Identification of a substrate-targeting domain in cyclin E necessary for phosphorylation of the retinoblastoma protein

Tolerance of Oncogene-Induced Replication Stress: A Fuel for Genomic Instability

Activation of oncogenes disturbs a wide variety of cellular processes and induces physiological dysregulation of DNA replication, widely referred to as replication stress (RS). Oncogene-induced RS can cause replication forks to stall or collapse, thereby leading to DNA damage. While the DNA damage response (DDR) can provoke an anti-tumor barrier to prevent the development of cancer, a small subset of cells triggers replication stress tolerance (RST), allowing precancerous cells to survive, thereby promoting clonal expansion and genomic instability (GIN). Genomic instability (GIN) is a hallmark of cancer, driving genetic alterations ranging from nucleotide changes to aneuploidy. These alterations increase the probability of oncogenic events and create a heterogeneous cell population with an enhanced ability to evolve. This review explores how major oncogenes such as RAS, cyclin E, and MYC induce RS through diverse mechanisms. Additionally, we delve into the strategies employed by normal and cancer cells to tolerate RS and promote GIN. Understanding the intricate relationship between oncogene activation, RS, and GIN is crucial to better understand how cancer cells emerge and to develop potential cancer therapies that target these vulnerabilities.

Cyclin E/CDK2: DNA Replication, Replication Stress and Genomic Instability

DNA replication must be precisely controlled in order to maintain genome stability. Transition through cell cycle phases is regulated by a family of Cyclin-Dependent Kinases (CDKs) in association with respective cyclin regulatory subunits. In normal cell cycles, E-type cyclins (Cyclin E1 and Cyclin E2, CCNE1 and CCNE2 genes) associate with CDK2 to promote G1/S transition. Cyclin E/CDK2 complex mostly controls cell cycle progression and DNA replication through phosphorylation of specific substrates. Oncogenic activation of Cyclin E/CDK2 complex impairs normal DNA replication, causing replication stress and DNA damage. As a consequence, Cyclin E/CDK2-induced replication stress leads to genomic instability and contributes to human carcinogenesis. In this review, we focus on the main functions of Cyclin E/CDK2 complex in normal DNA replication and the molecular mechanisms by which oncogenic activation of Cyclin E/CDK2 causes replication stress and genomic instability in human cancer.

A novel function for CDK2 activity at meiotic crossover sites

Genetic diversity in offspring is induced by meiotic recombination, which is initiated between homologs at >200 sites originating from meiotic double-strand breaks (DSBs). Of this initial pool, only 1-2 DSBs per homolog pair will be designated to form meiotic crossovers (COs), where reciprocal genetic exchange occurs between parental chromosomes. Cyclin-dependent kinase 2 (CDK2) is known to localize to so-called "late recombination nodules" (LRNs) marking incipient CO sites. However, the role of CDK2 kinase activity in the process of CO formation remains uncertain. Here, we describe the phenotype of 2 Cdk2 point mutants with elevated or decreased activity, respectively. Elevated CDK2 activity was associated with increased numbers of LRN-associated proteins, including CDK2 itself and the MutL homolog 1 (MLH1) component of the MutLγ complex, but did not lead to increased numbers of COs. In contrast, reduced CDK2 activity leads to the complete absence of CO formation during meiotic prophase I. Our data suggest an important role for CDK2 in regulating MLH1 focus numbers and that the activity of this kinase is a key regulatory factor in the formation of meiotic COs.

Cul3 regulates cyclin E1 protein abundance via a degron located within the N-terminal region of cyclin E

Cyclin E and its binding partner Cdk2 control the G1/S transition in mammalian cells. Increased levels of cyclin E are found in some cancers. Additionally, proteolytic removal of the cyclin E N-terminus occurs in some cancers and is associated with increased cyclin E-Cdk2 activity and poor clinical prognosis. Cyclin E levels are tightly regulated and controlled in part through ubiquitin-mediated degradation initiated by one of two E3 ligases, Cul1 and Cul3. Cul1 ubiquitylates phosphorylated cyclin E, but the mechanism through which Cul3 ubiquitylates cyclin E is poorly understood. In experiments to ascertain how Cul3 mediates cyclin E destruction, we identified a degron on cyclin E that Cul3 targets for ubiquitylation. Recognition of the degron and binding of Cul3 does not require a BTB domain-containing adaptor protein. Additionally, this degron is lacking in N-terminally truncated cyclin E. Our results describe a mechanism whereby N-terminally truncated cyclin E can avoid the Cul3-mediated degradation pathway. This mechanism helps to explain the increased activity that is associated with the truncated cyclin E variants that occurs in some cancers.

Novel Molecular Markers for Breast Cancer

The use of molecular biomarkers assures that breast cancer (BC) patients receive optimal treatment. Established biomarkers, such as estrogen receptor, progesterone receptor, HER2, and Ki67, have been playing significant roles in the subcategorization of BC to predict the prognosis and decide the specific therapy to each patient. Antihormonal therapy using 4-hydroxytamoxifen or aromatase inhibitors have been employed in patients whose tumor cells express hormone receptors, while monoclonal antibody to HER2 has been administered to HER2-positive BCs. Although new therapeutic agents have been developed in the past few decades, many patients still die of the disease due to relapse; thus, novel molecular markers that predict therapeutic failure and those that can be targets for specific therapy are expected. We have chosen four of such molecules by reviewing recent publications, which are cyclin E, B-Myb, Twist, and DMP1β. The oncogenicity of these molecules has been demonstrated in vivo and/or in vitro through studies using transgenic mice or siRNAs, and their expressions have been shown to be associated with shortened overall or disease-free survival of BC patients. The former three molecules have been shown to accelerate epithelial-mesenchymal transition that is often associated with cancer stem cell-ness and metastasis; all these four can be novel therapeutic targets as well. Thus, large prospective studies employing immunohistochemistry will be needed to establish the predictive values of these molecules in patients with BC.

RNA interference of Biot2 induces G1 phase arrest and apoptosis in mouse colorectal cancer cell line

Biot2 is a tumor-associated antigen, and it is a novel gene (GenBank EF100607) that was first identified with the SEREX technique and named by our laboratory. It is highly expressed in cancer cells and testis, with low or no expression in normal tissues. In our previous study, RNA interference of human Biot2 can inhibit tumor cell growth, and it is associated with poor prognosis of patients in clinical study; however, the mechanism of Biot2 that effects tumor growth is not yet clear. Here, in this study, we explore further the mechanism of Biot2 by silencing Biot2 in CT26 cells. It provides some theoretical basis for Biot2 as a new target for gene therapy. In CT26 cells, the expression of Biot2 was downregulated by Biot2-shRNA. It also promoted G₁ phase arrest, the expression of p16 and p21, and cell apoptosis. In the mouse model, the tumor volume and the expression of PCNA of the Biot2-shRNA group significantly decreased. These results suggest that silencing Biot2 in CT26 cells by RNA interference can inhibit cell growth in vitro and in vivo. It also induces cell cycle arrest in the G₁ phase and apoptosis throughout regulation of p16 and p21. Taken together, our data demonstrate that Biot2 can be a potential target of gene therapy.

In the wrong place at the wrong time: does cyclin mislocalization drive oncogenic transformation?

Cyclin-dependent kinases (CDKs) are regulated by both cyclin abundance and cyclin localization. Increased cyclin expression in cancer was first observed two decades ago, and its role in pathogenesis has been investigated in great depth. This Opinion article focuses on the spatial deregulation of cyclin expression and its potential link to oncogenesis. It describes the contexts in which particular cyclins have been reported to be mislocalized in neoplasia, reviews the mechanisms underlying the dynamic subcellular localization of CDK-cyclin complexes in normal cells, and discusses how these controls can be disrupted in cancer. It also outlines the mechanisms by which cyclin mislocalization might disrupt cell cycle control and interfere with faithful chromosome segregation. Finally, it discusses the extent to which cyclin mislocalization might facilitate tumorigenesis in human cancer.

Cyclin-specific docking motifs promote phosphorylation of yeast signaling proteins by G1/S Cdk complexes

BACKGROUND

The eukaryotic cell cycle begins with a burst of cyclin-dependent kinase (Cdk) phosphorylation. In budding yeast, several Cdk substrates are preferentially phosphorylated at the G1/S transition rather than later in the cell cycle when Cdk activity levels are high. These early Cdk substrates include signaling proteins in the pheromone response pathway. Two such proteins, Ste5 and Ste20, are phosphorylated only when Cdk is associated with the G1/S cyclins Cln1 and Cln2 and not G1, S, or M cyclins. The basis of this cyclin specificity is unknown.

RESULTS

Here we show that Ste5 and Ste20 have recognition sequences, or "docking" sites, for the G1/S cyclins. These docking sites, which are distinct from Clb5/cyclin A-binding "RXL" motifs, bind preferentially to Cln2. They strongly enhance Cln2-driven phosphorylation of each substrate in vivo and function largely independent of position and distance to the Cdk sites. We exploited this functional independence to rewire a Cdk regulatory circuit in a way that changes the target of Cdk inhibition in the pheromone response pathway. Furthermore, we uncover functionally active Cln2 docking motifs in several other Cdk substrates. The docking motifs drive cyclin-specific phosphorylation, and the cyclin preference can be switched by using a distinct motif.

CONCLUSIONS

Our findings indicate that some Cdk substrates are intrinsically capable of being phosphorylated by several different cyclin-Cdk forms, but they are inefficiently phosphorylated in vivo without a cyclin-specific docking site. Docking interactions may play a prevalent but previously unappreciated role in driving phosphorylation of select Cdk substrates preferentially at the G1/S transition.

Developmental downregulation of Xenopus cyclin E is phosphorylation and nuclear import dependent and is mediated by ubiquitination

Cyclins are regulatory subunits that bind to and activate catalytic Cdks. Cyclin E associates with Cdk2 to mediate the G1/S transition of the cell cycle. Cyclin E is overexpressed in breast, lung, skin, gastrointestinal, cervical, and ovarian cancers. Its overexpression correlates with poor patient prognosis and is involved in the etiology of breast cancer. We have been studying how cyclin E is normally downregulated during development in order to determine if disruption of similar mechanisms could either contribute to its overexpression in cancer, or be exploited to decrease its expression. In Xenopus laevis embryos, cyclin E protein level is high and constant until its abrupt destabilization by an undefined mechanism after the 12th cell cycle, which corresponds to the midblastula transition (MBT) and remodeling of the embryonic to the adult cell cycle. Since degradation of mammalian cyclin E is regulated by the ubiquitin proteasome system and is phosphorylation dependent, we examined the role of phosphorylation in Xenopus cyclin E turnover. We show that similarly to human cyclin E, phosphorylation of serine 398 and threonine 394 plays a role in cyclin E turnover at the MBT. Immunofluorescence analysis shows that cyclin E relocalizes from the cytoplasm to the nucleus preceding its degradation. When nuclear import is inhibited, cyclin E stability is markedly increased after the MBT. To investigate whether degradation of Xenopus cyclin E is mediated by the proteasomal pathway, we used proteasome inhibitors and observed a progressive accumulation of cyclin E in the cytoplasm after the MBT. Ubiquitination of cyclin E precedes its proteasomal degradation at the MBT. These results show that cyclin E destruction at the MBT requires both phosphorylation and nuclear import, as well as proteasomal activity.

Cyclin A promotes S-phase entry via interaction with the replication licensing factor Mcm7

Cyclin A is known to promote S-phase entry in mammals, but its critical targets in this process have not been defined. We derived a novel human cyclin A mutant (CycA-C1), which can activate cyclin-dependent kinase but cannot promote S-phase entry, and isolated replication licensing factor Mcm7 as a factor that interacts with the wild-type cyclin A but not with the mutant. We demonstrated that human cyclin A and Mcm7 interact in the chromatin fraction. To address the physiological significance of the cyclin A-Mcm7 interaction, we isolated an Mcm7 mutant (Mcm7-3) that is capable of association with CycA-C1 and found that it can also suppress the deficiency of CycA-C1 in promoting S-phase entry. Finally, RNA interference experiments showed that the CycA-C1 mutant is defective for the endogenous cyclin A function in S-phase entry and that this defect can be suppressed by the Mcm7-3 mutant. Our findings demonstrate that interaction with Mcm7 is essential for the function of cyclin A in promoting S-phase entry.

Regulator of G protein signaling 6 (RGS6) induces apoptosis via a mitochondrial-dependent pathway not involving its GTPase-activating protein activity

Regulator of G protein signaling 6 (RGS6) is a member of a family of proteins called RGS proteins, which function as GTPase-activating proteins (GAPs) for Gα subunits. Given the role of RGS6 as a G protein GAP, the link between G protein activation and cancer, and a reduction of cancer risk in humans expressing a RGS6 SNP leading to its increased translation, we hypothesized that RGS6 might function to inhibit growth of cancer cells. Here, we show a marked down-regulation of RGS6 in human mammary ductal epithelial cells that correlates with the progression of their transformation. RGS6 exhibited impressive antiproliferative actions in breast cancer cells, including inhibition of cell growth and colony formation and induction of cell cycle arrest and apoptosis by mechanisms independent of p53. RGS6 activated the intrinsic pathway of apoptosis involving regulation of Bax/Bcl-2, mitochondrial outer membrane permeabilization (MOMP), cytochrome c release, activation of caspases-3 and -9, and poly(ADP-ribose) polymerase cleavage. RGS6 promoted loss of mitochondrial membrane potential (ΔΨ(m)) and increases in reactive oxygen species (ROS). RGS6-induced caspase activation and loss of ΔΨ(m) was mediated by ROS, suggesting an amplification loop in which ROS provided a feed forward signal to induce MOMP, caspase activation, and cell death. Loss of RGS6 in mouse embryonic fibroblasts dramatically impaired doxorubicin-induced growth suppression and apoptosis. Surprisingly, RGS6-induced apoptosis in both breast cancer cells and mouse embryonic fibroblasts does not require its GAP activity toward G proteins. This work demonstrates a novel signaling action of RGS6 in cell death pathways and identifies it as a possible therapeutic target for treatment of breast cancer.

A role for the cell cycle phosphatase Cdc25a in vitamin D-dependent inhibition of adult rat vascular smooth muscle cell proliferation

We have explored the mechanism(s) underlying 1,25 dihydroxyvitamin D's (1,25(OH)(2)D) suppression of agonist-induced vascular smooth muscle cell (VSMC) proliferation. Quiescent cultured adult rat VSMC were treated with 1,25(OH)(2)D for 48h and endothelin (ET) or angiotensin II (AII) for the final 24h. We show that VSMC responded to 1,25(OH)(2)D or its less hypercalcemic analogue RO 25-6760 with ∼70% inhibition of ET-dependent (3)H-thymidine incorporation. The inhibition was linked to a comparable reduction in ET-stimulated cyclin-dependent kinase 2 (Cdk2) activity and suppression of an ET-induced Cdk2 activator, cell division cycle 25 homolog A (Cdc25A). Both 1,25(OH)(2)D and RO 25-6760 completely inhibited the ET-dependent increase in Cdc25A mRNA and protein levels, phosphatase and promoter activities. 1,25(OH)(2)D also suppressed AII-induced DNA synthesis, Cdk2 activity and Cdc25A gene transcription. Inhibition of Cdc25A gene expression using a siRNA approach resulted in significant inhibition of ET or AII-dependent Cdk2 activity and (3)H-thymidine incorporation. The Cdc25A siRNA-mediated inhibition of ET or AII-induced Cdk2 activity and DNA synthesis was not additive with that produced by 1,25(OH)(2)D treatment. These data demonstrate that 1,25(OH)(2)D inhibits VSMC proliferation through a Cdc25A-dependent mechanism and suggest that this hormone may prove useful in the management of disorders characterized by aberrant proliferation of VSMC in the vascular wall.

Altered subcellular localization of tumor-specific cyclin E isoforms affects cyclin-dependent kinase 2 complex formation and proteasomal regulation

In tumors, alternative translation and posttranslational proteolytic cleavage of full-length cyclin E (EL) produces tumorigenic low molecular weight cyclin E (LMW-E) isoforms that lack a portion of the EL amino-terminus containing a nuclear localization sequence. Therefore, we hypothesized that LMW-E isoforms have altered subcellular localization. To explore our hypothesis, we compared EL versus LMW-E localization in cell lysates and in vivo using fractionation and protein complementation assays. Our results reveal that LMW-E isoforms preferentially accumulate in the cytoplasm where they bind the cyclin E kinase partner, cyclin-dependent kinase 2 (Cdk2), and have associated kinase activity. The nuclear ubiquitin ligase Fbw7 targets Cdk2-bound cyclin E for degradation; thus, we examined if altered subcellular localization affected LMW-E degradation. We found that cytoplasmic LMW-E/Cdk2 was less susceptible to Fbw7-mediated degradation. One implication of our findings is that altered LMW-E and LMW-E/Cdk2 subcellular localization may lead to aberrant LMW-E protein interactions, regulation, and activity, ultimately contributing to LMW-E tumorigenicity.

Autophosphorylation-induced degradation of the Pho85 cyclin Pcl5 is essential for response to amino acid limitation

Pho85 cyclins (Pcls), activators of the yeast cyclin-dependent kinase (CDK) Pho85, belong together with the p35 activator of mammalian CDK5 to a distinct structural cyclin class. Different Pcls target Pho85 to distinct substrates. Pcl5 targets Pho85 specifically to Gcn4, a yeast transcription factor involved in the response to amino acid starvation, eventually causing the degradation of Gcn4. Pcl5 is itself highly unstable, an instability that was postulated to be important for regulation of Gcn4 degradation. We used hybrids between different Pcls to circumscribe the substrate recognition function to the core cyclin box domain of Pcl5. Furthermore, the cyclin hybrids revealed that Pcl5 degradation is uniquely dependent on two distinct degradation signals: one N-terminal and one C-terminal to the cyclin box domain. Whereas the C-terminal degradation signal is independent of Pho85, the N-terminal degradation signal requires phosphorylation of a specific threonine residue by the Pho85 molecule bound to the cyclin. This latter mode of degradation depends on the SCF ubiquitin ligase. Degradation of Pcl5 after self-catalyzed phosphorylation ensures that activity of the Pho85/Pcl5 complex is self-limiting in vivo. We demonstrate the importance of this mechanism for the regulation of Gcn4 degradation and for cell growth under conditions of amino acid starvation.

The N-terminal domain of the Drosophila retinoblastoma protein Rbf1 interacts with ORC and associates with chromatin in an E2F independent manner

Background

The retinoblastoma (Rb) tumor suppressor protein can function as a DNA replication inhibitor as well as a transcription factor. Regulation of DNA replication may occur through interaction of Rb with the origin recognition complex (ORC).

Principal Findings

We characterized the interaction of Drosophila Rb, Rbf1, with ORC. Using expression of proteins in Drosophila S2 cells, we found that an N-terminal Rbf1 fragment (amino acids 1–345) is sufficient for Rbf1 association with ORC but does not bind to dE2F1. We also found that the C-terminal half of Rbf1 (amino acids 345–845) interacts with ORC. We observed that the amino-terminal domain of Rbf1 localizes to chromatin in vivo and associates with chromosomal regions implicated in replication initiation, including colocalization with Orc2 and acetylated histone H4.

Conclusions/Significance

Our results suggest that Rbf1 can associate with ORC and chromatin through domains independent of the E2F binding site. We infer that Rbf1 may play a role in regulating replication directly through its association with ORC and/or chromatin factors other than E2F. Our data suggest an important role for retinoblastoma family proteins in cell proliferation and tumor suppression through interaction with the replication initiation machinery.

Activation of the p53-p21(Cip1) pathway is required for CDK2 activation and S-phase entry in primary rat hepatocytes

p53 plays a major role in the prevention of tumor development. It responds to a range of potentially oncogenic stresses by activating protective mechanisms, most notably cell-cycle arrest and apoptosis. The p53 gene is also induced during normal liver regeneration, and it has been hypothesized that p53 serve as a proliferative 'brake' to control excessive proliferation. However, it has lately been shown that p53 inhibition reduces hepatocyte growth factor-induced DNA synthesis of primary hepatocytes. Here we show that epidermal growth factor (EGF) activated p53 in a phosphatidylinositol-3 kinase-dependent way, and thus induced the cyclin-dependent kinase inhibitor p21(Cip1) in primary rat hepatocytes. p53 inactivation with a dominant-negative mutant (p53(V143A)) attenuated EGF-induced DNA synthesis and was associated with reduced CDK2 phosphorylation and retinoblastoma protein hyperphosphorylation. When p21(Cip1) was ectopically expressed in p53-inactivated cells, these effects were neutralized. In conclusion, our results demonstrate that in normal hepatocytes, EGF-induced expression of p53 is involved in regulating CDK2- and CDK4 activity, through p21(Cip1) expression.

CDK2 regulation through PI3K and CDK4 is necessary for cell cycle progression of primary rat hepatocytes

INTRODUCTION/OBJECTIVES

Cell cycle progression is driven by the coordinated regulation of cyclin-dependent kinases (CDKs). In response to mitogenic stimuli, CDK4 and CDK2 form complexes with cyclins D and E, respectively, and translocate to the nucleus in the late G(1) phase. It is an on-going discussion whether mammalian cells need both CDK4 and CDK2 kinase activities for induction of S phase.

METHODS AND RESULTS

In this study, we have explored the role of CDK4 activity during G(1) progression of primary rat hepatocytes. We found that CDK4 activity was restricted by either inhibiting growth factor induced cyclin D1-induction with the PI3K inhibitor LY294002, or by transient transfection with a dominant negative CDK4 mutant. In both cases, we observed reduced CDK2 nuclear translocation and reduced CDK2-Thr160 phosphorylation. Furthermore, reduced pRb hyperphosphorylation and reduced cellular proliferation were observed. Ectopic expression of cyclin D1 alone was not sufficient to induce CDK4 nuclear translocation, CDK2 activity or cell proliferation.

CONCLUSIONS

Thus, epidermal growth factor-induced CDK4 activity was necessary for CDK2 activation and for hepatocyte proliferation. These results also suggest that, in addition to regulating cyclin D1 expression, PI3K is involved in regulation of nuclear shuttling of cyclin-CDK complexes in G(1) phase.

Neoplastic hepatocyte growth associated with cyclin D1 redistribution from the cytoplasm to the nucleus in mouse hepatocarcinogenesis

Cyclin D1 overexpression is a frequent change in hepatocellular carcinomas (HCCs). Our present study demonstrated that cyclin D1 overexpression with abundant cyclin E, cdk4, cdk2, and p27Kip1 (p27) occurred in neoplastic hepatocytes from the early stage of mouse hepatocarcinogenesis. While cyclin D1 expression was mainly found in the cytoplasm of the tumor cells, it shifted to the nucleus in association with cell proliferation after the animals were subjected to a partial hepatectomy (PH), and then returned once more to the cytoplasm when the cells became quiescent. Inhibition of PI3 kinase (PI3K) by Ly294002 in mouse HCC cells in vitro suppressed the nuclear shift of cyclin D1 as well as cell proliferation, while PI3K activation by PTEN suppression failed to induce nuclear shift of cyclin D1, suggesting that PI3K activation is essential but not sufficient for the cyclin D1 nuclear shift. While MEK-ERK1/2 inhibition by PD98059 and mTOR inhibition by rapamycin affected the cyclin D1 nuclear shift and cell proliferation to a lesser extent, both these inhibitors reduced cyclin D1 levels. Finally, although p27, cdk4 and calmodulin (CaM) were detected in the cyclin D1 immunoprecipitates from both quiescent and proliferating HCC cells, Hsc70 and SSeCKS were detected only in the immunoprecipitate from quiescent cells, and p21Waf1/Cip1 (p21) was detected only in that from proliferating cells, suggesting that the cyclin D1 complex is different in quiescent and proliferating cells. These observations indicate that the nuclear/cytoplasmic localization of cyclin D1 plays an important role in the proliferation/quiescence of neoplastic hepatocytes.

Cdc7-Dbf4 phosphorylates MCM proteins via a docking site-mediated mechanism to promote S phase progression

Origins of DNA replication are licensed in G1 by recruiting the minichromosome maintenance (MCM) proteins to form a prereplicative complex (pre-RC). Prior to initiation of DNA synthesis from each origin, a preinitiation complex (pre-IC) containing Cdc45 and other proteins is formed. We report that Cdc7-Dbf4 protein kinase (DDK) promotes assembly of a stable Cdc45-MCM complex exclusively on chromatin in S phase. In this complex, Mcm4 is hyperphosphorylated. Studies in vitro using purified DDK and Mcm4 demonstrate that hyperphosphorylation occurs at the Mcm4 N terminus. However, the DDK substrate specificity is conferred by an adjacent DDK-docking domain (DDD), sufficient for facilitating efficient phosphorylation of artificial phosphoacceptors in cis. Genetic evidence suggests that phosphorylation of Mcm4 by DDK is important for timely S phase progression and for cell viability upon overproduction of Cdc45. We suggest that DDK docks on and phosphorylates MCM proteins at licensed origins to promote proper assembly of pre-IC.

Cyclin E in normal and neoplastic cell cycles

Cyclin E-Cdk2 has long been considered an essential and master regulator of progression through G1 phase of the cell cycle. Although recent mouse models have prompted a rethinking of cyclin E function in mammals, it remains clear that cyclin E impacts upon many processes central to cell division. Normal cells maintain strict control of cyclin E activity, and this is commonly disrupted in cancer cells. Moreover, cyclin E deregulation is thought to play a fundamental role in tumorigenesis. In this review, we discuss the regulation and functions of cyclin E in normal and neoplastic mammalian cells.

Cyclin E

E-type cyclins (cyclin E1 and cyclin E2) are expressed during the late G1 phase of the cell cycle until the end of the S-phase. The activity of cyclin E is limiting for the passage of cells through the restriction point "R" which marks a "point of no return" for cells entering the division cycle from a resting state or passing from G1 into S-phase. Expression of cyclin E is regulated on the level of gene transcription mainly by members of the E2F trrnscription factor family and by its degradation via the proteasome pathway. Cyclin E binds and activates the kinase Cdk2 and by phosphorylating its substrates, the so-called "pocket proteins", the cyclic/Cdk2 complexes initiate a cascade of events that leads to the expression of S-phase specific genes. Aside from this specific function as a regulator of S-phase-entry, cyclin E plays a direct role in the initiation of DNA replication, the control of genomic stability, and the centrosome cycle. Surprisingly, recent studies have shown that the once thought essential cyclin E is dispensable for the development of higher eukaryotes and for the mitotic division of eukaryotic cells. Nevertheless, high level cyclin E expression has been associated with the initiation or progression of different human cancers, in particular breast cancer but also leukemia, lymphoma and others. Transgenic mouse models in which cyclin E is constitutively expressed develop malignant diseases, supporting the notion of cyclin E as a dominant onco-protein.

Phosphorylation of cyclin D1 at Thr 286 during S phase leads to its proteasomal degradation and allows efficient DNA synthesis

Continuing proliferation requires regulation of cyclin D1 levels in each cell cycle phase. Growth factors stimulate high levels during G2 phase, which commits the cell to continue through G1 phase with sufficient cyclin D1 to initiate DNA synthesis. Upon entry into S phase, however, cyclin D1 levels rapidly decline. Our goal is to understand the mechanism and importance of this S-phase suppression. Here, we demonstrate that cyclin D1 levels decline during S phase due to reduced protein stability, without alterations in the rate of protein synthesis. This decline depends upon Thr 286, since mutation of this site eliminates the normal pattern of cyclin D1 suppression during S phase. As evidence that phosphorylation of Thr 286 is responsible for this decline, Thr 286 is shown to be more efficiently phosphorylated during S phase than in other cell cycle periods. Finally, high cyclin D1 levels during S phase are shown to inhibit DNA synthesis. This inhibitory activity presumably blocks the growth of cells with altered cyclin D1 expression characteristics. Abnormal stimulation of cyclin D1 might result in levels high enough to promote G1/S phase transition even in the absence of appropriate growth stimuli. In such cells, however, the levels of cyclin D1 would presumably be too high to be suppressed during S phase, resulting in the inhibition of DNA synthesis.

The structure of cyclin E1/CDK2: implications for CDK2 activation and CDK2-independent roles

Cyclin E, an activator of phospho-CDK2 (pCDK2), is important for cell cycle progression in metazoans and is frequently overexpressed in cancer cells. It is essential for entry to the cell cycle from G0 quiescent phase, for the assembly of prereplication complexes and for endoreduplication in megakaryotes and giant trophoblast cells. We report the crystal structure of pCDK2 in complex with a truncated cyclin E1 (residues 81-363) at 2.25 A resolution. The N-terminal cyclin box fold of cyclin E1 is similar to that of cyclin A and promotes identical changes in pCDK2 that lead to kinase activation. The C-terminal cyclin box fold shows significant differences from cyclin A. It makes additional interactions with pCDK2, especially in the region of the activation segment, and contributes to CDK2-independent binding sites of cyclin E. Kinetic analysis with model peptide substrates show a 1.6-fold increase in kcat for pCDK2/cyclin E1 (81-363) over kcat of pCDK2/cyclin E (full length) and pCDK2/cyclin A. The structural and kinetic results indicate no inherent substrate discrimination between pCDK2/cyclin E and pCDK2/cyclin A with model substrates.

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.

CDK2 and cyclin E knockout mice: lessons from breast cancer

The recent discovery that murine embryos can develop normally in spite of ablation of either CDK2 or cyclin E challenges the previously held dogma that cyclin E-CDK2 activity is strictly required for the cell-division cycle. However, genetic, cellular, biochemical and clinical evidence correlate aberrant cyclin E expression with tumorigenesis and poor patient prognosis, particularly in breast cancer. Thus cyclin E is a crucial regulator of estrogen-mediated growth signaling in breast tissue and, in spite of the apparent dispensability of cyclin E-CDK2 in development, the relationship between cyclin E and the development of breast cancer is convincing.

Phosphorylations of cyclin-dependent kinase 2 revisited using two-dimensional gel electrophoresis

To control the G1/S transition and the progression through the S phase, the activation of the cyclin-dependent kinase (CDK) 2 involves the binding of cyclin E then cyclin A, the activating Thr-160 phosphorylation within the T-loop by CDK-activating kinase (CAK), inhibitory phosphorylations within the ATP binding region at Tyr-15 and Thr-14, dephosphorylation of these sites by cdc25A, and release from Cip/Kip family (p27kip1 and p21cip1) CDK inhibitors. To re-assess the precise relationship between the different phosphorylations of CDK2, and the influence of cyclins and CDK inhibitors upon them, we introduce here the use of the high resolution power of two-dimensional gel electrophoresis, combined to Tyr-15- or Thr-160-phosphospecific antibodies. The relative proportions of the potentially active forms of CDK2 (phosphorylated at Thr-160 but not Tyr-15) and inactive forms (non-phosphorylated, phosphorylated only at Tyr-15, or at both Tyr-15 and Thr-160), and their respective association with cyclin E, cyclin A, p21, and p27, were demonstrated during the mitogenic stimulation of normal human fibroblasts. Novel observations modify the current model of the sequential CDK2 activation process: (i) Tyr-15 phosphorylation induced by serum was not restricted to cyclin-bound CDK2; (ii) Thr-160 phosphorylation engaged the entirety of Tyr-15-phosphorylated CDK2 associated not only with a cyclin but also with p27 and p21, suggesting that Cip/Kip proteins do not prevent CDK2 activity by impairing its phosphorylation by CAK; (iii) the potentially active CDK2 phosphorylated at Thr-160 but not Tyr-15 represented a tiny fraction of total CDK2 and a minor fraction of cyclin A-bound CDK2, underscoring the rate-limiting role of Tyr-15 dephosphorylation by cdc25A.

Cyclins E1 and E2 are required for endoreplication in placental trophoblast giant cells

In mammalian cells, cyclin E-CDK2 complexes are activated in the late G1 phase of the cell cycle and are believed to have an essential role in promoting S-phase entry. We have targeted the murine genes CCNE1 and CCNE2, encoding cyclins E1 and E2. Whereas single knockout mice were viable, double knockout embryos died around midgestation. Strikingly, however, these embryos showed no overt defects in cell proliferation. Instead, we observed developmental phenotypes consistent with placental dysfunction. Mutant placentas had an overall normal structure, but the nuclei of trophoblast giant cells, which normally undergo endoreplication and reach elevated ploidies, showed a marked reduction in DNA content. We derived trophoblast stem cells from double knockout E3.5 blastocysts. These cells retained the ability to differentiate into giant cells in vitro, but were unable to undergo multiple rounds of DNA synthesis, demonstrating that the lack of endoreplication was a cell-autonomous defect. Thus, during embryonic development, the needs for E-type cyclins can be overcome in mitotic cycles but not in endoreplicating cells.

Cyclin E ablation in the mouse

E type cyclins (E1 and E2) are believed to drive cell entry into the S phase. It is widely assumed that the two E type cyclins are critically required for proliferation of all cell types. Here, we demonstrate that E type cyclins are largely dispensable for mouse development. However, endoreplication of trophoblast giant cells and megakaryocytes is severely impaired in the absence of cyclin E. Cyclin E-deficient cells proliferate actively under conditions of continuous cell cycling but are unable to reenter the cell cycle from the quiescent G(0) state. Molecular analyses revealed that cells lacking cyclin E fail to normally incorporate MCM proteins into DNA replication origins during G(0)-->S progression. We also found that cyclin E-deficient cells are relatively resistant to oncogenic transformation. These findings define a molecular function for E type cyclins in cell cycle reentry and reveal a differential requirement for cyclin E in normal versus oncogenic proliferation.

Cyclic AMP-dependent phosphorylation of cyclin D3-bound CDK4 determines the passage through the cell cycle restriction point in thyroid epithelial cells

According to current concepts, the cell cycle commitment after restriction (R) point passage requires the sustained stimulation by mitogens of the synthesis of labile d-type cyclins, which associate with cyclin-dependent kinase (CDK) 4/6 to phosphorylate pRb family proteins and sequester the CDK inhibitor p27kip1. In primary cultures of dog thyroid epithelial cells, the cAMP-dependent cell cycle induced by a sustained stimulation by thyrotropin or forskolin differs from growth factor mitogenic pathways, as cAMP does not upregulate d-type cyclins but increases p27 levels. Instead, cAMP induces the assembly of required cyclin D3-CDK4 complexes, which associate with nuclear p27. In this study, the arrest of forskolin stimulation rapidly slowed down the entry of dog thyrocytes into S phase and the phosphorylation of pRb family proteins. The pRb kinase activity, but not the formation, of the cyclin D3-CDK4-p27 complex was strongly reduced. Using two-dimensional gel electrophoresis, a phosphorylated form of CDK4 was separated. It appeared in response to forskolin and was bound to both cyclin D3 and p27, presumably reflecting the activating Thr-172 phosphorylation of CDK4. Upon forskolin withdrawal or after cycloheximide addition, this CDK4 phosphoform unexpectedly persisted in p27 complexes devoid of cyclin D3 but it disappeared from the more labile cyclin D3 complexes. These data demonstrate that the assembly of the cyclin D3-CDK4-p27 holoenzyme and the subsequent phosphorylation and activation of CDK4 depend on distinct cAMP actions. This provides a first example of a crucial regulation of CDK4 phosphorylation by a mitogenic cascade and a novel mechanism of cell cycle control at the R point.

The RRASK motif in Xenopus cyclin B2 is required for the substrate recognition of Cdc25C by the cyclin B-Cdc2 complex

The FLRRXSK sequence is conserved in the second cyclin box fold of B-type cyclins. We show that this conserved sequence in Xenopus cyclin B2, termed the RRASK motif, is required for the substrate recognition by the cyclin B-Cdc2 complex of Cdc25C. Mutations to charged residues of the RRASK motif of cyclin B2 abolished its ability to activate Cdc2 kinase without affecting its capacity to bind to Cdc2. Cdc2 bound to the cyclin B2 RRASK mutant was not dephosphorylated by Cdc25C, and as a result, the complex was inactive. The cyclin B2 RRASK mutants can form a complex with the constitutively active Cdc2, but a resulting active complex did not phosphorylate a preferred substrate Cdc25C in vitro, although it can phosphorylate the non-specific substrate histone H1. The RRASK mutations prevented the interaction of Cdc25C with the cyclin B2-Cdc2 complex. Consistently, the RRASK mutants neither induced germinal vesicle breakdown in Xenopus oocyte maturation nor activated in vivo Cdc2 kinase during the cell cycle in mitotic extracts. These results suggest that the RRASK motif in Xenopus cyclin B2 plays an important role in defining the substrate specificity of the cyclin B-Cdc2 complex.

Molecular defects in the pathogenesis of pituitary tumours

The majority of pituitary adenomas are trophically stable and change relatively little in size over many years. A comparatively small proportion behave more aggressively and come to clinical attention through inappropriate hormone secretion or adverse effects on surrounding structures. True malignant behaviour with metastatic spread is very atypical. Pituitary adenomas that come to surgery are predominantly monoclonal in origin and roughly half are aneuploid, indicating either ongoing genetic instability or transition through a period of genetic instability at some time during their development. Few are associated with the classical mechanisms of tumour formation but it is generally believed that the majority harbour quantitative if not qualitative differences in molecular composition compared to the normal pituitary. Despite their prevalence and the ready availability of biopsy material, at the present time, the precise molecular pathogenesis of the majority of pituitary adenomas remains unclear. This review summarizes current thinking.

Identification of the novel role of pRB in eye cancer

Cancer of the eye, though relatively rare, can involve multiple areas. Retinoblastoma is the most common primary intraocular cancer in children, with 3-7 cases per million people per year worldwide. Uveal melanoma is the most common primary intraocular cancer in adults, predominately in whites, with annual incidence of six per million people in the United States and Europe. Despite the rarity of retinoblastoma, Knudson's two-hit hypothesis to explain its genesis was substantiated by elegant genetic studies and is viewed as a turning point in cancer research. pRB plays an important role in cell cycle and apoptosis, performing its function through interaction with transcription factors, p53, and MDM2. Unfortunately, advances in eye cancer treatment have not paralleled those in treatment of other sites of cancer. In spite of higher accuracy in early diagnosis, eye-cancer-specific mortalities have remained unchanged for decades, while overall cancer mortality rates have dramatically declined. An extensive literature search revealed that, except for retinoblastoma, few investigations had been done on the pRB pathway in eye cancers even though altered pRB expression has been associated with a number of cancers. Early detection of eye cancer is critical for the prognosis of both vision and survival. Mutation analysis should become an integral part of future management of patients with eye cancer. Characterization of the mutational pattern of RB1 is crucial in identifying predisposition for cancer of many sites including the eye. Furthermore, cost-effective and efficient genetic mutation screen testing methods, which can be used to categorize mutant RB1 carriers, are needed. Illumination of genetic insights can guide clinicians to develop a rational strategy for cancer treatment and help predict prognosis in cancer patients.

Identification of the nuclear localization signal in Xenopus cyclin E and analysis of its role in replication and mitosis

Cyclin-dependent kinase (Cdk)2/cyclin E is imported into nuclei assembled in Xenopus egg extracts by a pathway that requires importin-alpha and -beta. Here, we identify a basic nuclear localization sequence (NLS) in the N-terminus of Xenopus cyclin E. Mutation of the NLS eliminated nuclear accumulation of both cyclin E and Cdk2, and such versions of cyclin E were unable to trigger DNA replication. Addition of a heterologous NLS from SV40 large T antigen restored both nuclear targeting of Cdk2/cyclin E and DNA replication. We present evidence indicating that Cdk2/cyclin E complexes must become highly concentrated within nuclei to support replication and find that cyclin A can trigger replication at much lower intranuclear concentrations. We confirmed that depletion of endogenous cyclin E increases the concentration of cyclin B necessary to promote entry into mitosis. In contrast to its inability to promote DNA replication, cyclin E lacking its NLS was able to cooperate with cyclin B in promoting mitotic entry.

The oncogenic activity of cyclin E is not confined to Cdk2 activation alone but relies on several other, distinct functions of the protein

We have previously shown that cyclin E can malignantly transform primary rat embryo fibroblasts in cooperation with constitutively active Ha-Ras. In addition, we demonstrated that high level cyclin E expression potentiates the development of methyl-nitroso-urea-induced T-cell lymphomas in mice. To further investigate the mechanism underlying cyclin E-mediated malignant transformation, we have performed a mutational analysis of cyclin E function. Here we show that cyclin E mutants defective to form an active kinase complex with Cdk2 are unable to drive cells from G(1) into S phase but can still malignantly transform rat embryo fibroblasts in cooperation with Ha-Ras. In addition, Cdk2 activation is not a prerequisite for the ability of cyclin E to rescue yeast triple cln mutations. We also find that the oncogenic properties of cyclin E did not entirely correspond with its ability to interact with the negative cell cycle regulator p27(Kip1) or the pocket protein p130. These findings suggest that the oncogenic activity of cyclin E does not exclusively rely on its ability as a positive regulator of G(1) progression. Rather, we propose that cyclin E harbors other functions, independent of Cdk2 activation and p27(Kip1) binding, that contribute significantly to its oncogenic activity.

Cyclin E2, the cycle continues

The eukaryotic cell cycle is regulated by a family of serine/threonine protein kinases known as cyclin-dependent kinases (CDKs). The activation of a CDK is dependent on its association with a cyclin regulatory subunit. The formation of distinct cyclin-CDK complexes controls the progression through the first gap phase (G(1)) and initiation of DNA synthesis (S phase). These complexes are in turn regulated by protein phosphorylation and cyclin-dependent kinase inhibitors (CKIs). Cyclin E2 has emerged as the second member of the E-type cyclin family. Cyclin E2-associated kinase activity is regulated in a cell cycle dependent manner with peak activity at the G(1) to S transition. Ectopic expression of cyclin E2 in human cells accelerates G(1), suggesting that cyclin E2 is rate limiting for G(1) progression. Although the pattern and level of cyclin E2 expression in some primary tumor and normal tissue RNAs are distinct from cyclin E1, both E-type cyclins appear to have inherent functional redundancies. This functional redundancy has facilitated the rapid characterization of cyclin E2 and uncovered unique features associated with each E-type cyclin.

CAK-independent activation of CDK6 by a viral cyclin

In normal cells, activation of cyclin-dependent kinases (cdks) requires binding to a cyclin and phosphorylation by the cdk-activating kinase (CAK). The Kaposi's sarcoma-associated herpesvirus encodes a protein with similarity to D-type cyclins. This KSHV-cyclin activates CDK6, alters its substrate specificity, and renders CDK6 insensitive to inhibition by the cdk inhibitor p16(INK4a). Here we investigate the regulation of the CDK6/KSHV-cyclin kinase with the use of purified proteins and a cell-based assay. We find that KSHV-cyclin can activate CDK6 independent of phosphorylation by CAK in vitro. In addition, CAK phosphorylation decreased the p16(INK4a) sensitivity of CDK6/KSHV-cyclin complexes. In cells, expression of CDK6 or to a lesser degree of a nonphosphorylatable CDK6(T177A) together with KSHV-cyclin induced apoptosis, indicating that CDK6 activation by KSHV-cyclin can proceed in the absence of phosphorylation by CAK in vivo. Coexpression of p16 partially protected cells from cell death. p16 and KSHV-cyclin can form a ternary complex with CDK6 that can be detected by binding assays as well as by conformational changes in CDK6. The Kaposi's sarcoma-associated herpesvirus has adopted a clever strategy to render cell cycle progression independent of mitogenic signals, cdk inhibition, or phosphorylation by CAK.

Tumor-specific proteolytic processing of cyclin E generates hyperactive lower-molecular-weight forms

Cyclin E is a G(1) cyclin essential for S-phase entry and has a profound role in oncogenesis. Previously this laboratory found that cyclin E is overexpressed and present in lower-molecular-weight (LMW) isoforms in breast cancer cells and tumor tissues compared to normal cells and tissues. Such alteration of cyclin E is linked to poor patient outcome. Here we report that the LMW forms of cyclin E are hyperactive biochemically and they can more readily induce G(1)-to-S progression in transfected normal cells than the full-length form of the protein can. Through biochemical and mutational analyses we have identified two proteolytically sensitive sites in the amino terminus of human cyclin E that are cleaved to generate the LMW isoforms found in tumor cells. Not only are the LMW forms of cyclin E functional, as they phosphorylate substrates such as histone H1 and GST-Rb, but also their activities are higher than the full-length cyclin E. These nuclear localized LMW forms of cyclin E are also biologically functional, as their overexpression in normal cells increases the ability of these cells to enter S and G(2)/M. Lastly, we show that cyclin E is selectively cleaved in vitro by the elastase class of serine proteases to generate LMW forms similar to those observed in tumor cells. These studies suggest that the defective entry into and exit from S phase by tumor cells is in part due to the proteolytic processing of cyclin E, which generates hyperactive LMW isoforms whose activities have been modified from that of the full-length protein.

Cyclin E and its associated cdk activity do not cycle during early embryogenesis of the sea Urchin

Female sea urchins store their gametes as haploid eggs. The zygote enters S-phase 1 h after fertilization, initiating a series of cell cycles that lack gap phases. We have cloned cyclin E from the sea urchin Strongylocentrotus purpuratus. Cyclin E is synthesized during oogenesis, is present in the germinal vesicle, and is released into the egg cytoplasm at oocyte maturation. Cyclin E synthesis is activated at fertilization, although there is no increase in cyclin E protein levels due to continuous turnover of the protein. Cyclin E protein levels decline in morula embryos, while cyclin E mRNA levels remain high. After the blastula stage, cyclin E mRNA and protein levels are very low, and cyclin E expression is predominant only in cells that are actively dividing. These include cells in the left coelomic pouch, which forms the adult rudiment in the embryo. The cyclin E present in the egg is complexed with a protein kinase. Activity of the cyclin E/cdk2 changes little during the initial cell cycles. In particular, cyclin E-cdk2 levels remain high during both S-phase and mitosis. Our results suggest that progression through the early embryonic cell cycles in the sea urchin does not require fluctuations in cyclin E kinase activity.

Novel properties of the cyclin encoded by Human Herpesvirus 8 that facilitate exit from quiescence

Viral DNA replication is generally dependent upon circumventing host cell cycle control to force S phase entry in an otherwise quiescent cell. Here we describe novel attributes of the cyclin encoded by Human Herpesvirus 8 (K cyclin) that enable it to subvert the quiescent state. K cyclin is most similar to the mammalian D-type cyclins in primary sequence but displays properties more akin to those of cyclin E. K cyclin (like cyclin E) can autonomously couple with its cognate cdk subunit and localize to the nucleus. D-type cyclins require mitogen stimulated accessory factors (such as p21(Cip1) and p27(Kip1)) to facilitate both of these processes. A striking difference between K cyclin and mammalian cyclins is that K cyclin binding to cdk6 can substantially activate the catalytic activity of the complex without the requirement for cyclin H/cdk7 phosphorylation of the cdk T-loop; this phosphorylation is obligatory for endogenous cyclin/cdk activity. However, K cyclin/cdk6 complexes are not totally immune from cell cycle control since CAK phosphorylation is necessary for complete activation. Thus, CAK phosphorylated K cyclin/cdk6 targets multiple sites in the retinoblastoma protein (pRb) whereas the unphosphorylated complex targets a single site. The restricted substrate specificity of the non-CAK phosphorylated K cyclin/cdk6 complex is insufficient to enable K cyclin-mediated S phase entry. Thus, the viral K cyclin is reliant upon endogenous CAK activity to subvert the quiescent state.

Cyclin specificity: how many wheels do you need on a unicycle?

Cyclin-dependent kinase (CDK) activity is essential for eukaryotic cell cycle events. Multiple cyclins activate CDKs in all eukaryotes, but it is unclear whether multiple cyclins are really required for cell cycle progression. It has been argued that cyclins may predominantly act as simple enzymatic activators of CDKs; in opposition to this idea, it has been argued that cyclins might target the activated CDK to particular substrates or inhibitors. Such targeting might occur through a combination of factors, including temporal expression, protein associations, and subcellular localization.

The N-terminal helix of Xenopus cyclins A and B contributes to binding specificity of the cyclin-CDK complex

Mitotic cyclins A and B contain a conserved N-terminal helix upstream of the cyclin box fold that contributes to a significant interface between cyclin and cyclin-dependent kinase (CDK). To address its contribution on cyclin-CDK interaction, we have constructed mutants in conserved residues of the N-terminal helix of Xenopus cyclins B2 and A1. The mutants showed altered binding affinities to Cdc2 and/or Cdk2. We also screened for mutations in the C-terminal lobe of CDK that exhibited different binding affinities for the cyclin-CDK complex. These mutations were at residues that interact with the cyclin N-terminal helix motif. The cyclin N-terminal helix mutations have a significant effect on the interaction between the cyclin-CDK complex and specific substrates, Xenopus Cdc6 and Cdc25C. These results suggest that the N-terminal helix of mitotic cyclins is required for specific interactions with CDKs and that to interact with CDK, specific substrates Cdc6 and Cdc25C require the CDK to be associated with a cyclin. The interaction between the cyclin N-terminal helix and the CDK C-terminal lobe may contribute to binding specificity of the cyclin-CDK complex.

The retinoblastoma gene: a prototypic and multifunctional tumor suppressor

Genome instability has been implicated in the generation of multiple somatic mutations that underlie cancer. Germline mutation in the retinoblastoma (RB) gene leads to tumor formation in both human and experimental animal models, and reintroduction of wild-type RB is able to suppress neoplastic phenotypes. Rb governs the passage of cells through the G1 phase-restriction point and this control is lost in most cancer cells. Rb has also been shown to promote terminal differentiation and prevent cell cycle reentry. Recent studies implicate Rb in mitotic progression, faithful chromosome segregation, checkpoint control, and chromatin remodeling, suggesting that Rb may function in the maintenance of genome integrity. It is likely that Rb suppresses tumor formation by virtue of its multiple biological activities. A single protein capable of performing multiple antioncogenic functions may be a common characteristic of other tumor suppressors including p53 and BRCA1/2.

A U-rich element in the 5' untranslated region is necessary for the translation of p27 mRNA

Increased translation of p27 mRNA correlates with withdrawal of cells from the cell cycle. This raised the possibility that antimitogenic signals might mediate their effects on p27 expression by altering complexes that formed on p27 mRNA, regulating its translation. In this report, we identify a U-rich sequence in the 5' untranslated region (5'UTR) of p27 mRNA that is necessary for efficient translation in proliferating and nonproliferating cells. We show that a number of factors bind to the 5'UTR in vitro in a manner dependent on the U-rich element, and their availability in the cytosol is controlled in a growth- and cell cycle-dependent fashion. One of these factors is HuR, a protein previously implicated in mRNA stability, transport, and translation. Another is hnRNP C1 and C2, proteins implicated in mRNA processing and the translation of a specific subset of mRNAs expressed in differentiated cells. In lovastatin-treated MDA468 cells, the mobility of the associated hnRNP C1 and C2 proteins changed, and this correlated with increased p27 expression. Together, these data suggest that the U-rich dependent RNP complex on the 5'UTR may regulate the translation of p27 mRNA and may be a target of antimitogenic signals.

Poly(A) polymerase phosphorylation is dependent on novel interactions with cyclins

We have previously shown that poly(A) polymerase (PAP) is negatively regulated by cyclin B-cdc2 kinase hyperphosphorylation in the M phase of the cell cycle. Here we show that cyclin B(1) binds PAP directly, and we demonstrate further that this interaction is mediated by a stretch of amino acids in PAP with homology to the cyclin recognition motif (CRM), a sequence previously shown in several cell cycle regulators to target specifically G(1)-phase-type cyclins. We find that PAP interacts with not only G(1)- but also G(2)-type cyclins via the CRM and is a substrate for phosphorylation by both types of cyclin-cdk pairs. PAP's CRM shows novel, concentration-dependent effects when introduced as an 8-mer peptide into binding and kinase assays. While higher concentrations of PAP's CRM block PAP-cyclin binding and phosphorylation, lower concentrations induce dramatic stimulation of both activities. Our data not only support the notion that PAP is directly regulated by cyclin-dependent kinases throughout the cell cycle but also introduce a novel type of CRM that functionally interacts with both G(1)- and G(2)-type cyclins in an unexpected way.

Analysis of CAK activities from human cells

The cdk-activating kinase (CAK) activates cyclin-dependent kinases (cdks) that control cell-cycle progression by phosphorylating a threonine residue conserved in cdks. CAK from humans contains p40MO15 (cdk7), cyclin H and MAT1, which are also subunits of transcription factor IIH where they phosphorylate the C-terminal domain of the large subunit of RNA polymerase II. In contrast, budding yeast Cak1p is a monomeric enzyme without C-terminal domain kinase activity. Here, we analyze CAK activities in HeLa cells using cdk2-affinity chromatography. In addition to MO15, a second CAK activity was detected that runs on gel filtration at 30-40 kDa. This activity phosphorylated and activated cdk2 and cdk6. Furthermore, this 'small CAK' activity resembled Cak1p rather than MO15 in terms of substrate specificity, reactivity to antibodies against MO15 and Cak1p, and sensitivity to 5'-fluorosulfonylbenzoyladenosine, an irreversible inhibitory ATP analog. Our findings suggest the presence of at least two different CAK activities in human cells.

Functions of Pho85 cyclin-dependent kinases in budding yeast

Pho85 is a multifunctional cyclin-dependent kinase (Cdk) in Saccharomyces cerevisiae that has emerged as an important model for the role of Cdks in both cell cycle control and other processes. Pho85 was originally discovered as a regulator of phosphate metabolism but roles for Pho85 in glycogen biosynthesis, actin regulation and cell cycle progression have since been discovered. Ten genes encoding known or putative Pho85 cyclins (Pcls) have been identified and the Pcls appear to target Pho85 to specific cellular functions and substrates. In this chapter, we review the functions of the various Pcl-Pho85 complexes in budding yeast. We focus on the known biological roles of Pho85 with an emphasis on Pho85 substrates and cyclin-Cdk specificity.

A screen for modifiers of cyclin E function in Drosophila melanogaster identifies Cdk2 mutations, revealing the insignificance of putative phosphorylation sites in Cdk2

In higher eukaryotes, cyclin E is thought to control the progression from G1 into S phase of the cell cycle by associating as a regulatory subunit with cdk2. To identify genes interacting with cyclin E, we have screened in Drosophila melanogaster for mutations that act as dominant modifiers of an eye phenotype caused by a Sevenless-CycE transgene that directs ectopic Cyclin E expression in postmitotic cells of eye imaginal disc and causes a rough eye phenotype in adult flies. The majority of the EMS-induced mutations that we have identified fall into four complementation groups corresponding to the genes split ends, dacapo, dE2F1, and Cdk2(Cdc2c). The Cdk2 mutations in combination with mutant Cdk2 transgenes have allowed us to address the regulatory significance of potential phosphorylation sites in Cdk2 (Thr 18 and Tyr 19). The corresponding sites in the closely related Cdk1 (Thr 14 and Tyr 15) are of crucial importance for regulation of the G2/M transition by myt1 and wee1 kinases and cdc25 phosphatases. In contrast, our results demonstrate that the equivalent sites in Cdk2 play no essential role.

Transforming growth factor beta(1) selectively inhibits the cyclic AMP-dependent proliferation of primary thyroid epithelial cells by preventing the association of cyclin D3-cdk4 with nuclear p27(kip1)

Dog thyroid epithelial cells in primary culture constitute a physiologically relevant model of positive control of DNA synthesis initiation and G0-S prereplicative phase progression by cAMP as a second messenger for thyrotropin (thyroid-stimulating hormone [TSH]). As previously shown in this system, the cAMP-dependent mitogenic pathway differs from growth factor cascades as it stimulates the accumulation of p27(kip1) but not cyclins D. Nevertheless, TSH induces the nuclear translocations and assembly of cyclin D3 and cdk4, which are essential in cAMP-dependent mitogenesis. Here we demonstrate that transforming growth factor beta(1) (TGFbeta(1)) selectively inhibits the cAMP-dependent cell cycle in mid-G1 and various cell cycle regulatory events, but it weakly affects the stimulation of DNA synthesis by epidermal growth factor (EGF), hepatocyte growth factor, serum, and phorbol esters. EGF+serum and TSH did not interfere importantly with TGFbeta receptor signaling, because they did not affect the TGFbeta-induced nuclear translocation of Smad 2 and 3. TGFbeta inhibited the phosphorylation of Rb, p107, and p130 induced by TSH, but it weakly affected the phosphorylation state of Rb-related proteins in EGF+serum-treated cells. TGFbeta did not inhibit c-myc expression. In TSH-stimulated cells, TGFbeta did not affect the expression of cyclin D3, cdk4, and p27(kip1), nor the induced formation of cyclin D3-cdk4 complexes, but it prevented the TSH-induced relocalization of p27(kip1) from cdk2 to cyclin D3-cdk4. It prevented the nuclear translocations of cdk4 and cyclin D3 without altering the assembly of cyclin D3-cdk4 complexes probably formed in the cytoplasm, where they were prevented from sequestering nuclear p27(kip1) away from cdk2. This study dissociates the assembly of cyclin D3-cdk4 complexes from their nuclear localization and association with p27(kip1). It provides a new mechanism of regulation of proliferation by TGFbeta, which points out the subcellular location of cyclin D-cdk4 complexes as a crucial factor integrating mitogenic and antimitogenic regulations in an epithelial cell in primary culture.