results reveal that diverse growth signals work through Rb to regulate entry into S phase, and they indicate that absence of Rb protein produces a constitutive DNA replication signal capable of activating a p53-associated apoptotic response.

Murine models of neoplasia: functional analysis of the tumour suppressor genes Rb-1 and p53

Loss of function of one or both of the two tumour suppressor genes p53 and RB-1 has been recognised as an important step in the development of a variety of human neoplasias for some time. By virtue of the ability to manipulate the genome of murine embryonic stem cells in culture, it has become possible to generate strains of mice which bear inactivations of the murine counterparts of these genes. This article attempts to bring together some of the many results obtained from these murine strains which are shedding light both on the normal role played by both of these genes and the consequences of their dysfunction. Surprisingly neither gene product is revealed to have an indispensable role at the level of the single cell. Hence, even though the Rb-1 gene product clearly has an important role in cell cycle regulation animals constitutively deficient in this gene develop relatively normally for the first 10 days of embryogenesis. It is only at and beyond this stage of development that a requirement for Rb-1 becomes clear, in the regulation of certain cell populations through control of both proliferation and apoptosis. That loss of function of Rb-1 is associated with tumorigenesis is confirmed by the development of tumours of the pituitary gland within heterozygotes. The retinas of these animals, the target organ for tumorigenesis in human RB-1 heterozygotes, remain unaffected. The majority of mice homozygous for an inactivating p53 mutation survive to birth, but then rapidly succumb to tumorigenesis. Heterozygotes also develop tumours, but with a delayed time course and altered spectrum. Analysis of several tissue types from the mutant animals has shown p53 to be crucial for the normal induction of apoptosis following DNA damage, and it is thought that failure of this process is a key predisposing step towards tumorigenesis within the mutant animals. Finally, studies on these and other transgenic strains have revealed interactions between pathways governed by these two genes. For example, the fate of Rb-1 deficient cells has been shown, in some tissues at least, to be dependent upon the functional status of p53.

Abnormalities of retinoblastoma gene expression in hematological malignancies

The human retinoblastoma gene product which is involved in cell cycle control and also acts as a transcriptional repressor of genes involved in growth control, is constitutively expressed as a phosphoprotein in normal hemopoietic cells. Abnormalities of the retinoblastoma gene expression leading to loss of protein expression either due to structural changes, mutations or transcriptional abnormalities have been found in a variety of hematological malignancies. There is evidence that loss of Rb protein expression is particularly associated with tumour progression and an adverse response to therapy which may be linked to the biological effect of Rb protein loss on the growth characteristics of tumour cells.

Transgenic and gene knockout mice in cancer research

Transgenic animal technology, and the use of germline manipulation for the creation of targeted gene mutations, has resulted in a plethora of murine models for cancer research. Our understanding of some of the important issues regarding the mechanisms controlling cell division, differentiation and death has dramatically advanced in recent years through exploitation of these techniques to generate transgenic mice. In particular, the generation of mice with targeted mutations in genes encoding proteins of oncological interest has proved to be a useful way of elucidating the function of these gene products in vivo. Transgenic mouse models have provided some insight into the complex oncogenic events contributing to cellular dysregulation and the loss of growth control that can lead to tumorigenesis. These animal studies have highlighted the fact that there are many different stages at which the loss of cell cycle control can occur, as a result of mutations affecting proteins anywhere from the cell surface to the nucleus. Although mutations affecting growth factors, growth factor receptors, signal transduction molecules, cytoplasmic proteins or nuclear proteins might appear to be very distinct, the end result of these changes may be accelerated and unchecked cell growth ultimately leading to cancer. It is beyond the scope of this review to mention every animal model that has been developed for cancer research, especially since many of the early studies have been covered extensively in previous reviews. This article will instead focus on a small selection of transgenic and knockout animal models which exemplify how proteins from distinct localisations along multiple pathways can contribute to loss of cell cycle control and the pathogenesis of cancer.

Cells en route to apoptosis are characterized by the upregulation of c-fos, c-myc, c-jun, cdc2, and RB phosphorylation, resembling events of early cell-cycle traverse

Density-arrested quiescent murine Balb/c-3T3 cells are dependent upon growth factors for their survival. Withdrawal of serum from their medium induces rapid cell death, the mechanism of which is not yet fully understood. We have studied the effect of serum deprivation on density-inhibited quiescent Swiss 3T3 cells and found that they undergo rapid cell death upon total withdrawal of serum. The nature of this cell death is similar to apoptosis, as shown by cellular and nuclear morphology and DNA fragmentation into oligonucleosomal fragments. Investigating the regulation of early cell-cycle genes during this process, we found that c-myc, c-jun, c-fos, and cdc2 protein presence is induced after serum deprivation, when the phosphorylated form of the RB protein also appears. The upregulation of these genes' protein products is coupled with the appearance of PCNA, a proliferation-specific nuclear antigen, as well as significant incorporation of BrdU, which may reflect DNA repair activity; in situ analysis shows that BrdU-positive cells are also positive for DNA fragmentation. These results suggest that en route to apoptosis, cells undergo events typical of early cell-cycle traverse by expressing early G1 genes and may even experience the late G1/S phase boundary, as shown by the presence of PCNA. However, the demonstrated ability of these cells to traverse the G1 phase of the cell cycle seems to be an abortive event, since they die shortly afterwards.

A subset of p53-deficient embryos exhibit exencephaly

Defects in neural tube formation are among the most common malformations leading to infant mortality. Although numerous genetic loci appear to contribute to the defects observed in humans and in animal model systems, few of the genes involved have been characterized at the molecular level. Mice lacking the p53 tumour suppressor gene are predisposed to tumours, but the viability of these animals indicates that p53 function is not essential for embryonic development. Here, we demonstrate that a fraction of p53-deficient embryos in fact do not develop normally. These animals display defects in neural tube closure resulting in an overgrowth of neural tissue in the region of the mid-brain, a condition known as exencephaly.

B-lymphoid potential in pre-liver mouse embryo

Increasing interest in the initial steps of hemopoiesis in the embryo is prompted by the lack of information about the primordial origin and the nature of hemopoietic stem cells. It appears critical to understand the emergence, diversification and differentiation potential of hemopoietic cells in the preliver embryo. In-vitro studies of B-potential in the mouse embryo prior to liver colonization show that, contrary to interpretations prevalent 10 years ago, there are two sources of B-cell progenitors (and most probably stem cells) at these stages: the yolk sac and the paraaortic splanchnopleura within the embryo proper. The analysis of the phenotype and differentiation potential of precursors from both sources shows them to be very similar, while different from the fetal liver progenitors. Indications for an independent generation of precursors in both sites are reviewed.

Modulation of retinoblastoma gene in normal adult hematopoiesis: peak expression and functional role in advanced erythroid differentiation

The retinoblastoma (RB) gene specifies a nuclear phosphoprotein (pRb 105), which is a prototype tumor suppressor inactivated in a variety of human tumors. Recent studies suggest that RB is also involved in embryonic development of murine central nervous and hematopoietic systems. We have investigated RB expression and function in human adult hematopoiesis--i.e., in liquid suspension culture of purified quiescent hematopoietic progenitor cells (HPCs) induced by growth factor stimulus to proliferation and unilinage differentiation/maturation through the erythroid or granulocytic lineage. In the initial HPC differentiation stages, the RB gene is gradually induced at the mRNA and protein level in both erythroid and granulopoietic cultures. In late HPC differentiation and then precursor maturation, RB gene expression is sustained in the erythroid lineage, whereas it is sharply downmodulated in the granulocytic series. Functional studies were performed by treatment of HPC differentiation culture with phosphorothioate antisense oligomer targeting Rb mRNA; coherent with the expression pattern, oligomer treatment of late HPCs causes a dose-dependent and selective inhibition of erythroid colony formation. These observations suggest that the RB gene plays an erythroid- and stage-specific functional role in normal adult hematopoiesis, particularly at the level of late erythroid HPCs.

Inhibitors of mammalian G1 cyclin-dependent kinases

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Study of the role of retinoblastoma protein in terminal differentiation of murine erythroleukemia cells

Hexamethylenebisacetamide-induced terminal differentiation of Friend virus-transformed murine erythroleukemia (MEL) cells can be inhibited by okadaic acid, an inhibitor of type 1 and type 2A protein phosphatases. The inhibition is shown to be correlated with prevention of dephosphorylation of retinoblastoma protein (pRB) in cells and bypass of G1 prolongation in the cell cycle. These results suggest that pRB-mediated G1 prolongation is necessary for MEL cells to commit to terminal differentiation. However, further experiments demonstrate that the simple cell cycle exit is not sufficient for commitment to terminal differentiation. Induction of dephosphorylation of pRB and subsequent G1 prolongation by forskolin does not lead MEL cells to differentiate. Additional pRB has been expressed in MEL cells by transfection with a neo-resistant plasmid containing RB cDNA under the control of a cytomegalovirus promoter. Exogenously expressed pRB is hyperphosphorylated in logarithmically growing MEL cells without any noticeable change in growth rate between the transfected cell line and the parental cell line. This result suggests that pRB in MEL cells is regulated by protein kinases and protein phosphatases and not by transcription.

Cells differentiating into neuroectoderm undergo apoptosis in the absence of functional retinoblastoma family proteins

The retinoblastoma (RB) protein is present at low levels in early mouse embryos and in pluripotent P19 embryonal carcinoma cells; however, the levels of RB rise dramatically in neuroectoderm formed both in embryos and in differentiating cultures of P19 cells. To investigate the effect of inactivating RB and related proteins p107 and p130, we transfected P19 cells with genes encoding mutated versions of the adenovirus E1A protein that bind RB and related proteins. When these E1A-expressing P19 cells were induced to differentiate into neuroectoderm, there was a striking rise in the expression of c-fos and extensive cell death. The ultrastructural and biochemical characteristics of the dying cells were indicative of apoptosis. The dying cells were those committed to the neural lineages because neurons and astrocytes were lost from differentiating cultures. Cell death was dependent on the ability of the E1A protein to bind RB and related proteins. Our results suggest that proteins of the RB family are essential for the development of the neural lineages and that the absence of functional RB activity triggers apoptosis of differentiating neuroectodermal cells.

Cyclin D1 is dispensable for G1 control in retinoblastoma gene-deficient cells independently of cdk4 activity

To elucidate the regulator-versus-target relationship in the cyclin D1/cdk4/retinoblastoma protein (pRB) pathway, we examined fibroblasts from RB-1 gene-deficient and RB-1 wild-type littermate mouse embryos (ME) and in human tumor cell lines that differed in the status of the RB-1 gene. The RB+/+ and RB-/- ME fibroblasts expressed similar protein levels of D-type cyclins, cdk4, and cdk6, showed analogous spectra and abundance of cellular proteins complexed with cdk4 and/or cyclins D1 and D2, and exhibited comparable associated kinase activities. Of the two human cell lines established from the same sarcoma biopsy, the RB-positive SKUT1B cells contained cdk4 that was mainly associated with D-type cyclins, contrary to a predominant cdk4-p16INK4 complex in the RB-deficient SKUT1A cells. Antibody-mediated neutralization of cyclin D1 arrested the RB-positive ME and SKUT1B cells in G1, whereas this cyclin appeared dispensable in the RB-deficient ME and SKUT1A cells. Lack of requirement for cyclin D1 therefore correlated with absence of functional pRB, regardless of whether active cyclin D1/cdk4 holoenzyme was present in the cells under study. Consistent with a potential role of cyclin D/cdk4 in phosphorylation of pRB, monoclonal anti-cyclin D1 antibodies supporting the associated kinase activity failed to significantly affect proliferation of RB-positive cells, whereas the antibody DCS-6, unable to coprecipitate cdk4, efficiently inhibited G1 progression and prevented pRB phosphorylation in vivo. These data provide evidence for an upstream control function of cyclin D1/cdk4, and a downstream role for pRB, in the order of events regulating transition through late G1 phase of the mammalian cell division cycle.

Pathophysiological aspects of tumor development

Neoplastic transformation is one possible consequence of genomically disturbed intracellular feedback mechanisms normally governing life, differentiation, function and death of an individual cell. Neoplastic growth can be thought of as the abnormal activation of the mitotic program and/or the inactivation of programs for growth-inhibition and apoptosis. This article reviews the current knowledge on three types, or families, of proteins that act on different levels of subcellular organization and are involved in controlling the integrity of the genome, survival and death: i) the DNA-binding nuclear protein p53 inducing cell cycle arrest and apoptosis, ii) the bcl-2 family of proteins acting as regulators of prolonged survival and programmed cell death and iii) APO-1/Fas, a cell surface receptor transducing an apoptotic signal delivered either by the cell itself (cis death) or by another cell (trans death). Although much is still unknown, especially concerning the functional linkages of these three principles, the data available allow a fascinating insight into the society of cells, which we are, after all.

Maintaining the neuronal phenotype after injury in the adult CNS. Neurotrophic factors, axonal growth substrates, and gene therapy

Multiple genetic and epigenetic events determine neuronal phenotype during nervous system development. After the mature mammalian neuronal phenotype has been determined it is usually static for the remainder of life, unless an injury or degenerative event occurs. Injured neurons may suffer one of three potential fates: death, persistent atrophy, or recovery. The ability of an injured adult neuron to recover from injury in adulthood may be determined by events that also influence neuronal phenotype during development, including expression of growth-related genes and responsiveness to survival and growth signals in the environment. The latter signals include neurotrophic factors and substrate molecules that promote neurite growth. Several adult CNS regions exhibit neurotrophic-factor responsiveness, including the basal forebrain, entorhinal cortex, hippocampus, thalamus, brainstem, and spinal cord. The specificity of neurotrophic-factor responsiveness in these regions parallels patterns observed during development. In addition, neurons of several CNS regions extend neurites after injury when presented with growth-promoting substrates. When both neurotrophic factors and growth-promoting substrates are provided to adult rats that have undergone bilateral fimbria-fornix lesions, then partial morphological and behavioral recovery can be induced. Gene therapy is one useful tool for providing these substances. Thus, the mature CNS remains robustly responsive to signals that shape nervous system development, and is highly plastic when stimulated by appropriate cues.

Massive cell death of immature hematopoietic cells and neurons in Bcl-x-deficient mice

bcl-x is a member of the bcl-2 gene family, which may regulate programmed cell death. Mice were generated that lacked Bcl-x. The Bcl-x-deficient mice died around embryonic day 13. Extensive apoptotic cell death was evident in postmitotic immature neurons of the developing brain, spinal cord, and dorsal root ganglia. Hematopoietic cells in the liver were also apoptotic. Analyses of bcl-x double-knockout chimeric mice showed that the maturation of Bcl-x-deficient lymphocytes was diminished. The life-span of immature lymphocytes, but not mature lymphocytes, was shortened. Thus, Bcl-x functions to support the viability of immature cells during the development of the nervous and hematopoietic systems.

Death before birth: clues from gene knockouts and mutations

A survey of mouse gene knockouts, transgene insertions and spontaneous mutations that are lethal prenatally reveals that surprisingly few developmental disturbances lead to death of the embryo and early foetus. These disturbances include failure to establish and maintain a vascular circulation, and failure to make the transition from yolk-sac-based to liver-based haematopoiesis. The embryo must also establish gestation-dependent routes of nutritional interaction with the mother, including implantation, formation of a yolk-sac vascular circulation, and formation of a chorioallantoic placenta. A number of embryonic organ and body systems, including the central nervous system, gut, lungs, urogenital system and musculoskeletal system, appear to have little or no survival value in utero.

Genetic instability as a consequence of inappropriate entry into and progression through S-phase

The stability of the mammalian genome depends on the proper function of G1 and G2 cell cycle control mechanisms. Two tumor suppressors, p53 and retinoblastoma (Rb), play key roles in progression from G1 into S-phase. We address the mechanisms by which these proteins mediate a G1 arrest in response to DNA damage and limiting metabolic conditions. Gamma-irradiation induced a prolonged, p53-dependent G1 arrest associated with a long-term increase in the levels of the cdk-inhibitor p21WAFl/Cipl (p21). Microinjection of linear plasmid DNA also caused a G1 arrest. The p53-dependent arrest induced by inhibitors of UMP biosynthesis was reversible and occurred in the absence of detectable DNA damage. Both arrest mechanisms contribute to limiting the formation and propagation of damaged genomes. Cells containing mutant p53 but wild-type Rb do not generate methotrexate (Mtx) resistant variants. However, pre-treatment with DNA damaging agents prior to drug selection resulted in resistant clones containing amplified dihydrofolate reductase (DHFR) genes, suggesting that DNA breakage is a rate limiting step for gene amplification. The Mtx-induced arrest did not occur in cells with non-functional Rb. Rb acts as a negative regulator of the E2F transcription factors, and Rb-deficient primary mouse embryo fibroblasts (MEFs) produced elevated levels of mRNA and protein for key E2F target genes. Failure to prevent entry into S-phase in Rb-/- MEFs exposed to DNA-damaging or nutrient limiting conditions caused apoptosis and correlated with p53 induction. Taken together, these findings indicate a link between p53 and Rb function and suggest that their coordination insures correct entry into S-phase, minimizing the emergence of genetic variants.

P53, cell cycle control and apoptosis: implications for cancer

Cellular proliferation depends on the rates of both cell division and cell death. Tumors frequently have decreased cell death as a primary mode of increased cell proliferation. Genetic changes resulting in loss of programmed cell death (apoptosis) are likely to be critical components of tumorigenesis. Many of the gene products which appear to control apoptotic tendencies are regulators of cell cycle progression; thus, cell cycle control and cell death appear to be tightly linked processes. P53 protein is an example of a gene product which affects both cell cycle progression and apoptosis. The ability of p53 overexpression to induce apoptosis may be a major reason why tumor cells frequently disable p53 during the transformation process. Unfortunately, the same genetic changes which cause loss of apoptosis during tumor development, may also result in tumor cell resistance to anti-neoplastic therapies which kill tumor cells by apoptosis. Elucidation of the genetic and biochemical controls of these cellular responses may provide insights into ways to induce cell death and thus hopefully suggest new targets for improving therapeutic index in the treatment of malignancies.

Scatter factor/hepatocyte growth factor is essential for liver development

Polypeptide growth factors are important effectors of cell growth and differentiation in vitro and are thought to be critical for processes such as specification of cell fate, tissue growth and organogenesis in vivo. Scatter factor/hepatocyte growth factor (SF/HGF) is the prototype of an emerging family of growth factors that resemble in their domain structure and mechanism of activation the blood proteinase plasminogen. The cellular responses of SF/HGF are mediated by the c-Met tyrosine kinase receptor. Here we report that mice lacking SF/HGF fail to complete development and die in utero. The mutation affects the embryonic liver, which is reduced in size and shows extensive loss of parenchymal cells. In addition, development of the placenta, particularly of trophoblast cells, is impaired. Thus, SF/HGF is essential for the development of several epithelial organs.

Delayed development of retinoblastoma associated with loss of a maternal allele on chromosome 13

Loss of heterozygosity (LOH) on chromosome 13, which is associated with the functional inactivation of the retinoblastoma (RB) gene, is critical for the development of RB. To date, we have found that LOH-negative tumors develop earlier than LOH-positive tumors in hereditary cases of RB, an observation which suggests that loss of one allele on chromosome 13 may be disadvantageous with respect to growth of RB tumors. In this study, the parental origin of the lost allele on chromosome 13 and the age at operation of 13 patients with non-hereditary RB tumors that had been enucleated at the same stage were studied, in an attempt to determine whether there are any differences between tumors with loss of a maternal allele on chromosome 13 and tumors with loss of a paternal allele. Six tumors had lost the maternal allele and 7 tumors had lost the paternal allele on chromosome 13. The age (average 694 days) of patients at operation in the case of tumors with loss of the paternal allele was significantly lower than the age (average 1,079 days) of patients at operation for removal of tumors with loss of the maternal allele. RB tumors that had lost the maternal allele on chromosome 13 developed later than tumors that had lost the paternal allele. The possibility is discussed that loss of the maternal allele on chromosome 13 might be disadvantageous for growth of RB tumors.

p53, Rb and bcl-2 expression during the cell cycle: a study in phytohaemagglutinin stimulated lymphocytes and microwave irradiated lymphoid tissue sections

AIMS

To determine the expression of p53, Rb, and bcl-2 during the cell cycle in stimulated peripheral blood lymphocytes (PBLs) and microwave heated reactive lymphoid tissue sections.

METHODS

The expression of p53, Rb and bcl-2 proteins in paraffin wax embedded tonsil tissue sections was detected by immunohistochemistry using an (APAAP) technique following microwave irradiation. Flow cytometric analysis as performed on phytohaemagglutinin (PHA) stimulated PBLs, with simultaneous S fraction determination.

RESULTS

Expression of p53 protein was detected in reactive tonsil germinal centre cells, in some suprabasal cells in the surface and cryptic epithelium, and in some endothelial cells. Analysis of p53 in PHA stimulated PBLs revealed expression of p53 by non-tumoral activated lymphocytes. Rb protein expression was increased in PHA stimulated PBLs and was usually detected in most germinal centre B cells, in isolated paracortical cells, in a fraction of endothelial cells, and in most epithelial suprabasal cells. Expression of bcl-2 in stimulated lymphocytes was inversely correlated with proliferation. This confirms findings in reactive tonsil tissue samples, where proliferating cells located in the germinal centres and paracortical area are mostly bcl-2 negative.

CONCLUSIONS

Expression of these three oncogenic and tumour suppressor proteins varies during the cell cycle in non-tumoral cells. Consequently, tumoral growth fraction must be taken into account when analysing dysregulation of these three genes in lymphomas and other tumours. The p53 protein may be detected in benign conditions, as its expression is not synonymous with malignancy or mutation of the p53 gene.

The retinoblastoma tumor suppressor protein

Loss of the retinoblastoma protein, pRb, appears to have a role in several human tumor types. Mice lacking pRb have been produced as models of human disease, but have a different spectrum of affected tissues. Recent work shows that the tumorigenic effects of pRb may be revealed only after additional genetic alterations, such as loss of p53. New targets/effectors of pRb have been identified recently, and the system of kinases that inactivate pRb is proving to be complex.

Amplification of the E2F1 transcription factor gene in the HEL erythroleukemia cell line

The E2F transcription factor plays an important regulatory role in cell proliferation, mediating the expression of genes whose products are essential for inducing resting cells to enter the cell cycle and synthesize DNA. To investigate the possible involvement of E2F in hematopoietic malignancies, we isolated genomic clones encompassing the human E2F1 gene. We then used fluorescence in situ hybridization to localize E2F1 to human chromosome 20q11, telomeric to the p107 locus, a gene whose product is related to the retinoblastoma gene product (pRb). This finding contrasts with the 1p36 and 6q22 chromosomal locations previously assigned E2F2 and E2F3, two additional members of the E2F family. Although deletions or structural rearrangements of E2F1 were not detected in 14 primary acute leukemia or myelodysplasia samples with structural abnormalities of chromosome 20q11, the gene was amplified and overexpressed in HEL erythroleukemia cells and translocated to other chromosomes in several established human leukemia cell lines. This study provides the first evidence of gene amplification involving a member of the E2F family of transcription factors. We propose that E2F1 overexpression in erythroid progenitors may stimulate abnormal cell proliferation by overriding negative regulatory signals mediated by tumor suppressor proteins such as pRb.

The 'LXCXE' hydropathic superfamily of ligands for retinoblastoma protein: a proposal

The present study reports structural similarities between viral oncoproteins, growth factors belonging to the insulin family, members of the steroid/thyroid receptor superfamily, a D-type cyclin, the Elf-1 transcription factor and Bcl oncoproteins in regions that have been shown or proposed to mediate complex formation of these proteins with the tumor suppressor retinoblastoma protein (RB). This relationship predicts a common intracellular pathway for mitogenic signals and molecules promoting cell survival. Conversely, the structural evidence described here suggests that RB may play a central role both at the boundary between negative and positive cell growth regulation as well as in developmental decisions between cell death and cell survival.

Disruption of retinoblastoma protein function by coexpression of its C pocket fragment

The growth suppression function of the retinoblastoma protein (RB) is mediated by its interaction with a variety of cellular proteins. RB contains at least two protein-binding pockets: the large A/B pocket, which interacts with E2F and the D-type cyclins, and the C pocket, which interacts with the nuclear c-Abl tyrosine kinase. The large A/B pocket and the C pocket are shown here to be functionally distinct and can be occupied simultaneously. A complex containing E2F, RB, and c-Abl is detected in vivo and can be assembled in vitro. We propose that the biological activity of RB not only depends on the inhibition of its targets but also on its ability to properly assemble specific protein complexes. Consistent with this hypothesis, a fragment of RB, SE delta, containing only the C pocket is shown to act as a dominant-negative inhibitor of RB function. SE delta does not have growth inhibitory activity of its own. When coexpressed with full-length RB, SE delta does not disrupt the RB-E2F or RB-D2 complexes nor does it affect the expression, phosphorylation, or nuclear tethering of the full-length RB. SE delta does compete with RB for binding to c-Abl and is fully capable of inhibiting the c-Abl tyrosine kinase. Thus, SE delta can inactivate RB while maintaining the inhibition of E2F and c-Abl. These results suggest that the inhibition of RB-binding proteins is not sufficient to suppress cell growth and that the assembly of RB-mediated protein complexes is also important for the promotion of cell-cycle arrest.

The tsA58 simian virus 40 large tumor antigen disrupts megakaryocyte differentiation in transgenic mice

Thrombocytopenia is a condition of multiple etiologies affecting the megakaryocyte lineage. To perturb this lineage in transgenic mice, the tsA58 mutation of the simian virus 40 large tumor antigen was targeted to megakaryocytes using the platelet factor 4 promoter. Ten of 17 transgenic lines generated exhibited low platelet levels, each line displaying a distinct, heritable level of thrombocytopenia. Within a line, the degree of the platelet reduction correlated directly with transgene zygosity. The platelet level could be further reduced by the inactivation of one copy of the endogenous retinoblastoma gene. Western blot analysis detected large tumor antigen protein in the most severely affected lines; less affected lines were below the level of detection. Platelets and megakaryocytes from thrombocytopenic mice exhibited morphological abnormalities. Mice with either normal or reduced platelet levels developed megakaryocytic malignancies with a mean age of onset of about 8 months. There was no correlation between severity of thrombocytopenia and onset of malignancy. These mice provide a defined genetic model for thrombocytopenia, and for megakaryocytic neoplasia, and implicate the retinoblastoma protein in the process of megakaryocyte differentiation.

Deregulated expression of E2F-1 induces S-phase entry and leads to apoptosis

E2F-1, the first gene product identified among a family of E2F transcription factors, is thought to play a critical role in G1/S progression of the cell cycle. Transcriptional activities of E2F are modulated during the cell cycle, mainly by the formation of complexes between E2F and several key regulators of cell cycle such as the retinoblastoma protein and related proteins. To further understand the roles of E2F in the cell cycle progression, we have overexpressed exogenous E2F-1 by using a tetracycline-controlled expression system. We have found that the induced expression of E2F-1 in Rat-2 fibroblasts promotes S-phase entry and subsequently leads to apoptosis. The apoptosis occurs in an E2F-1 dose-dependent manner. Cells resistant to the induction of apoptosis have lost the ability to express exogenous E2F-1. Cells growing in low serum are more sensitive to the E2F-1-mediated cell death. Overexpression of E2F-1 mutants that impair DNA binding or transactivation does not alter cell cycle progression or induce apoptosis. These results define a novel pathway to apoptosis and demonstrate that premature S-phase entry is associated with apoptotic cell death.

DP and E2F proteins: coordinating transcription with cell cycle progression

Transcriptional control during the G1/S transition is important in regulating cell cycle progression. The transcription factor DRTF1/E2F is believed to play a crucial role in this process by integrating the activity of the machinery that drives the cell cycle with the transcription apparatus. Being the point of convergence for growth-promoting and growth-inhibitory signals, it is a pivotal cellular target for molecules which subvert normal cell cycle control, such as oncoviral proteins. Recent studies have indicated that members of two distinct families of proteins, DP and E2F, interact combinatorially as DP/E2F heterodimers in DRTF1/E2F. The activities of both DP and E2F proteins are under cell cycle control, being influenced by the level of phosphorylation imparted through the cell cycle regulated activity of cyclin-dependent kinases. Both DP and E2F proteins are endowed with proto-oncogenic activity and, conversely, have been implicated in regulating apoptosis. Current evidence suggests therefore that the activity of DRTF1/E2F is instrumental in regulating progression through the cell cycle.

p53 and Rb: their cellular roles

Within the past year considerable new insights have been gained into the roles the p53 and retinoblastoma tumour suppressors play in determining the fate of cells through their regulation of cell cycle progression, apoptosis and gene expression. Key advances have been achieved in the identification and characterization of functional domains and through functional knockout studies.

Control of cell proliferation during plant development

Knowledge of the control of cell division in eukaryotes has increased tremendously in recent years. The isolation and characterization of the major players from a number of systems and the study of their interactions have led to a comprehensive understanding of how the different components of the cell cycle apparatus are brought together and assembled in a fine-tuned machinery. Many parts of this machine are highly conserved in organisms as evolutionary distant as yeast and animals. Some key regulators of cell division have also been identified in higher plants and have been shown to be functional homologues of the yeast or animal proteins. Although still in its early days, investigations into the regulation of these molecules have provided some clues on how cell division is coupled to plant development.

Deregulated expression of E2F-1 induces S-phase entry and leads to apoptosis

E2F-1, the first gene product identified among a family of E2F transcription factors, is thought to play a critical role in G1/S progression of the cell cycle. Transcriptional activities of E2F are modulated during the cell cycle, mainly by the formation of complexes between E2F and several key regulators of cell cycle such as the retinoblastoma protein and related proteins. To further understand the roles of E2F in the cell cycle progression, we have overexpressed exogenous E2F-1 by using a tetracycline-controlled expression system. We have found that the induced expression of E2F-1 in Rat-2 fibroblasts promotes S-phase entry and subsequently leads to apoptosis. The apoptosis occurs in an E2F-1 dose-dependent manner. Cells resistant to the induction of apoptosis have lost the ability to express exogenous E2F-1. Cells growing in low serum are more sensitive to the E2F-1-mediated cell death. Overexpression of E2F-1 mutants that impair DNA binding or transactivation does not alter cell cycle progression or induce apoptosis. These results define a novel pathway to apoptosis and demonstrate that premature S-phase entry is associated with apoptotic cell death.

Deregulated transcription factor E2F-1 expression leads to S-phase entry and p53-mediated apoptosis

E2F-1 is a transcription factor suspected of activating genes required for S phase and a known target for the action of RB, the retinoblastoma gene product. Its induction in quiescent fibroblasts led to S-phase entry followed by apoptosis. E2F-1-mediated apoptosis was suppressed by coexpression of wild-type RB or a transdominant negative mutant species of p53. In contrast, coexpression of a naturally occurring loss-of-function RB mutant or wild-type p53 did not suppress the induction of apoptosis under these conditions. Thus, deregulated E2F-1 activity gives rise to proliferative and apoptotic signals. p53 appears to participate in the execution of the latter.