Overexpression of the replication licensing regulators hCdt1 and hCdc6 characterizes a subset of non-small-cell lung carcinomas: synergistic effect with mutant p53 on tumor growth and chromosomal instability--evidence of E2F-1 transcriptional control over hCdt1

Replication licensing ensures once per cell cycle replication and is essential for genome stability. Overexpression of two key licensing factors, Cdc6 and Cdt1, leads to overreplication and chromosomal instability (CIN) in lower eukaryotes and recently in human cell lines. In this report, we analyzed hCdt1, hCdc6, and hGeminin, the hCdt1 inhibitor expression, in a series of non-small-cell lung carcinomas, and investigated for putative relations with G(1)/S phase regulators, tumor kinetics, and ploidy. This is the first study of these fundamental licensing elements in primary human lung carcinomas. We herein demonstrate elevated levels (more than fourfold) of hCdt1 and hCdc6 in 43% and 50% of neoplasms, respectively, whereas aberrant expression of hGeminin was observed in 49% of cases (underexpression, 12%; overexpression, 37%). hCdt1 expression positively correlated with hCdc6 and E2F-1 levels (P = 0.001 and P = 0.048, respectively). Supportive of the observed link between E2F-1 and hCdt1, we provide evidence that E2F-1 up-regulates the hCdt1 promoter in cultured mammalian cells. Interestingly, hGeminin overexpression was statistically related to increased hCdt1 levels (P = 0.025). Regarding the kinetic and ploidy status of hCdt1- and/or hCdc6-overexpressing tumors, p53-mutant cases exhibited significantly increased tumor growth values (Growth Index; GI) and aneuploidy/CIN compared to those bearing intact p53 (P = 0.008 for GI, P = 0.001 for CIN). The significance of these results was underscored by the fact that the latter parameters were independent of p53 within the hCdt1-hCdc6 normally expressing cases. Cumulatively, the above suggest a synergistic effect between hCdt1-hCdc6 overexpression and mutant-p53 over tumor growth and CIN in non-small-cell lung carcinomas.

Crystal structure of the coiled-coil dimerization motif of geminin: structural and functional insights on DNA replication regulation

We have determined the crystal structure of the coiled-coil domain of human geminin, a DNA synthesis inhibitor in higher eukaryotes. We show that a peptide encompassing the five heptad repeats of the geminin leucine zipper (LZ) domain is a dimeric parallel coiled coil characterized by a unique pattern of internal polar residues and a negatively charged surface that may target the basic domain of interacting partners. We show that the LZ domain itself is not sufficient to inhibit DNA synthesis but upstream and downstream residues are required. Analysis of a functional form of geminin by density sedimentation indicates an oligomeric structure. X-ray solution scattering experiments performed on a non-functional form of geminin having upstream basic residues and the LZ domain show a tetramer structure. Altogether, these results give a consistent identification and mapping of geminin interacting regions onto structurally important domains. They also suggest that oligomerization properties of geminin may be implicated in its inhibitory activity of DNA synthesis.

Geminin Has Dimerization, Cdt1-binding, and Destruction Domains That Are Required for Biological Activity

Geminin is an unstable regulatory protein that affects both cell division and cell differentiation. Geminin inhibits a second round of DNA synthesis during S and G(2) phase by binding the essential replication protein Cdt1. Geminin is also required for entry into mitosis, either by preventing replication abnormalities or by down-regulating the checkpoint kinase Chk1. Geminin overexpression during embryonic development induces ectopic neural tissue, inhibits eye formation, and perturbs the segmental patterning of the embryo. In order to define the structural and functional domains of the geminin protein, we generated over 40 missense and deletion mutations and tested their phenotypes in biological and biochemical assays. We find that geminin self-associates through the coiled-coil domain to form dimers and that dimerization is required for activity. Geminin contains a typical bipartite nuclear localization signal that is also required for its destruction during mitosis. Nondegradable mutants of geminin interfere with DNA replication in succeeding cell cycles. Geminin's Cdt1-binding domain lies immediately adjacent to the dimerization domain and overlaps it. We constructed two nonbinding mutants in this domain and found that they neither inhibited replication nor permitted entry into mitosis, indicating that this domain is necessary for both activities. We identified several missense mutations in geminin's Cdt1 binding domain that were deficient in their ability to inhibit replication yet were still able to allow mitotic entry, suggesting that these are separate functions of geminin.

Cdt1 and geminin are down-regulated upon cell cycle exit and are over-expressed in cancer-derived cell lines

Licensing origins for replication upon completion of mitosis ensures genomic stability in cycling cells. Cdt1 was recently discovered as an essential licensing factor, which is inhibited by geminin. Over-expression of Cdt1 was shown to predispose cells for malignant transformation. We show here that Cdt1 is down-regulated at both the protein and RNA level when primary human fibroblasts exit the cell cycle into G0, and its expression is induced as cells re-enter the cell cycle, prior to S phase onset. Cdt1's inhibitor, geminin, is similarly down-regulated upon cell cycle exit at both the protein and RNA level, and geminin protein accumulates with a 3-6 h delay over Cdt1, following serum re-addition. Similarly, mouse NIH3T3 cells down-regulate Cdt1 and geminin mRNA and protein when serum starved. Our data suggest a transcriptional control over Cdt1 and geminin at the transition from quiescence to proliferation. In situ hybridization and immunohistochemistry localize Cdt1 as well as geminin to the proliferative compartment of the developing mouse gut epithelium. Cdt1 and geminin levels were compared in primary cells vs. cancer-derived human cell lines. We show that Cdt1 is consistently over-expressed in cancer cell lines at both the protein and RNA level, and that the Cdt1 protein accumulates to higher levels in individual cancer cells. Geminin is similarly over-expressed in the majority of cancer cell lines tested. The relative ratios of Cdt1 and geminin differ significantly amongst cell lines. Our data establish that Cdt1 and geminin are regulated at cell cycle exit, and suggest that the mechanisms controlling Cdt1 and geminin levels may be altered in cancer cells.

Molecular structure of human geminin

The origin licensing repressor geminin is a unique bifunctional protein providing a molecular link between cellular proliferation, differentiation and genomic stability. Here we report the first molecular structure of human geminin, determined by EM and image processing at a resolution of 17.5 A. The geminin molecule is a tetramer formed by two dimers with monomers interacting via coiled-coil domains. The unusual structural organization of geminin provides molecular insight into its bifunctional nature.

Geminin predicts adverse clinical outcome in breast cancer by reflecting cell-cycle progression

Geminin inhibits DNA replication by preventing Cdt1 from loading minichromosome maintenance (MCM) proteins onto DNA. The present study has investigated whether the frequency of geminin expression predicts clinical outcome in breast cancer. Immunohistochemistry was used first to examine geminin expression in normal and malignant breast tissue (n = 67). Correlations with cell-cycle parameters, pathological features, and clinical outcome were then determined using an invasive breast carcinoma tissue microarray (n = 165). Breast carcinomas were scanned for mutations (n = 61) and copy number imbalances (n = 241) of the geminin gene. Finally, the cell cycle distribution of geminin in breast cancer cells was investigated in vivo and in vitro. Despite a putative tumour suppressor function, it was found that increased geminin expression is a powerful independent indicator of adverse prognosis in invasive breast cancer. Both poor overall survival (p = 0.0002) and the development of distant metastases (p = 0.005) are predicted by high geminin expression, which performs better in this patient cohort than traditional factors currently used to determine prognosis and appropriate therapy. No mutations or deletions of the geminin gene and no evidence that a high frequency of protein expression is related to gene amplification were found. It is shown that geminin is expressed from S to M phase in breast carcinoma tissue and cell lines, disappearing at the metaphase--anaphase transition. While MCM proteins identify all non-quiescent cells, geminin identifies the sub-fraction that have entered S phase, but not exited mitosis, thereby indicating the rate of cell-cycle progression. It is suggested that this explains its unexpected value as a prognostic marker in breast cancer.

Loss of Geminin induces rereplication in the presence of functional p53

Strict regulation of DNA replication is essential to ensure proper duplication and segregation of chromosomes during the cell cycle, as its deregulation can lead to genomic instability and cancer. Thus, eukaryotic organisms have evolved multiple mechanisms to restrict DNA replication to once per cell cycle. Here, we show that inactivation of Geminin, an inhibitor of origin licensing, leads to rereplication in human normal and tumor cells within the same cell cycle. We found a CHK1-dependent checkpoint to be activated in rereplicating cells accompanied by formation of gammaH2AX and RAD51 nuclear foci. Abrogation of the checkpoint leads to abortive mitosis and death of rereplicated cells. In addition, we demonstrate that the induction of rereplication is dependent on the replication initiation factors CDT1 and CDC6, and independent of the functional status of p53. These data show that Geminin is required for maintaining genomic stability in human cells.

A dimerized coiled-coil domain and an adjoining part of geminin interact with two sites on Cdt1 for replication inhibition

Geminin is a cellular protein that associates with Cdt1 and inhibits Mcm2-7 loading during S phase. It prevents multiple cycles of replication per cell cycle and prevents episome replication. It also directly inhibits the HoxA11 transcription factor. Here we report that geminin forms a parallel coiled-coil homodimer with atypical residues in the dimer interface. Point mutations that disrupt the dimerization abolish interaction with Cdt1 and inhibition of replication. An array of glutamic acid residues on the coiled-coil domain surface interacts with positive charges in the middle of Cdt1. An adjoining region interacts independently with the N-terminal 100 residues of Cdt1. Both interactions are essential for replication inhibition. The negative residues on the coiled-coil domain and a different part of geminin are also required for interaction with HoxA11. Therefore a rigid cylinder with negative surface charges is a critical component of a bipartite interaction interface between geminin and its cellular targets.

DNA replication licensing

The DNA replication licensing system ensures that chromosomal DNA is replicated precisely once before cell division occurs. A DNA helicase must be loaded on origin DNA for replication to initiate. Considerable evidence suggests that the MCM complex acts as a replicative helicase in eukaryotes. When the MCM complex is loaded on the chromatin, the replication origin is formally defined as being licensed for replication. Licensing takes place several hours before origins are activated to undergo replication in S-phase. Genetic and biochemical studies show that the licensing process is well conserved in eukaryotes. Cyclin Dependent Kinases (CDKs), the master regulators of the cell cycle, coordinate the initiation of the two key cell cycle events, replication of DNA and its segregation at mitosis. Eukaryotes have developed complex regulatory mechanisms to ensure that origin licensing is coordinated with these events so that genome integrity is preserved during successive cell divisions.

DNA replication licensing and cell cycle kinetics of oligodendroglial tumours

The convergence point of growth-signalling pathways that control cell proliferation is the initiation of genome replication, the core of which is the assembly of pre-replicative complexes (pre-RCs), resulting in chromatin being ‘licensed’ for DNA replication in the subsequent S phase. The Mcm2–7 complex is a core constituent of the pre-RC, whose recruitment to replication origins is dependent on the Cdt1 loading factor. Geminin is a potent inhibitor of the initiation of DNA replication by preventing Mcm2–7 assembly at origins via its interaction with Cdt1, ensuring genomic integrity through suppression of re-initiation events in S phase. Here we investigate the regulation of Ki67, Mcm2, p21, caspase 3 and Geminin in a series of 55 oligodendrogliomas to provide an integrated picture of how cellular proliferation and programmed cell death are dysregulated in these tumours. Geminin does not behave as an inhibitor of cell proliferation, its labelling index rising with increasing growth fraction as defined by Ki67 or Mcm2 expression. Geminin is expressed in a higher proportion of cells in higher grade tumours (P<0.001) and shows a strong correlation to proliferation and replication licensing (P<0.01), but not apoptosis. Increasing tumour anaplasia is not associated with loss of Geminin. Importantly, the G1 phase of the proliferative cell cycle, as assessed by the Geminin/Ki67 ratio, shortens with increasing anaplasia, providing new potential algorithms for prognostic assessment. Origin licensing proteins thus provide powerful novel tools for assessment of tumour cell cycle kinetics in routinely processed surgical biopsy material.

Rereplication by depletion of geminin is seen regardless of p53 status and activates a G2/M checkpoint

Genomic DNA replication is tightly controlled to ensure that DNA replication occurs once per cell cycle; loss of this control leads to genomic instability. Geminin, a DNA replication inhibitor, plays an important role in regulation of DNA replication. To investigate the role of human geminin in the maintenance of genomic stability, we eliminated geminin by RNA interference in human cancer cells. Depletion of geminin led to overreplication and the formation of giant nuclei in cells that had wild-type or mutant p53. We found that overreplication caused by depletion of geminin activated both Chk1 and Chk2, which then phosphorylated Cdc25C on Ser216, resulting in its sequestration outside the nucleus, thus inhibiting cyclin B-Cdc2 activity. This activated G(2)/M checkpoint prevented cells with overreplicated DNA from entering mitosis. Addition of caffeine, UCN-01, or inhibitors of checkpoint pathways or silencing of Chk1 suppressed the accumulation of overreplicated cells and promoted apoptosis. From these results, we conclude that geminin is required for suppressing overreplication in human cells and that a G(2)/M checkpoint restricts the proliferation of cells with overreplicated DNA.

Geminin: a major DNA replication safeguard in higher eukaryotes

Eukaryotes have evolved multiple mechanisms to restrict DNA replication to once per cell cycle. These mechanisms prevent relicensing of origins of replication after initiation of DNA replication in S phase until the end of mitosis. Most of our knowledge of mechanisms controlling prereplication complex (preRC) formation are based on studies from yeast and Xenopus, while much less is known for mammalian cells. Here we discuss our recent data demonstrating that Geminin is required for preventing rereplication in human normal and cancer cells.

Structural basis for inhibition of the replication licensing factor Cdt1 by geminin

To maintain chromosome stability in eukaryotic cells, replication origins must be licensed by loading mini-chromosome maintenance (MCM2-7) complexes once and only once per cell cycle. This licensing control is achieved through the activities of geminin and cyclin-dependent kinases. Geminin binds tightly to Cdt1, an essential component of the replication licensing system, and prevents the inappropriate reinitiation of replication on an already fired origin. The inhibitory effect of geminin is thought to prevent the interaction between Cdt1 and the MCM helicase. Here we describe the crystal structure of the mouse geminin-Cdt1 complex using tGeminin (residues 79-157, truncated geminin) and tCdt1 (residues 172-368, truncated Cdt1). The amino-terminal region of a coiled-coil dimer of tGeminin interacts with both N-terminal and carboxy-terminal parts of tCdt1. The primary interface relies on the steric complementarity between the tGeminin dimer and the hydrophobic face of the two short N-terminal helices of tCdt1 and, in particular, Pro 181, Ala 182, Tyr 183, Phe 186 and Leu 189. The crystal structure, in conjunction with our biochemical data, indicates that the N-terminal region of tGeminin might be required to anchor tCdt1, and the C-terminal region of tGeminin prevents access of the MCM complex to tCdt1 through steric hindrance.

The replicative regulator protein geminin on chromatin in the HeLa cell cycle

Geminin is believed to have a major function in the regulation of genome replication and cell proliferation. Published evidence shows that geminin specifically interacts with Cdt1 to block its function in the assembly of prereplication complexes. However, in proliferating HeLa cells geminin and Cdt1 are co-expressed during a relatively short time at the G(1)-to-S phase transition. Under these conditions, nearly all Cdt1 and a major part of geminin are bound to chromatin and reside at the same or closely adjacent sites as shown here by chromatin immunoprecipitation. Cdt1 is rapidly degraded early in S phase, but geminin remains bound to the chromatin sites. One function that chromatin-bound geminin could perform is to prevent access to Cdt1 that may escape S phase-dependent degradation or is synthesized in excess. Indeed, Cdt1 continues to be synthesized in HeLa cells in S phase but never accumulates because of the efficient degradation. Therefore, geminin can be eliminated by RNA interference without detectable effects on cell cycle parameters.

Self-assembling protein microarrays

Protein microarrays provide a powerful tool for the study of protein function. However, they are not widely used, in part because of the challenges in producing proteins to spot on the arrays. We generated protein microarrays by printing complementary DNAs onto glass slides and then translating target proteins with mammalian reticulocyte lysate. Epitope tags fused to the proteins allowed them to be immobilized in situ. This obviated the need to purify proteins, avoided protein stability problems during storage, and captured sufficient protein for functional studies. We used the technology to map pairwise interactions among 29 human DNA replication initiation proteins, recapitulate the regulation of Cdt1 binding to select replication proteins, and map its geminin-binding domain.

Human geminin promotes pre-RC formation and DNA replication by stabilizing CDT1 in mitosis

Geminin is an unstable inhibitor of DNA replication that negatively regulates the licensing factor CDT1 and inhibits pre-replicative complex (pre-RC) formation in Xenopus egg extracts. Here we describe a novel function of Geminin. We demonstrate that human Geminin protects CDT1 from proteasome-mediated degradation by inhibiting its ubiquitination. In particular, Geminin ensures basal levels of CDT1 during S phase and its accumulation during mitosis. Consistently, inhibition of Geminin synthesis during M phase leads to impairment of pre-RC formation and DNA replication during the following cell cycle. Moreover, we show that inhibition of CDK1 during mitosis, and not Geminin depletion, is sufficient for premature formation of pre-RCs, indicating that CDK activity is the major mitotic inhibitor of licensing in human cells. Taken together with recent data from our laboratory, our results demonstrate that Geminin is both a negative and positive regulator of pre-RC formation in human cells, playing a positive role in allowing CDT1 accumulation in G2-M, and preventing relicensing of origins in S-G2.

Geminin coordinates cell cycle and developmental control

Growth and differentiation are two major themes in embryonic development. Numerous cell divisions have to be regulated on the path from a unicellular embryo, the zygote, to the multicellular structures of a mature being. Numerous functions, specializations and cellular identities have to be generated, in order to form a complex and mature animal. Numerous mechanisms have to control the correct assignment and acquisition of cellular fates, as well as the right timing and allocation of cells. Therefore, a strict coordination has to occur between embryonic patterning and the cell cycle. From this point of view, dual roles or mutual interactions of typical proliferation and developmental control genes are likely. Recently, new light was shed on these issues by identifying the nuclear protein Geminin as a molecular coordinator between the cell cycle and axial patterning. We summarize the role of Geminin in cell cycle, in the embryonic patterning controlled by Hox genes, providing insights into cell cycle regulators in embryonic development, and conversely, typical developmental control genes in cell cycle regulation.

Cell cycle regulation of the licensing activity of Cdt1 in Xenopus laevis

Cdt1 is a conserved replication factor required in licensing the chromosome for a single round of DNA synthesis. The activity of Cdt1 is inhibited by geminin. The mechanism by which geminin interferes with Cdt1 activity is unknown. It is thought that geminin binds to and sequestrate Cdt1. We show that geminin does not interfere with the chromatin association of Cdt1 and that inhibition of DNA synthesis by geminin is observed following its accumulation on chromatin. The binding of geminin to chromatin has been investigated during S phase. We demonstrate that loading of geminin onto chromatin requires Cdt1, suggesting that geminin is targeted at replication origins. We also show that geminin binds chromatin at the transition from the pre-replication to pre-initiation complexes, which overlaps with the release of Cdt1. This regulation is strikingly different from that observed in somatic cells where the chromatin binding of these proteins is mutually exclusive. In contrast to somatic cells, we further show that geminin is stable during the early embryonic cell cycles. These results suggest a specific regulation of origin firing adapted to the rapid cell cycles of Xenopus and indicate that periodic degradation of geminin is not relevant to licensing during embryonic development.

Regulation of Geminin and Cdt1 expression by E2F transcription factors

Geminin and Cdt1 play an essential role in the initiation of DNA replication, by regulating the chromatin loading of the MCM complex. In this study, we showed that the transcription of human Geminin and Cdt1, as well as that of MCM7, is activated by transcription factors E2F1-4, but not by factors E2F5-7. Analysis of various Geminin and Cdt1 promoter constructs showed that an E2F-responsive sequence in the vicinity of the transcription initiation site is necessary for the transcriptional activation. The promoter activity for human Geminin was activated by the E7, but not E6, oncogene of human papillomavirus type 16. While E2F1-induced activation of human Cdt1 gene transcription was suppressed by pRb, but not by p107 or p130, its E2F4-induced activation was suppressed by pRb, p107, and p130. Furthermore, the promoter activities of human Geminin and Cdt1 were demonstrated to be growth-dependent. Taken together, the results demonstrate that Geminin and Cdt1 constitute targets for various members of the E2F family of transcription factors, and that expression of Geminin and Cdt1 is perhaps mediated by the activation of a pRb/E2F pathway.

Proteolysis of DNA replication licensing factor Cdt1 in S-phase is performed independently of geminin through its N-terminal region

Licensing of replication origins is carefully regulated in a cell cycle to maintain genome integrity. Using an in vivo ubiquitination assay and temperature-sensitive cell lines we demonstrate that Cdt1 in mammalian cells is degraded through ubiquitin-dependent proteolysis in S-phase. siRNA experiments for Geminin indicate that Cdt1 is degraded in the absence of Geminin. The N terminus of Cdt1 is required for its nuclear localization, associates with cyclin A, but is dispensable for the association of Cdt1 with Geminin in cells. This region is responsible for proteolysis of Cdt1 in S-phase. On the other hand, the N terminus-truncated Cdt1 is stable in S-phase, and associates with the licensing inhibitor, Geminin. High level expression of this form of Cdt1 brings about cells having higher DNA content. Proteasome inhibitors stabilize Cdt1 in S-phase, and the protein is detected in the nucleus in a complex with Geminin. This form of Cdt1 associates with chromatin as tightly as that of G1-cells, when no Geminin is detected. Our data show that proteolysis and Geminin binding independently inactivate Cdt1 after the onset of S-phase to prevent re-replication.

Protein kinase CK2 phosphorylates the cell cycle regulatory protein Geminin

Geminin contributes to cell cycle regulation by a timely inhibition of Cdt1p, the loading factor required for the assembly of pre-replication complexes. Geminin is expressed during S and G2 phase of the HeLa cell cycle and phosphorylated soon after its synthesis. We show here that Geminin is an excellent substrate for protein kinase CK2 in vitro; and that the highly specific CK2 inhibitor tetrabromobenzotriazole (TBB) blocks the phosphorylation of Geminin in HeLa protein extracts and HeLa cells in vivo. The sites of CK2 phosphorylation are located in the carboxyterminal region of Geminin, which carries several consensus sequence motifs for CK2. We also show that a minor phosphorylating activity in protein extracts can be attributed to glycogen synthase kinase 3 (GSK3), which most likely targets a central peptide in Geminin. Treatment of HeLa cells with TBB does not interfere with the ability of Geminin to interact with the loading factor Cdt1.

Geminin is targeted for repression by the retinoblastoma tumor suppressor pathway through intragenic E2F sites

The geminin protein is a critical regulator of DNA replication. It functions to control replication fidelity by blocking the assembly of prereplication complexes in the S and G(2) phases of the cell cycle. Geminin protein levels, which are low in G(0)/G(1) and increase at the G(1)/S transition, are controlled through coordinate transcriptional and proteolytic regulation. Here we show that geminin is regulated transcriptionally by the retinoblastoma tumor suppressor (RB)/E2F pathway. Initially, we observed that the activation of RB led to the repression of geminin transcription. Conversely, Rb-null mouse embryonic fibroblasts have enhanced the expression of geminin relative to wild type mouse embryonic fibroblasts. Similarly, an acute loss of Rb in mouse adult fibroblasts deregulated geminin RNA and protein levels. To delineate the responsible regulatory motifs, luciferase reporter constructs containing fragments of the geminin promoter were generated. An analysis of the critical regulatory cis-acting elements in the geminin promoter indicated that intragenic E2F sites down-stream of the first exon were responsible for RB-mediated repression of geminin. The direct analysis of the endogenous geminin promoter revealed that these intragenic E2F sites are occupied by E2F proteins, and the mutation of these sites eliminates responsiveness to RB. Together, these data link the expression of geminin to the RB/E2F pathway and represent the first promoter analysis of this important regulator of DNA replication.

Negative regulation of geminin by CDK-dependent ubiquitination controls replication licensing

The replication licensing system ensures the precise duplication of chromosomal DNA in each cell cycle. In metazoans, a small protein called geminin plays a central role in negatively regulating licensing late in the cell cycle. Recent work using Xenopus egg extracts shows how geminin activity is downregulated on exit from metaphase in a process that requires mitotic cyclin-dependent kinases (CDKs). Geminin is polyubiquitinated by the Anaphase Promoting Complex, but instead of being proteolysed-the normal fate of polyubiquitinated proteins-much of the geminin is deubiquitinated, leaving it inactive. These results suggest a simple model for how precise chromosome duplication is ensured in the Xenopus model system.

Peptide binding to Geminin and inhibitory for DNA replication

Geminin binds to Cdt1 to ensure that DNA replication occurs only once during the cell cycle. To identify the peptide that binds to Geminin and thereby modifies the latter's ability to alter the DNA replication activity in human cancer cells, we screened a phage display library of random peptides in successive cycles of phage library panning and found one peptide sequence that bound to the 31-111 amino acid residues of Geminin. Delivery of this peptide sequence into the nucleus of HCT116 human colon cancer cells resulted in the suppression of BrdU incorporation. These results provide new insights into the function of Geminin and further validate Geminin as a potential therapeutic target in tumors.

The cell-cycle regulator geminin inhibits Hox function through direct and polycomb-mediated interactions

Embryonic development is tightly controlled. The clustered genes of the Hox family of homeobox proteins play an important part in regulating this development and also proliferation. They specify embryonic structures along the body axis, and are associated with normal and malignant cell growth. The cell-cycle regulator geminin controls replication by binding to the licensing factor Cdt1, and is involved in neural differentiation. Here, we show that murine geminin associates transiently with members of the Hox-repressing polycomb complex, with the chromatin of Hox regulatory DNA elements and with Hox proteins. Gain- and loss-of-function experiments in the chick neural tube demonstrate that geminin modulates the anterior boundary of Hoxb9 transcription, which suggests a polycomb-like activity for geminin. The interaction between geminin and Hox proteins prevents Hox proteins from binding to DNA, inhibits Hox-dependent transcriptional activation of reporter and endogenous downstream target genes, and displaces Cdt1 from its complex with geminin. By establishing competitive regulation, geminin functions as a coordinator of developmental and proliferative control.