Defining the minimal portion of the retinoblastoma protein that serves as an efficient substrate for cdk4 kinase/cyclin D1 complex

The DNA deaminase APOBEC3B interacts with the cell-cycle protein CDK4 and disrupts CDK4-mediated nuclear import of Cyclin D1

Apolipoprotein B mRNA editing enzyme catalytic subunit-like protein 3B (APOBEC3B or A3B), as other APOBEC3 members, is a single-stranded (ss)DNA cytosine deaminase with antiviral activity. A3B is also overexpressed in multiple tumor types, such as carcinomas of the bladder, cervix, lung, head/neck, and breast. A3B generates both dispersed and clustered C-to-T and C-to-G mutations in intrinsically preferred trinucleotide motifs (TCA/TCG/TCT). A3B-catalyzed mutations are likely to promote tumor evolution and cancer progression and, as such, are associated with poor clinical outcomes. However, little is known about cellular processes that regulate A3B. Here, we used a proteomics approach involving affinity purification coupled to MS with human 293T cells to identify cellular proteins that interact with A3B. This approach revealed a specific interaction with cyclin-dependent kinase 4 (CDK4). We validated and mapped this interaction by co-immunoprecipitation experiments. Functional studies and immunofluorescence microscopy experiments in multiple cell lines revealed that A3B is not a substrate for CDK4-Cyclin D1 phosphorylation nor is its deaminase activity modulated. Instead, we found that A3B is capable of disrupting the CDK4-dependent nuclear import of Cyclin D1. We propose that this interaction may favor a more potent antiviral response and simultaneously facilitate cancer mutagenesis.

Genetic evidence for p75NTR-dependent tetraploidy in cortical projection neurons from adult mice

A subpopulation of chick retinal projection neurons becomes tetraploid during development, an event prevented by blocking antibodies against p75 neurotrophin receptor (p75(NTR)). We have used an optimized flow cytometric assay, based on the analysis of unfixed brain cell nuclei, to study whether p75(NTR)-dependent neuronal tetraploidization takes place in the cerebral cortex, giving rise to projection neurons as well. We show that 3% of neurons in both murine neocortex and chick telencephalic derivatives are tetraploid, and that in the mouse ~85% of these neurons express the immediate early genes Erg-1 and c-Fos, indicating that they are functionally active. Tetraploid cortical neurons (65-80%) express CTIP2, a transcription factor specific for subcortical projection neurons in the mouse neocortex. During the period in which these neurons are born, p75(NTR) is detected in differentiating neurons undergoing DNA replication. Accordingly, p75(NTR)-deficient mice contain a reduced proportion of both NeuN and CTIP2-positive neocortical tetraploid neurons, thus providing genetic evidence for the participation of p75(NTR) in the induction of neuronal tetraploidy in the mouse neocortex. In the striatum tetraploidy is mainly associated with long-range projection neurons as well since ~80% of tetraploid neurons in this structure express calbindin, a marker of neostriatal-matrix spiny neurons, known to establish long-range projections to the substantia nigra and globus pallidus. In contrast, only 20% of tetraploid cortical neurons express calbindin, which is mainly expressed in layers II-III, where CTIP2 is absent. We conclude that tetraploidy mainly affects long-range projection neurons, being facilitated by p75(NTR) in the neocortex.

Phospho-Rb mediating cell cycle reentry induces early apoptosis following oxygen-glucose deprivation in rat cortical neurons

The aim of this study was to investigate the relationship between cell cycle reentry and apoptosis in cultured cortical neurons following oxygen-glucose deprivation (OGD). We found that the percentage of neurons with BrdU uptake, TUNEL staining, and colocalized BrdU uptake and TUNEL staining was increased relative to control 6, 12 and 24 h after 1 h of OGD. The number of neurons with colocalized BrdU and TUNEL staining was decreased relative to the number of TUNEL-positive neurons at 24 h. The expression of phosphorylated retinoblastoma protein (phospho-Rb) was significantly increased 6, 12 and 24 h after OGD, parallel with the changes in BrdU uptake. Phospho-Rb and TUNEL staining were colocalized in neurons 6 and 12 h after OGD. This colocalization was strikingly decreased 24 h after OGD. Treatment with the cyclin-dependent kinase inhibitor roscovitine (100 μM) decreased the expression of phospho-Rb and reduced neuronal apoptosis in vitro. These results demonstrated that attempted cell cycle reentry with phosphorylation of Rb induce early apoptosis in neurons after OGD and there must be other mechanisms involved in the later stages of neuronal apoptosis besides cell cycle reentry. Phosphoralated Rb may be an important factor which closely associates aberrant cell cycle reentry with the early stages of neuronal apoptosis following ischemia/hypoxia in vitro, and pharmacological interventions for neuroprotection may be useful directed at this keypoint.

Crystal structure of human CDK4 in complex with a D-type cyclin

The cyclin D1-cyclin-dependent kinase 4 (CDK4) complex is a key regulator of the transition through the G(1) phase of the cell cycle. Among the cyclin/CDKs, CDK4 and cyclin D1 are the most frequently activated by somatic genetic alterations in multiple tumor types. Thus, aberrant regulation of the CDK4/cyclin D1 pathway plays an essential role in oncogenesis; hence, CDK4 is a genetically validated therapeutic target. Although X-ray crystallographic structures have been determined for various CDK/cyclin complexes, CDK4/cyclin D1 has remained highly refractory to structure determination. Here, we report the crystal structure of CDK4 in complex with cyclin D1 at a resolution of 2.3 A. Although CDK4 is bound to cyclin D1 and has a phosphorylated T-loop, CDK4 is in an inactive conformation and the conformation of the heterodimer diverges from the previously known CDK/cyclin binary complexes, which suggests a unique mechanism for the process of CDK4 regulation and activation.

Differential p38 mitogen-activated protein kinase-controlled hypophosphorylation of the retinoblastoma protein induced by nitric oxide in neuroblastoma cells

In this report we show that exogenous NO added to human neuroblastoma NB69 cells inhibits cell proliferation and downregulates the epidermal growth factor receptor (EGFR) and its downstream signaling pathways. These comprise the 3-phosphoinositide-dependent kinase 1/Akt/glycogen synthase kinase-3beta pathway, the mitogen-activated protein kinase (MAPK)/extracellular-regulated kinases 1 and 2 pathway, and the phospholipase Cgamma pathway. In contrast, NO enhances the EGFR-controlled p38MAPK pathway. We also show that NO enhances the activation of the cAMP-responsive element binding protein, a transcription factor controlled by p38MAPK, as demonstrated using 4-(4-fluorophenyl)-2-(4-hydroxyphenyl)-5-(4-pyridyl)1H-imidazole (SB202190), a p38MAPK inhibitor. These processes are accompanied by the NO-mediated hypophosphorylation of the retinoblastoma protein (pRb), preferentially at Ser795 compared to Ser780 and Ser807/811, and the downregulation of p27(KIP1), p21(CIP1/WAF1), and p16(INK4a), although NO downregulated p16(INK4a) only when the p38MAPK activity was suppressed. The p38MAPK pathway controls the phosphorylation status of pRb as SB202190 enhances the hypophosphorylation of pRb. We reverted the inhibitory action of NO on EGFR and pRb phosphorylation in living cells using cell-permeable reducing agents, which suggested that reversible S-nitrosation controls these proteins. Our results support the notion that NO negatively modulates the p38MAPK-controlled phosphorylation of pRb, inducing the subsequent arrest of the cell cycle at the G1/S transition.

Xenografts of human hepatocellular carcinoma: a useful model for testing drugs

PURPOSE

Our aims were to establish and characterize primary human hepatocellular carcinoma xenografts. They were used to screen new drugs and improve our current treatment regimens used in hepatocellular carcinoma.

EXPERIMENTAL DESIGN

Primary hepatocellular carcinomas were used to create the xenografts. Western blotting was used to determine the changes in proteins in these xenografts before and after therapies. Apoptotic and cell proliferation were analyzed by immunohistochemistry.

RESULTS

Seven lines of xenografts were established from primary human hepatocellular carcinomas. Lines 4-1318, 2-1318, 2006, and 26-1004 grew rapidly in severe combined immunodeficient (SCID) mice and doubled its volume every 48 to 72 hours. Series 5-1318 (5-1318, 30-1004, and 29-1104) grew relatively slowly in SCID mice and required approximately 6 to 10 days to double its tumor volume. Western blot analysis revealed that the growth rate of these xenografts was associated with abnormal expression of proteins associated with the cell cycle, signaling pathways, and tumor suppressor genes. Although hepatocellular carcinoma xenografts expressed the receptors for androgens, estrogens, and progesterone, their growth rate was not affected by either castration or sex steroid hormone supplementation. Cisplatin, oxaliplatin, vitamin D analogue EB1089, and Iressa had no effects on the growth rate in SCID mice. Although 5-fluorouracil exerted mild growth inhibition of these xenografts, i.p. delivery of 2-chloroethyl-3-sarcosinamide-1-nitrosourea (SarCNU) or doxorubicin resulted in a significant growth inhibition. Doxorubicin-induced growth suppression was associated with elevation of p53 and p21(Cip1/Waf1). In addition to up-regulation of p53 and p21(Cip1/Waf1), SarCNU also increased the levels of phosphorylated cdc-2 at Tyr15.

CONCLUSION

Hepatocellular carcinoma xenografts are powerful tools for screening drugs and SarCNU may be useful in the treatment of this fatal disease.

Limited redundancy in phosphorylation of retinoblastoma tumor suppressor protein by cyclin-dependent kinases in acute lymphoblastic leukemia

Cyclin-dependent kinases (CDKs) successively phosphorylate the retinoblastoma protein (RB) at the restriction point in G1 phase. Hyperphosphorylation results in functional inactivation of RB, activation of the E2F transcriptional program, and entry of cells into S phase. RB unphosphorylated at serine 608 has growth suppressive activity. Phosphorylation of serines 608/612 inhibits binding of E2F-1 to RB. In Nalm-6 acute lymphoblastic leukemia extracts, serine 608 is phosphorylated by CDK4/6 complexes but not by CDK2. We reasoned that phosphorylation of serines 608/612 by redundant CDKs could accelerate phospho group formation and determined which G1 CDK contributes to serine 612 phosphorylation. Here, we report that CDK4 complexes from Nalm-6 extracts phosphorylated in vitro the CDK2-preferred serine 612, which was inhibited by p16INK4a, and fascaplysin. In contrast, serine 780 and serine 795 were efficiently phosphorylated by CDK4 but not by CDK2. The data suggest that the redundancy in phosphorylation of RB by CDK2 and CDK4 in Nalm-6 extracts is limited. Serine 612 phosphorylation by CDK4 also occurred in extracts of childhood acute lymphoblastic leukemia cells but not in extracts of mobilized CD34+ hemopoietic progenitor cells. This phenomenon could contribute to the commitment of childhood acute lymphocytic leukemia cells to proliferate and explain their refractoriness to differentiation-inducing agents.

Resveratrol inhibits proliferation of human epidermoid carcinoma A431 cells by modulating MEK1 and AP-1 signalling pathways

Resveratrol (trans-3,4',5-trihydroxystilbene) is a naturally occurring polyphenolic phytoalexin found in grapes, and has been shown to inhibit the growth of various types of cancer cells. We investigated the mechanism of the antiproliferative effect of resveratrol in A431-transformed keratinocytes harbouring mutant p53, and show that it is accompanied by G1 cell cycle arrest, which coincides with a marked inhibition of G1 cell cycle regulatory proteins, including cyclins A and D1 and cyclin-dependent kinase (CDK)6 and p53-independent induction of p21WAF1. Cell cycle arrest was also associated with the accumulation of hypophosphorylated Rb and p27KIP1. Resveratrol inhibited mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK)1 > extracellular signal-regulated protein kinase (ERK)1/2 signalling, downregulated c-Jun, and suppressed activating protein (AP)-1 DNA-binding and promoter activity. In addition, the inhibition of MEK1 > ERK1/2 signalling appears to be independent of retinoblastoma protein (pRb) hypophosphorylation in A431 cells, as PD098059 did not suppress pRb phosphorylation. Our results demonstrate that resveratrol affects multiple cellular targets in A431 cells, and that the downregulation of both AP-1 and pRb contributes to its antiproliferative activity in these cells.

Downregulation of Fer induces PP1 activation and cell-cycle arrest in malignant cells

Fer is a nuclear and cytoplasmic intracellular tyrosine kinase. Herein we show that Fer is required for cell-cycle progression in malignant cells. Decreasing the level of Fer using the RNA interference (RNAi) approach impeded the proliferation of prostate and breast carcinoma cells and led to their arrest at the G0/G1 phase. At the molecular level, knockdown of Fer resulted in the activation of the retinoblastoma protein (pRB), and this was reflected by profound hypo-phosphorylation of pRB on both cyclin-dependent kinase CDK4 and CDK2 phosphorylation sites. Dephosphorylation of pRB was not seen upon the direct targeting of either CDK4 or CDK2 expression, and was only partially achieved by the simultaneous depletion of these two kinases. Amino-acid sequence analysis revealed two protein phosphatase 1 (PP1) binding motifs in the kinase domain of Fer and the association of Fer with the pRB phosphatase PP1alpha was verified using co-immunoprecipitation analysis. Downregulation of Fer potentiated the activation of PP1alpha and overexpression of Fer decreased the enzymatic activity of that phosphatase. Our findings portray Fer as a regulator of cell-cycle progression in malignant cells and as a potential target for cancer intervention.

Selectivity and potency of cyclin-dependent kinase inhibitors

Members of the cyclin-dependent kinase (CDK) family play key roles in various cellular processes. There are 11 members of the CDK family known till now. CDKs are activated by forming noncovalent complexes with cyclins such as A-, B-, C-, D- (D1, D2, and D3), and E-type cyclins. Each isozyme of this family is responsible for particular aspects (cell signaling, transcription, etc) of the cell cycle, and some of the CDK isozymes are specific to certain kinds of tissues. Aberrant expression and overexpression of these kinases are evidenced in many disease conditions. Inhibition of isozymes of CDKs specifically can yield beneficiary treatment modalities with minimum side effects. More than 80 3-dimensional structures of CDK2, CDK5, and CDK6 complexed with inhibitors have been published. This review provides an understanding of the structural aspects of CDK isozymes and binding modes of various known CDK inhibitors so that these kinases can be better targeted for drug discovery and design. The amino acid residues that constitute the cyclin binding region, the substrate binding region, and the area around the adenosine triphosphate (ATP) binding site have been compared for CDK isozymes. Those amino acids at the ATP binding site that could be used to improve the potency and subtype specificity have been described.

Structure of the Rb C-terminal domain bound to E2F1-DP1: a mechanism for phosphorylation-induced E2F release

The retinoblastoma (Rb) protein negatively regulates the G1-S transition by binding to the E2F transcription factors, until cyclin-dependent kinases phosphorylate Rb, causing E2F release. The Rb pocket domain is necessary for E2F binding, but the Rb C-terminal domain (RbC) is also required for growth suppression. Here we demonstrate a high-affinity interaction between RbC and E2F-DP heterodimers shared by all Rb and E2F family members. The crystal structure of an RbC-E2F1-DP1 complex reveals an intertwined heterodimer in which the marked box domains of both E2F1 and DP1 contact RbC. We also demonstrate that phosphorylation of RbC at serines 788 and 795 destabilizes one set of RbC-E2F-DP interactions directly, while phosphorylation at threonines 821 and 826 induces an intramolecular interaction between RbC and the Rb pocket that destabilizes the remaining interactions indirectly. Our findings explain the requirement of RbC for high-affinity E2F binding and growth suppression and establish a mechanism for the regulation of Rb-E2F association by phosphorylation.

Acquisition of essential somatic cell cycle regulatory protein expression and implied activity occurs at the second to third cell division in mouse preimplantation embryos

It is clear that G1-S phase control is exerted after the mouse embryo implants into the uterus 4.5 days after fertilization (E4.5); null mutants of genes that control cell cycle commitment such as max, rb (retinoblastoma), and dp1 are embryonic lethal after implantation with proliferation phenotypes. But, a number of studies of genes mediating proliferation control in the embryo after fertilization-implantation have yielded confusing results. In order to understand when embryos might first exert G1-S phase regulatory control, we assayed preimplantation mouse embryos for the acquisition of expression of mRNA, protein, and phospho-protein for max, Rb, and DP-1, and for the proliferation-promoting phospho-protein forms of mycC (thr58/ser62) and Rb (ser795). The key findings are that: (1) DP-1 protein was present in the nucleus as early as the four-cell stage onwards, (2) max protein was in the nucleus, suggesting function from the four-cell stage onwards, (3) both mycC and Rb all form protein was present at increasing quantities in the cytoplasm from the 2 cell and 4/8 cell stage, respectively, (4) the phosphorylated form of mycC phospho was present in the nucleus at high levels from the two-cell stage through blastocyst-stage, and (5) the phosphorylated form of Rb was detected at low levels in the two-cell stage embryo and was highly expressed at the 4/8-cell stage through the blastocyst stage. Taken together, these data suggest that activation of mycC phospho/max dimer pairs, (E2F)/DP-1 dimer pairs, and repression of Rb inhibition of cell cycle progression via phosphorylation at ser795 occurs at the earliest stages of embryonic development. In addition, the presence of max, mycC phospho, DP-1, and Rb phospho in the nuclei of embryonic and placental lineage cells in the blastocyst and in trophoblast stem cells suggests that a similar type of cell cycle regulation is present throughout preimplantation development and in both embryonic and extra-embryonic cell lineages.

Docking-dependent regulation of the Rb tumor suppressor protein by Cdk4

Phosphorylation of target proteins by cyclin D1-Cdk4 requires both substrate docking and kinase activity. In addition to the ability of cyclin D1-Cdk4 to catalyze the phosphorylation of consensus sites within the primary amino acid sequence of a substrate, maximum catalytic activity requires the enzyme complex to anchor at a site remote from the phospho-acceptor site. A novel Cdk4 docking motif has been defined within a stretch of 19 amino acids from the C-terminal domain of the Rb protein that are essential for Cdk4 binding. Mutation or deletion of the docking motif prevents Cdk4-dependent phosphorylation of full-length Rb protein or C-terminal Rb fragments in vitro and in cells, while a peptide encompassing the Cdk4 docking motif specifically inhibits Cdk4-dependent phosphorylation of Rb. Cyclin D1-Cdk4 can overcome the growth-suppressive activity of Rb in both cell cycle progression and colony formation assays; however, while mutants of Rb in which the Cdk4 docking site has been either deleted or mutated retain growth suppressor activity, they are resistant to inactivation by cyclin D1-Cdk4. Finally, binding of Cdk4 to its docking site can inhibit cleavage of exogenous and endogenous Rb in response to distinct apoptotic signals. The Cdk4 docking motif in Rb gives insight into the mechanism by which enzyme specificity is ensured and highlights a role for Cdk4 docking in maintaining the Rb protein in a form that favors cell survival rather than apoptosis.

Mitogen-induced Rapid Phosphorylation of Serine 795 of the Retinoblastoma Gene Product in Vascular Smooth Muscle Cells Involves ERK Activation

We examined the relationship between mitogen-activated MEK (mitogen and extracellular signal-regulated protein kinase kinase) and phosphorylation of the gene product encoded by retinoblastoma (hereafter referred to as Rb) in vascular smooth muscle cells. Brief treatment of the cells with 100 nm angiotensin II or 1 microm serotonin resulted in serine phosphorylation of Rb that was equal in magnitude to that induced by treating cells for 20 h with 10% fetal bovine serum ( approximately 3 x basal). There was no detectable rapid phosphorylation of two close cousins of Rb, p107 and p130. Phosphorylation state-specific antisera demonstrated that the rapid phosphorylation occurred on Ser(795), but not on Ser(249), Thr(252), Thr(373), Ser(780), Ser(807), or Ser(811). Phosphorylation of Rb Ser(795) peaked at 10 min, lagging behind phosphorylation of MEK and ERK (extracellular signal-regulated protein kinase). Rb Ser(795) phosphorylation could be blocked by PD98059, a MEK inhibitor, and greatly attenuated by apigenin, an inhibitor of the Ras --> Raf --> MEK --> ERK pathway. The effect also appears to be mediated by CDK4. Immunoprecipitation/immunoblot studies revealed that serotonin and angiotensin II induced complex formation between CDK4, cyclin D1, and phosphorylated ERK. These studies show a rapid, novel, and selective phosphorylation of Rb Ser(795) by mitogens and demonstrate an unexpected rapid linkage between the actions of the Ras --> Raf --> MEK --> ERK pathway and the phosphorylation state of Rb.

Cyclin C/cdk3 promotes Rb-dependent G0 exit

G0 is a physiological state occupied by resting or terminally differentiated cells that have exited the cell cycle. In contrast to the well-characterized cyclin/cdk-mediated inactivation of pRb that controls the G1/S transition, little is known about regulation of the G0/G1 transition. However, pRb is likely to participate in this process because its acute somatic inactivation is sufficient for G0-arrested cells to re-enter the cell cycle. One physiological regulator of this event may be cyclin C because its highest mRNA levels occur during G0 exit. Here we show that a non-cdk8-associated cellular pool of cyclin C combines with cdk3 to stimulate pRb phosphorylation at S807/811 during the G0/G1 transition, and that this phosphorylation is required for cells to exit G0 efficiently. Thus, G1 entry is regulated in an analogous fashion to S phase entry, but involves a distinct cyclin/cdk combination.

Cell cycle arrest by monochloramine through the oxidation of retinoblastoma protein

Impairment of cell cycle control has serious effects on inflammation, tissue repair, and carcinogenesis. We report here the G1 cell cycle arrest by monochloramine (NH2Cl), a physiological oxidant derived from activated neutrophils, and its mechanism. When Jurkat cells were treated with NH2Cl (70 microM, 10 min) and incubated for 24 h, the S phase population decreased significantly with a slight increase in the hypodiploid cell population. The G0/ G1 phase and G2/M phase populations did not show marked changes. Three hours after NH2Cl treatment, the retinoblastoma protein (pRB) was dephosphorylated especially at Ser780 and Ser795, both of which are important phosphorylation sites for the G1 checkpoint function. The phosphorylation at Ser807/811 showed no apparent change. The expression of cyclins, cyclin-dependent kinases, and cyclin-dependent kinase inhibitors showed no apparent change. Moreover, the kinase activity that phosphorylates pRB remained constant even after NH2Cl treatment. The protein phosphatase activity that dephosphorylates pRB showed a marginal increase. Notably, when the recombinant pRB was oxidized by NH2Cl in vitro, the oxidized pRB became difficult to be phosphorylated by kinases, especially at Ser780 and Ser795, but not at Ser807/811. Amino acid analysis of oxidized pRB showed methionine oxidation to methionine sulfoxide. The NH2Cl-treated Jurkat cell proteins also showed a decrease in methionine. These observations suggested that direct pRB oxidation was the major cause of NH2Cl-induced cell cycle arrest. In the presence of 2 mM NH4+, NaOCl (200 microM) or activated neutrophils also induced a G1 cell cycle arrest. As protein methionine oxidation has been reported in inflammation and aging, cell cycle modulation by pRB oxidation may occur in various pathological conditions.

Physical interaction between pRb and cdk9/cyclinT2 complex

Cyclin-dependent kinase 9 (cdk9) is a multifunctional kinase with roles in different cellular pathways such as transcriptional elongation, differentiation and apoptosis. Cdk9/cyclin T differs functionally from other cdk/cyclin complexes that regulate cell cycle progression, but maintains structural affinity with those complexes. In addition, previous reports have demonstrated that the cdk9 complex is able to phosphorylate p56/pRb in vitro. In this report we show in vitro and in vivo interaction between cdk9/cyclinT2 and the protein product of the retinoblastoma gene (pRb) in human cell lines. The interaction involves the region composed of residues 129-195 of cdk9, cyclinT2 (1-642 aa) and the C-terminal domain of pRb (835-928 aa). We located the minimal region of cdk9 phosphorylation on the C-terminus of pRb, by identifying the residues between 793 and 834. This region contains at least three proline-directed serines (sp), S795, S807 and S811, which have been reported to be phosphorylated in vivo and which could be targeted by the cdk9 complex. These data suggest that, in logarithmically growing cells, cdk9/cyclin T2 and pRb are located in a nuclear multiprotein complex probably involved in transduction of cellular signals to the basal transcription machinery and that one of these signals could be the cdk9 phosphorylation of pRb.

Mutant mouse models reveal the relative roles of E2F1 and E2F3 in vivo

The E2F1, -2, and -3 transcription factors are key downstream targets of the retinoblastoma protein (pRB) tumor suppressor that drive expression of proliferation-associated genes. Here we use mutant mouse strains to investigate E2F3's role in vivo. We show that E2F3 is essential for embryonic viability in the pure 129/Sv background but the presence of C57BL/6 alleles yields some adult survivors. Although growth retarded, surviving E2f3(-/-) animals are initially healthy. However, they die prematurely, exhibiting no obvious tumor phenotype but with the typical signs of congestive heart failure. The defects are completely distinct from those arising in E2f1 mutant mice (S. J. Field et al., Cell 85:549-561; 1996; L. Yamasaki et al., Cell 85:537-548, 1996), supporting the prevailing view that these E2Fs must have some unique biological functions in vivo. To test this model, we examined the phenotypes of E2f1 E2f3 compound mutant mice. Almost all of the developmental and age-related defects arising in the individual E2f1 or E2f3 mice were exacerbated by the mutation of the other E2f. Thus, E2F1 and E2F3 appear to play critical, overlapping roles in the development and maintenance of a variety of tissues. Importantly, this study did identify one major difference in the properties of E2F1 and E2F3: either alone or in combination with E2F1 loss, E2f3 mutation did not increase the incidence of tumor formation. These data strongly suggest that tumor suppression is a specific property of E2F1 and not E2F3.

Regulation of the retinoblastoma tumor suppressor protein by cyclin/cdks

The retinoblastoma tumor suppressor protein (pRB) is a paradigm for understanding cell cycle- and proliferation-dependent transcription and how deregulation of this process contributes to the neoplastic process in humans. The ability of pRB to regulate transcription, and consequently cell proliferation and differentiation, is regulated by the activity of cyclin/cdks. In general, phosphorylation of pRB by cyclin/cdks inactivates pRB-mediated transcriptional inhibition and growth suppression. However, it is apparent that pRB is a multi-functional protein that can inhibit transcription through various mechanisms. This review focuses on recent data to suggest that different pRB functions are progressively and cooperatively inactivated by multiple cyclin/cdk complexes during G1- and S-phase. The implications of such a model for pRB-mediated tumor suppression are discussed.

Retinoblastoma: the disease, gene and protein provide critical leads to understand cancer

Retinoblastoma has contributed much to the understanding of cancer. The protein product of the RB gene, pRB, is a multifaceted regulator of transcription which controls the cell cycle, differentiation and apoptosis in normal development of specific tissues. Elucidating the mechanisms in which pRB plays a critical role will enable novel therapies and strategies for prevention, not only for retinoblastoma, but for cancer in general.

The 100-kDa proteolytic fragment of RB is retained predominantly within the nuclear compartment of apoptotic cells

The retinoblastoma tumor suppressor protein (RB) has been shown to play a role in regulating the eukaryotic cell cycle, promoting cellular differentiation, and modulating programmed cell death. Although regulation of RB tumor suppressor activity is mediated by reversible phosphorylation, an additional posttranslational modification involves the cleavage of 42 residues from the carboxy terminus of RB during the onset of drug-induced or receptor-mediated apoptosis. We now demonstrate that a recombinant p100cl RB species localizes to the nucleus where it may retain wildtype "pocket" protein binding activity. In addition, using immunocytochemistry, we show that cleavage of the endogenous RB protein occurs in vivo in human cells and that p100cl is predominantly retained within the nuclear compartment of cells during early apoptosis. We also show that the carboxy-terminal cleavage of RB is detected immediately following caspase-3 and PARP cleavage during FAS-mediated apoptosis of MCF10 cells. These findings suggest that this cleavage event may be a component of a downstream cascade during programmed cell death.

Analysis of site-specific phosphorylation of the retinoblastoma protein during cell cycle progression

Differential phosphorylation of the retinoblastoma protein plays a pivotal role in cell cycle regulation. The retinoblastoma protein is specifically phosphorylated during the cell cycle by cyclin-dependent kinase complexes which intersect with many cellular signaling networks. Since the loss of the retinoblastoma signaling pathways occurs in a wide variety of human tumors, understanding the significance of site-specific phosphorylation can clarify the role of selected cyclin-dependent kinase complexes during cell cycle progression. Here we describe the phosphospecificity and cellular characterization of a panel of polyclonal antibodies that recognize unique phosphorylation sites within the retinoblastoma protein. These reagents were used to validate authentic cellular retinoblastoma phosphorylation sites at amino acids 780, 795, and 807/811 correlating with the G1-S transition.

Retinoblastoma protein contains a C-terminal motif that targets it for phosphorylation by cyclin-cdk complexes

Stable association of certain proteins, such as E2F1 and p21, with cyclin-cdk2 complexes is dependent upon a conserved cyclin-cdk2 binding motif that contains the core sequence ZRXL, where Z and X are usually basic. In vitro phosphorylation of the retinoblastoma tumor suppressor protein, pRB, by cyclin A-cdk2 and cyclin E-cdk2 was inhibited by a short peptide spanning the cyclin-cdk2 binding motif present in E2F1. Examination of the pRB C terminus revealed that it contained sequence elements related to ZRXL. Site-directed mutagenesis of one of these sequences, beginning at residue 870, impaired the phosphorylation of pRB in vitro. A synthetic peptide spanning this sequence also inhibited the phosphorylation of pRB in vitro. pRB C-terminal truncation mutants lacking this sequence were hypophosphorylated in vitro and in vivo despite the presence of intact cyclin-cdk phosphoacceptor sites. Phosphorylation of such mutants was restored by fusion to the ZRXL-like motif derived from pRB or to the ZRXL motifs from E2F1 or p21. Phospho-site-specific antibodies revealed that certain phosphoacceptor sites strictly required a C-terminal ZRXL motif whereas at least one site did not. Furthermore, this residual phosphorylation was sufficient to inactivate pRB in vivo, implying that there are additional mechanisms for directing cyclin-cdk complexes to pRB. Thus, the C terminus of pRB contains a cyclin-cdk interaction motif of the type found in E2F1 and p21 that enables it to be recognized and phosphorylated by cyclin-cdk complexes.