Dual roles of the retinoblastoma protein in cell cycle regulation and neuron differentiation

pRb controls proliferation, differentiation, and death of skeletal muscle cells and other lineages during embryogenesis

Mice deficient for the RB gene (RB-/-), prior to death at embryonic day 14.5, show increased cell death in all tissues that normally express RB1: the nervous system, liver, lens, and skeletal muscle precursor cells. We have generated transgenic mice (RBlox) that express low levels of pRb, driven by an RB1 minigene. RBlox/RB-/- mutant fetuses die at birth with specific skeletal muscle defects, including increased cell death prior to myoblast fusion, shorter myotubes with fewer myofibrils, reduced muscle fibers, accumulation of elongated nuclei that actively synthesized DNA within the myotubes, and reduction in expression of the late muscle-specific genes MCK and MRF4. Thus, insufficient pRb results in failure of myogenesis in vivo, manifest in two ways. First, the massive apoptosis of myoblasts implicates a role of pRb in cell survival. Second, surviving myotubes failed to develop normally and accumulated large polyploid nuclei, implicating pRb in permanent withdrawal from the cell cycle. These results demonstrate a role for pRb during terminal differentiation of skeletal muscles in vivo and place pRb at a nodal point that controls cell proliferation, differentiation, and death.

Lack of neurotrophin-3 results in death of spinal sensory neurons and premature differentiation of their precursors

To understand mechanisms resulting in the absence of two-thirds of spinal sensory neurons in mice lacking NT-3, we have compared dorsal root ganglia development in normal and mutant embryos. The reduction in neurons, achieved by E13, results from several deficits: first, elevated neuronal apoptosis significantly reduces neuronal numbers; second, elevated neurogenesis between E11 and E12, without changes in rates of precursor proliferation or apoptosis, depletes the precursor pool; consequently, the reduced precursor pool prevents increases in neuronal numbers between E12 and E13, when most neurons are born in normal animals. Although deficits occur before final target innervation, we show that NT-3 is expressed at all stages in regions accessible to these neurons or their axons and is only restricted to final targets after innervation.

Retinoblastoma protein positively regulates terminal adipocyte differentiation through direct interaction with C/EBPs

To define a mechanism by which retinoblastoma protein (Rb) functions in cellular differentiation, we studied primary fibroblasts from the lung buds of wild-type (RB+/+) and null-mutant (RB-/-) mouse embryos. In culture, the RB+/+ fibroblasts differentiated into fat-storing cells, either spontaneously or in response to hormonal induction; otherwise syngenic RB-/- fibroblasts cultured in identical conditions did not. Ectopic expression of normal Rb, but not Rb with a single point mutation, enabled RB-/- fibroblasts to differentiate into adipocytes. Rb appears in murine fibroblasts to activate CCAAT/enhancer-binding proteins (C/EBPs), a family of transcription factors crucial for adipocyte differentiation. Physical interaction between Rb and C/EBPs was demonstrated by reciprocal coimmunoprecipitation, but occurred only in differentiating cells. Wild-type Rb also enhanced the binding of C/EBP to cognate DNA sequences in vitro and the transactivation of a C/EBPbeta-responsive promoter in cells. Taken together, these observations establish a direct and positive role for Rb in terminal differentiation. Such a role contrasts with the function of Rb in arresting cell cycle progression in G1 by negative regulation of other transcription factors like E2F-1.

Skeletal muscle cells lacking the retinoblastoma protein display defects in muscle gene expression and accumulate in S and G2 phases of the cell cycle

Viral oncoproteins that inactivate the retinoblastoma tumor suppressor protein (pRb) family both block skeletal muscle differentiation and promote cell cycle progression. To clarify the dependence of terminal differentiation on the presence of the different pRb-related proteins, we have studied myogenesis using isogenic primary fibroblasts derived from mouse embryos individually deficient for pRb, p107, or p130. When ectopically expressed in fibroblasts lacking pRb, MyoD induces an aberrant skeletal muscle differentiation program characterized by normal expression of early differentiation markers such as myogenin and p21, but attenuated expression of late differentiation markers such as myosin heavy chain (MHC). Similar defects in MHC expression were not observed in cells lacking either p107 or p130, indicating that the defect is specific to the loss of pRb. In contrast to wild-type, p107-deficient, or p130-deficient differentiated myocytes that are permanently withdrawn from the cell cycle, differentiated myocytes lacking pRb accumulate in S and G2 phases and express extremely high levels of cyclins A and B, cyclin-dependent kinase (Cdk2), and Cdc2, but fail to readily proceed to mitosis. Administration of caffeine, an agent that removes inhibitory phosphorylations on inactive Cdc2/cyclin B complexes, specifically induced mitotic catastrophe in pRb-deficient myocytes, consistent with the observation that the majority of pRb-deficient myocytes arrest in S and G2. Together, these findings indicate that pRb is required for the expression of late skeletal muscle differentiation markers and for the inhibition of DNA synthesis, but that a pRb-independent mechanism restricts entry of differentiated myocytes into mitosis.

A new member of the hsp90 family of molecular chaperones interacts with the retinoblastoma protein during mitosis and after heat shock

A gene encoding a new heat shock protein that may function as a molecular chaperone for the retinoblastoma protein (Rb) was characterized. The cDNA fragment was isolated by using the yeast two-hybrid system and Rb as bait. The open reading frame of the longest cDNA codes for a protein with substantial sequence homology to members of the hsp90 family. Antibodies prepared against fusions between glutathione S-transferase and portions of this new heat shock protein specifically recognized a 75-kDa cellular protein, hereafter designated hsp75, which is expressed ubiquitously and located in the cytoplasm. A unique LxCxE motif in hsp75, but not in other hsp90 family members, appears to be important for binding to the simian virus 40 T-antigen-binding domain of hypophosphorylated Rb, since a single mutation changing the cysteine to methionine abolishes the binding. In mammalian cells, Rb formed complexes with hsp75 under two special physiological conditions: (i) during M phase, when the envelope that separates the nuclear and cytoplasmic compartments broke down, and (ii) after heat shock, when hsp75 moved from its normal cytoplasmic location into the nucleus. In vitro, hsp75 had a biochemical activity to refold denatured Rb into its native conformation. Taken together, these results suggest that Rb may be a physiological substrate for the hsp75 chaperone molecule. The discovery of a heat shock protein that chaperones Rb identifies a mechanism, in addition to phosphorylation, by which Rb is regulated in response to progression of the cell cycle and to external stimuli.

Early loss of the retinoblastoma gene is associated with impaired growth inhibitory innervation during melanotroph carcinogenesis in Rb+/- mice

To better understand the cell lineage-specific character of retinoblastoma (Rb) gene inactivation during tumor formation, the earliest stages of spontaneous melanotroph carcinogenesis in Rb+/- heterozygous mice have been subjected to sequential analyses. The first atypical cells are detected in the pituitary intermediate lobe during a period corresponding to the cessation of melanotroph proliferation between 35 and 60 days after birth. Atypical cells contain no wild-type copy of the Rb gene and synchronously form early atypical proliferates (EAP) in the subsequent 30-60 day period. In contrast to surrounding mature melanotrophs with the wild-type Rb gene, Rb-negative cells in EAP continue to proliferate well past postnatal day 60, and fail to be innervated by growth inhibitory dopaminergic nerve terminals. Atypical melanotrophs remain competent for dopamine D2 receptor stimulation and undergo S-phase apoptosis in close proximity to nerve terminals. These results indicate a key role for the Rb protein in the onset of neuron-neuroendocrine cell interactions. This role may explain cell-type-specific neuroendocrine carcinogenesis associated with inactivation of the ubiquitously expressed Rb gene.

Inactivation of the mouse Brca1 gene leads to failure in the morphogenesis of the egg cylinder in early postimplantation development

BRCA1 is proposed to be a tumor suppressor gene. To explore the biological function of BRCA1, a partial deletion (amino acids 300-361) of mouse Brca1 exon 11 was introduced into the genome of embryonic stem (ES) cells by homologous recombination. Mice carrying one mutated allele of Brca1 appear normal and are fertile up to 10 months of age without any sign of illness. However, no viable progeny homozygous for the Brca1 mutant allele were obtained. Detailed analysis of large numbers of embryos at different stages of development indicated that the homozygous mutant concepti are severely retarded in growth as early as embryonic day 4.5 (E4.5) and are resorbed completely by E8.5. Although the homozygotes at E5.5-E6.5 are able to synthesize DNA and display distinguishable embryonic and extraembryonic structures, they fail to differentiate and form egg cylinders. Consequently, they were unable to form primitive streaks and undergo gastrulation. Consistent with these in vivo results, blastocysts homozygous for mutated Brca1 alleles are at a considerable disadvantage when grown in vitro. These observations suggest that Brca1 has an important role in the early development of mouse embryos.

Cell cycle regulation by the retinoblastoma family of growth inhibitory proteins

The retinoblastoma family of growth-inhibitory proteins act by binding and inhibiting several proteins with growth-stimulatory activity, the most prominent of which is the cellular transcription factor E2F. In higher organisms, progression through the cell division cycle is accompanied by the cyclical activation of a number of protein kinases, the cyclin-dependent kinases. Phosphorylation of retinoblastoma family proteins by these cyclin-dependent kinases leads to release of the associated growth-stimulatory proteins which in turn mediate progression through the cell division cycle.

The latency-related gene of bovine herpesvirus 1 encodes a product which inhibits cell cycle progression

Bovine herpesvirus 1 (BHV-1) establishes a latent infection in the sensory ganglionic neurons of cattle. The exclusive viral RNA expressed in a latent infection is the latency-related (LR) RNA, suggesting that it regulates some aspect of a latent infection. During the course of a productive infection, alphaherpesviruses induce certain events which occur during cell cycle progression. Consequently, we hypothesized that a BHV-1 infection might induce events in neurons which occur during cell cycle progression. In agreement with this hypothesis, cyclin A was detected in neurons of trigeminal ganglia when rabbits were infected. Neuronal cell cycle progression or inappropriate expression of cyclin A leads to apoptosis, suggesting that a viral factor inhibits the deleterious effects of cyclin A expression. The BHV-1 LR gene inhibited cell cycle progression and proliferation of human osteosarcoma cells. Antibodies directed against cyclin A or the LR protein coprecipitated the LR protein or cyclin A, respectively, suggesting that the two proteins interact with each other. We conclude that LR gene products inhibit cell cycle progression and hypothesize that this activity enhances the survival of infected neurons.

Tumor induction and tissue atrophy in mice lacking E2F-1

The retinoblastoma tumor suppressor protein (pRB) is a transcriptional repressor that regulates gene expression by physically associating with transcription factors such as E2F family members. Although pRB and its upstream regulators are commonly mutated in human cancer, the physiological role of the pRB-E2F pathway is unknown. To address the function of E2F-1 and pRB/E2F-1 complexes in vivo, we have produced mice homozygous for a nonfunctional E2F-1 allele. Mice lacking E2F-1 are viable and fertile, yet experience testicular atrophy and exocrine gland dysplasia. Surprisingly, mice lacking E2F-1 develop a broad and unusual spectrum of tumors. Although overexpression of E2F-1 in tissue culture cells can stimulate cell proliferation and be oncogenic, loss of E2F-1 in mice results in tumorigenesis, demonstrating that E2F-1 also functions as a tumor suppressor.

Induction of DNA synthesis and apoptosis in cardiac myocytes by E1A oncoprotein

Beginning during the second half of gestation, increasing numbers of cardiac myocytes withdraw from the cell cycle such that DNA synthesis is no longer detectable in these cells by neonatal day 17 in vivo. The mechanisms that exclude these and other terminally differentiated cells from the cell division cycle are poorly understood. To begin to explore the molecular basis of the barrier to G1/S progression in cardiac myocytes, we used adenoviruses to express wild-type and mutant E1A proteins in primary cultures from embryonic day 20 rats. While most of these cardiac myocytes are ordinarily refractory to DNA synthesis, even in the presence of serum growth factors, expression of wild-type E1A stimulates DNA synthesis in up to 94% or almost all successfully transduced cells. Rather than complete the cell cycle, however, these cells undergo apoptosis. Apoptosis is limited to those cells that engage in DNA synthesis, and the kinetics of the two processes suggest that DNA synthesis precedes apoptosis. Mutations in E1A that disable it from binding Rb and related pocket proteins have little effect on its ability to stimulate DNA synthesis in cardiac myocytes. In contrast, mutants that are defective in binding the cellular protein p300 stimulate DNA synthesis 2.4-4.1-fold less efficiently, even in the context of retained E1A pocket protein binding. In the absence of ElA pocket protein binding, the usual situation in the cell, loss of p300 binding severely decreases the ability of ElA to stimulate DNA synthesis. These results suggest that the barrier to G1/S progression in cardiac myocytes is mediated. at least in part, by the same molecules that gate the G1/S transition in actively cycling cells, and that p300 or related family members play an important role in this process.

Retinoblastoma protein directly interacts with and activates the transcription factor NF-IL6

The biological function of the retinoblastoma protein (RB) in the cell division cycle has been extensively documented, but its apparent role in differentiation remains largely unexplored. To investigate how RB is involved in differentiation, the U937 large-cell lymphoma line was induced to differentiate along a monocyte/macrophage lineage. During differentiation RB was found to interact directly through its simian virus 40 large tumor antigen (T antigen)-binding domain with NF-IL6, a member of the CAAT/enhancer-binding protein (C/EBP) family of transcription factors. NF-IL6 utilizes two distinct regions to bind to the hypophosphorylated form of RB in vitro and in cells. Wild-type but not mutant RB enhanced both binding activity of NF-IL6 to its cognate DNA sequences in vitro and promoter transactivation by NF-IL6 in cells. These findings indicate a novel biochemical function of RB: it activates, by an apparent chaperone-like activity, specific transcription factors important for differentiation. This contrasts with its sequestration and inactivation of other transcription factors, such as E2F-1, which promote progression of the cell cycle. Such disparate mechanisms may help to explain the dual role of RB in cell differentiation and the cell division cycle.

The cellular effects of E2F overexpression

The product of the retinoblastoma tumor-suppressor gene (RB) is a ubiquitously expressed, 105-kDa nuclear phosphoprotein (pRB). The pRB protein negatively regulates the cellular G1/S phase transition, and it is at this point in the cell cycle that it is thought to play its role as a tumor suppressor. The growth-inhibitory effects of pRB are exerted, at least in part, through the E2F family of transcription factors. This chapter reviews the insights into the mechanism of action of the E2F family members that have been obtained through overexpression studies. Studies in RB-/- SAOS-2 cells have provided evidence in support of the hypothesis that the E2F family members are negatively regulated by pRB and the related protein p130. In particular, the results obtained are consistent with the earlier biochemical data which suggested that E2F1 is regulated primarily by pRB, and E2F4 by p130. Results relating to p107 are also discussed. Consistent with the proposed role of pRB and E2F1 as coregulators of entry into S phase, experiments have demonstrated that overexpression of E2F1 is sufficient to override the cell cycle arrests caused by serum deprivation of fibroblasts or transforming growth factor-beta (TGF-beta) treatment of mink lung epithelial cells. However, at least in the case of the serum deprivation induced arrest, the ultimate result of E2F1 overexpression is death by p53-dependent apoptosis. In light of this and other data, a model is discussed as to how functional inactivation of pRB and p53 might cooperate to promote tumorigenesis. A number of studies have demonstrated the oncogenic potential of E2F family members, at least under certain conditions. This is, again, in keeping with the notion that these proteins play a critical role in controlling proliferation.

Retinoblastoma gene in mouse neural development

The retinoblastoma gene (Rb) was the first tumor suppressor gene to be cloned [Dryja et al., 1986; Friend et al., 1986; Lee et al., 1987], and, as a consequence, has been studied intensively within the context of cell cycle regulation and oncogenesis. However, a number of recent findings indicate that the retinoblastoma gene product (pRb) likely plays an essential role not only in controlling entry into the cell cycle, but also in the terminal differentiation of a number of different cell types [Lee et al., 1994; Gu et al., 1993]. In particular, the phenotype of the Rb nullizygous mice, created by a number of groups using homologous recombination [Jacks et al., 1992: Clarke et al., 1992; Lee et al., 1992], indicates that pRb is essential for normal development of the nervous and hematopoietic systems and may even function to regulate apoptosis [Haas-Kogan et al., 1995]. Although this paper briefly reviews the traditional role of pRB in regulation of cellular proliferation, we focus on the role of pRB in neuronal development and apoptosis. Recent reviews have been published on the role of pRb in cell cycle and transcriptional regulation [Hamel et al., 1992; Cobrinik et al., 1992; Kouzarides, 1993; Hollingsworth et al., 1993; Helin and Harlow, 1993; Sherr, 1994], as well as the relationship between pRb and p53 [Picksley and Lane, 1994; White, 1994].

Apoptosis and the cell cycle

Apoptosis is an evolutionarily conserved 'suicide' programme present in all metazoan cells. Despite its highly conserved nature, it is only recently that any of the molecular mechanisms underlying apoptosis have been identified. Several lines of reasoning indicate that apoptosis and cell proliferation coincide to some degree: many oncogenes that promote cell cycle progression also induce apoptosis; damage to the cell cycle or to DNA integrity is a potent trigger of apoptosis; and the key tumour suppressor proteins, p105rb and p53, exert direct effects both on cell viability and on cell cycle progression. There is less evidence, however, to indicate that apoptosis and the cell cycle share common molecular mechanisms. Moreover, the interleukin-1 beta converting enzyme (ICE) family of cysteine proteases is now known to play a key role in apoptosis but has no discernible role in the cell cycle, arguing that the two processes are discrete.

Dual retinoblastoma-binding proteins with properties related to a negative regulator of ras in yeast

The retinoblastoma protein (Rb) interacts with multiple cellular proteins that mediate its cellular function. We have identified nine polypeptides that bind to the T-binding domains of Rb using an Rb affinity resin. RbAp48 and RbAp46 are quantitatively the major Rb-associated proteins purified by this approach. RbAp48 was characterized previously and was found to be related to MSI1, a negative regulator of Ras in the yeast Saccharomyces cerevisiae. Here we report the cloning and characterization of RbAp46. RbAp46 shares 89.4% amino acid identity with RbAp48. The internal WD repeats, which are found in a growing number of eukaryotic proteins, are conserved between RbAp46 and RbAp48. Like RbAp48, RbAp46 forms a complex with Rb both in vitro and in vivo and suppresses the heat-shock sensitivity of the yeast RAS2Val-19 strains. We have also isolated the murine cDNA homologs of RbAp48 and RbAp46. Although both mRNA can be detected in all mouse tissues, their mRNA levels vary dramatically between different tissues. No significant differences were observed in the expression patterns of these genes in most tissues except thymus, testis, and ovary/uterus, in which 2-fold differences were observed. Interestingly, the mouse and human RbAp48 amino acid sequences are completely identical, and the mouse and human RbAp46 differ only by one conserved amino acid substitution. These results suggest that RbAp48 and RbAp46 may have shared as well as unique functions in the regulation of cell proliferation and differentiation.

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.

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.

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.

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.

The retinoblastoma protein binds to RIZ, a zinc-finger protein that shares an epitope with the adenovirus E1A protein

The retinoblastoma protein (Rb) is a target of viral oncoproteins. To explore the hypothesis that viral proteins may be structural mimics of cellular proteins, we have searched cDNA libraries for Rb-binding proteins. We report here the cloning of a cDNA for the protein RIZ from rat and human cells. RIZ is a 250-kDa nuclear protein containing eight zinc-finger motifs. It contains an Rb-binding motif that shares an antigenic epitope with the C terminus of E1A. A domain is conserved between RIZ and the PRDI-BF1/Blimp-1 differentiation factor. Other motifs of RIZ include putative GTPase and SH3 (src homology domain 3) domains. RIZ is preferentially expressed in both adult and embryonic rat neuroendocrine tissues. It is also expressed in human retinoblastoma cells and at low levels in all other human cell lines examined. While the function of RIZ is not yet clear, its structure and pattern of expression suggest a role for RIZ in transcriptional regulation during neuronal differentiation and pathogenesis of retinoblastoma.

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.

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.