Biomedical subjects

Gary D Kao

Publications and source records attributed to Gary D Kao.

18 recordsLinked to original sources

PubMed2006

A novel histone deacetylase pathway regulates mitosis by modulating Aurora B kinase activity.

Histone deacetylase (HDAC) inhibitors perturb the cell cycle and have great potential as anti-cancer agents, but their mechanism of action is not well established. HDACs classically function as repressors of gene expression, tethered to sequence-specific transcription factors. Here we report that HDAC3 is a critical, transcription-independent regulator of mitosis. HDAC3 forms a complex with A-Kinase-Anchoring Proteins AKAP95 and HA95, which are targeted to mitotic chromosomes. Deacetylation of H3 in mitosis requires AKAP95/HA95 and HDAC3 and provides a hypoacetylated H3 tail that is the preferred substrate for Aurora B kinase. Phosphorylation of H3S10 by Aurora B leads to dissociation of HP1 proteins from methylated H3K9 residues on mitotic heterochromatin. This transcription-independent pathway, involving interdependent changes in histone modification and protein association, is required for normal progression through mitosis and is an unexpected target of HDAC inhibitors, a class of drugs currently in clinical trials for treating cancer.

A Kinase Anchor Proteins↗

PubMed2006

Radiosensitizing effects of the prenyltransferase inhibitor AZD3409 against RAS mutated cell lines.

Mutations at the H-, N- and K-ras loci are among the most frequent genetic alterations in human cancers. In this study, we have investigated the effect of AZD3409, a novel, peptidomimetic prenyltransferase inhibitor (PTI), on the radiosensitivity of cells with mutated ras alleles. AZD3409, developed by AstraZeneca, inhibits both farnesyl- and geranylgeranyl transferase in cell free systems. AZD3409 inhibits the growth of a variety of human cancer cell lines, including cells that express mutant alleles of either K- or H- ras and was well tolerated when administered orally to healthy volunteers in a phase I clinical trial. We have previously shown that PTI can radiosensitize human and rodent cancer cell lines that express activated RAS. Here we assessed the ability of AZD3409 to radiosensitize human cancer cell lines in vivo and in vitro and the activation state of RAS proteins in treated cells. Once daily oral administration of AZD3409 to nude mice bearing PSN-1 and MiaPaCa-2 human pancreatic cancer xenografts expressing mutant K-ras was well tolerated and resulted in a supra-additive reduction in clonogenic cell survival after irradiation. Similarly, AZD3409 reduced clonogenic survival in cells that express either mutant K- or H- ras in vitro. We next examined the effect of AZD3409 on the processing and activation of K- and H-RAS. AZD3409-mediated radiosensitization, both in vivo and in vitro, correlates with a decrease in H-RAS processing without detectable effect on K-RAS processing. RAS activation assays show that the decreased H-RAS processing is accompanied by decreased H-RAS activation in cell lines with mutations in either K- or H-ras. However, no decrease in K-RAS activation was detected. Thus, radiosensitization of human cancer cells that express mutated K-RAS occurred under conditions where AZD3409 inihibits the activation of farneyslated H-RAS, but did not inhibit K-RAS activation.

Animals↗

PubMed2006

Zebrafish as a "biosensor"? Effects of ionizing radiation and amifostine on embryonic viability and development.

The zebrafish (Danio rerio) has emerged as a popular vertebrate model system for cancer and treatment-related research. Benefits include ease of care, rapid development, optical clarity of embryos, which allows visualization of major organ systems, and opportunities for genetic manipulation. However, specific parameters of radiation sensitivity have not been systematically documented. We investigated the effects of radiation and a radiomodifier on zebrafish viability and embryonic development. Embryos were exposed to gamma-radiation (5, 10, or 20 Gy) at sequential times postfertilization and serially assessed for viability and morphologic abnormalities. As expected, lethality and morphologic perturbations were more pronounced earlier in embryogenesis and with higher radiation doses and were partially reversed by amifostine. The effects of radiation and concurrent treatment with amifostine on the developmental organization of the eye and brain were striking. Radiation resulted in hypocellularity and disorganization of the cellular layers of the retina, effects partially reversed by amifostine, as well as lens opacification. Radiation strikingly reduced the volume of brain, but the volume loss was substantially blocked by amifostine. Increased terminal deoxyribonucleotidyl transferase-mediated dUTP nick end labeling signal was noted in both the irradiated eye and brain, but reduced by amifostine. Finally, irradiating embryos resulted in caspase activation detectable in 96-well microplates, which was proportional to the number of embryos and radiation dose; the degree of activation was markedly reduced by amifostine. These results together suggest the power and versatility of the zebrafish in assessing the effects of radiation and radiomodifiers on organ and tissue development.

Amifostine↗

PubMed2006

EGFR tyrosine kinase inhibitors decrease VEGF expression by both hypoxia-inducible factor (HIF)-1-independent and HIF-1-dependent mechanisms.

Epidermal growth factor receptor (EGFR) inhibitors can decrease vascular endothelial growth factor (VEGF) expression and tumor angiogenesis. In the current study, we investigate the molecular pathways by which this occurs using two drugs that have been used in the clinic, gefitinib (Iressa) and erlotinib (Tarceva). The decrease in VEGF expression by gefitinib in SQ20B squamous cell carcinoma cells was opposed by adenoviral expression of Akt in these cells. The hypoxia-inducible factor-1 (HIF-1) binding site located at approximately -1 kbp in the VEGF promoter was not required for down-regulation of promoter activity by gefitinib under normoxia. Furthermore, the drug decreased activity of a reporter containing the -88/+54 region. In a gel shift assay, gefitinib led to decreased retardation of a labeled DNA oligonucleotide probe corresponding to the -88/-66 region of the VEGF promoter, which contains Sp1 binding sites. These effects of gefitinib on VEGF promoter activity and DNA binding were both reversed by Akt expression. Phosphorylation of Sp1 was decreased in the presence of gefitinib. Gefitinib also decreases VEGF expression by decreasing HIF-1alpha expression. This occurs due to decreased protein translation without any change in the level of HIF-1alpha mRNA. Together, these results suggest that gefitinib decreases VEGF expression both by decreasing Sp1 binding to the proximal core VEGF promoter and by down-regulating HIF-1alpha expression. Similar results were obtained with erlotinib in SQ20B and gefitinib in HSC3 squamous carcinoma cells. These results indicate that there are at least two separate mechanisms by which EGFR inhibitors decrease VEGF expression.

Binding Sites↗

PubMed2006

Gamma radiation increases endonuclease-dependent L1 retrotransposition in a cultured cell assay.

Long Interspersed Elements (LINE-1s, L1s) are the most active mobile elements in the human genome and account for a significant fraction of its mass. The propagation of L1 in the human genome requires disruption and repair of DNA at the site of integration. As Barbara McClintock first hypothesized, genotoxic stress may contribute to the mobilization of transposable elements, and conversely, element mobility may contribute to genotoxic stress. We tested the ability of genotoxic agents to increase L1 retrotransposition in a cultured cell assay. We observed that cells exposed to gamma radiation exhibited increased levels of L1 retrotransposition. The L1 retrotransposition frequency was proportional to the number of phosphorylated H2AX foci, an indicator of genotoxic stress. To explore the role of the L1 endonuclease in this context, endonuclease-deficient tagged L1 constructs were produced and tested for their activity in irradiated cells. The activity of the endonuclease-deficient L1 was very low in irradiated cells, suggesting that most L1 insertions in irradiated cells still use the L1 endonuclease. Consistent with this interpretation, DNA sequences that flank L1 insertions in irradiated cells harbored target site duplications. These results suggest that increased L1 retrotransposition in irradiated cells is endonuclease dependent. The mobilization of L1 in irradiated cells potentially contributes to genomic instability and could be a driving force for secondary mutations in patients undergoing radiation therapy.

Aminoglycosides↗

PubMed2006

Histone deacetylase inhibitor-mediated radiosensitization of human cancer cells: class differences and the potential influence of p53.

Histone deacetylase inhibitors (HDI) are emerging as potentially useful components of the anticancer armamentarium and as useful tools to dissect mechanistic pathways. HDIs that globally inhibit histone deacetylases (HDAC) have radiosensitizing effects, but the relative contribution of specific HDAC classes remains unclear. Newly characterized HDIs are now available that preferentially inhibit specific HDAC classes, including SK7041 (inhibits class I HDACs) and splitomicin (inhibits class III HDACs). We investigated in human cancer cells the relative radiosensitizations that result from blocking specific HDAC classes. We found that trichostatin A (TSA; inhibitor of both class I and II HDACs) was the most effective radiosensitizer, followed by the class I inhibitor SK7041, whereas splitomicin (inhibitor of class III) had least effect. Interestingly, radiosensitization by TSA in cell lines expressing p53 was more pronounced than in isogenic lines lacking p53. Radiosensitization of cells expressing p53 by TSA was reduced by pifithrin-alpha, a small-molecule inhibitor of p53. In contrast, the radiosensitization by TSA of cells expressing low levels of p53 was enhanced by transfection of wild-type p53-expressing vector or pretreatment with leptomycin B, an inhibitor of nuclear export that increased intracellular levels of p53. These effects on radiosensitization were respectively muted or not seen in cells treated with SK7041 or splitomicin. To our knowledge, this may be among the first systematic investigations of the comparative anticancer effects of inhibiting specific classes of HDACs, with results suggesting differences in the degrees of radiosensitization, which in some cell lines may be influenced by p53 expression.

Amides↗

PubMed2006

Akt1 activation can augment hypoxia-inducible factor-1alpha expression by increasing protein translation through a mammalian target of rapamycin-independent pathway.

The phosphoinositide 3-kinase (PI3K)/Akt pathway is commonly activated in cancer; therefore, we investigated its role in hypoxia-inducible factor-1alpha (HIF-1alpha) regulation. Inhibition of PI3K in U87MG glioblastoma cells, which have activated PI3K/Akt activity secondary to phosphatase and tensin homologue deleted on chromosome 10 (PTEN) mutation, with LY294002 blunted the induction of HIF-1alpha protein and its targets vascular endothelial growth factor and glut1 mRNA in response to hypoxia. Introduction of wild-type PTEN into these cells also blunted HIF-1alpha induction in response to hypoxia and decreased HIF-1alpha accumulation in the presence of the proteasomal inhibitor MG132. Akt small interfering RNA (siRNA) also decreased HIF-1alpha induction under hypoxia and its accumulation in normoxia in the presence of dimethyloxallyl glycine, a prolyl hydroxylase inhibitor that prevents HIF-1alpha degradation. Metabolic labeling studies showed that Akt siRNA decreased HIF-1alpha translation in normoxia in the presence of dimethyloxallyl glycine and in hypoxia. Inhibition of mammalian target of rapamycin (mTOR) with rapamycin (10-100 nmol/L) had no significant effect on HIF-1alpha induction in a variety of cell lines, a finding that was confirmed using mTOR siRNA. Furthermore, neither mTOR siRNA nor rapamycin decreased HIF-1alpha translation as determined by metabolic labeling studies. Therefore, our results indicate that Akt can augment HIF-1alpha expression by increasing its translation under both normoxic and hypoxic conditions; however, the pathway we are investigating seems to be rapamycin insensitive and mTOR independent. These observations, which were made on cells grown in standard tissue culture medium (10% serum), were confirmed in PC3 prostate carcinoma cells. We did find that rapamycin could decrease HIF-1alpha expression when cells were cultured in low serum, but this seems to represent a different pathway.

Cell Line, Tumor↗

PubMed2005

Regulation of histone deacetylase 4 expression by the SP family of transcription factors.

Histone deacetylases mediate critical cellular functions but relatively little is known about mechanisms controlling their expression, including expression of HDAC4, a class II HDAC implicated in the modulation of cellular differentiation and viability. Endogenous HDAC4 mRNA, protein levels and promoter activity were all readily repressed by mithramycin, suggesting regulation by GC-rich DNA sequences. We validated consensus binding sites for Sp1/Sp3 transcription factors in the HDAC4 promoter through truncation studies and targeted mutagenesis. Specific and functional binding by Sp1/Sp3 at these sites was confirmed with chromatin immunoprecipitation (ChIP) and electromobility shift assays (EMSA). Cotransfection of either Sp1 or Sp3 with a reporter driven by the HDAC4 promoter led to high activities in SL2 insect cells (which lack endogenous Sp1/Sp3). In human cells, restored expression of Sp1 and Sp3 up-regulated HDAC4 protein levels, whereas levels were decreased by RNA-interference-mediated knockdown of either protein. Finally, variable levels of Sp1 were in concordance with that of HDAC4 in a number of human tissues and cancer cell lines. These studies together characterize for the first time the activity of the HDAC4 promoter, through which Sp1 and Sp3 modulates expression of HDAC4 and which may contribute to tissue or cell-line-specific expression of HDAC4.

Animals↗

PubMed2005

Protein kinase C delta stimulates apoptosis by initiating G1 phase cell cycle progression and S phase arrest.

Overexpression of protein kinase C delta (PKCdelta) stimulates apoptosis in a wide variety of cell types through a mechanism that is incompletely understood. PKCdelta-deficient cells are impaired in their response to DNA damage-induced apoptosis, suggesting that PKCdelta is required to mount an appropriate apoptotic response under conditions of stress. The mechanism through which it does so remains elusive. In addition to effects on cell survival, PKCdelta elicits pleiotropic effects on cellular proliferation. We now provide the first evidence that the ability of PKCdelta to stimulate apoptosis is intimately linked to its ability to stimulate G(1) phase cell cycle progression. Using an adenoviral-based expression system to express PKCalpha,-delta, and -epsilon in epithelial cells, we demonstrate that a modest increase in PKCdelta activity selectively stimulates quiescent cells to initiate G(1) phase cell cycle progression. Rather than completing the cell cycle, PKCdelta-infected cells arrest in S phase, an event that triggers caspase-dependent apoptotic cell death. Apoptosis was preceded by the activation of cell cycle checkpoints, culminating in the phosphorylation of Chk-1 and p53. Strikingly, blockade of S phase entry using the phosphatidylinositol 3-kinase inhibitor LY294002 prevented checkpoint activation and apoptosis. In contrast, inhibitors of mitogen-activated protein kinase cascades failed to prevent apoptosis. These findings demonstrate that the biological effects of PKCdelta can be extended to include positive regulation of G(1) phase cell cycle progression. Importantly, they reveal the existence of a novel, cell cycle-dependent mechanism through which PKCdelta stimulates cell death.

Adenoviridae↗

PubMed2005

Newly identified roles for an old guardian: profound deficiency of the mitotic spindle checkpoint protein BubR1 leads to early aging and infertility.

The mitotic spindle checkpoint ensures proper chromosome segregation, and thereby guards against the deleterious effects of aneuploidy. The protein components of the check-point machinery include evolutionarily conserved proteins such as BubR1. While the molecular and cellular biology of this checkpoint is becoming increasingly clarified, the ultimate consequences for overall health of deficiency of specific components such as BubR1 are much less clear--in part due to the embryonic lethality of complete knockouts. Through a clever combination of hypomorphic and knockout alleles, Baker and colleagues were able to engineer mice with graduated levels of BubR1 protein. In doing so, they established the threshold permitting survival to adulthood, but even more intriguingly, they discovered critical roles for BubR1 in preventing early aging and infertility.

Aging↗

PubMed2005

Mitotic spindle checkpoint inactivation by trichostatin a defines a mechanism for increasing cancer cell killing by microtubule-disrupting agents.

Microtubule-disrupting agents such as the taxanes comprise some of the most clinically useful chemotherapeutic agents and invoke the spindle checkpoint in proliferating cells. A robust spindle checkpoint in turn may forestall mitotic catastrophe, potentially providing a mechanism that permits cancer cells to survive transient exposure to these drugs. Previous reports on G2-M cell cycle progression by histone deacetylase inhibitors suggested a potential role in modulating the therapeutic efficacy of microtubule-disrupting agents. As both classes of agents are generally administered in clinical trials as pulse treatments, we investigated in human cancer cells the effects of brief treatments with the histone deacetylase inhibitor trichostatin A (TSA) alone or with nocodazole or paclitaxel (Taxol) on cell cycle progression and the spindle checkpoint. Treatment of synchronized cells with 200 ng/ml of TSA alone for eight hours to completely block class I and II HDACs did not interfere with progression into mitosis with chromosomal condensation as confirmed by MPM-2 expression. TSA treatment at this concentration surprisingly did not interfere with formation of the mitotic spindle or centrosomal separation, but instead led to missegregation of chromosomes, suggesting effects on the spindle checkpoint. Consistent with this hypothesis, TSA abrogated the phosphorylation and kinetochore localization of the mitotic checkpoint protein BubR1 and the phosphorylation of histone H3 after paclitaxel and nocodazole treatment. These effects in turn led to rapid cell death and considerably reduced clonogenic survival. These results together suggest that by inactivating the spindle checkpoint, TSA can potentiate the lethal effects of microtubule-disrupting drugs, a strategy that might be usefully exploited for optimizing anticancer therapy.

Antineoplastic Agents↗

PubMed2005

Caspase-mediated specific cleavage of BubR1 is a determinant of mitotic progression.

The fidelity of chromosomal duplication is monitored by cell cycle checkpoints operational during mitosis. One such cell cycle delay is invoked by microtubule-targeting agents such as nocodazole or paclitaxel (Taxol) and is mediated by mitotic checkpoint proteins that include BubR1. Relatively little is known about the regulation of expression and stability of BubR1 (or other checkpoint proteins) and how these factors dictate the durability of the cell cycle delay. We report here that treatment of HeLa cells with spindle-disrupting agents resulted in caspase activation and precipitated the cleavage of BubR1. This mechanism ultimately leads to reduced levels of full-length protein, which are accompanied by abrogation of the mitotic block; the checkpoint abrogation is substantially accelerated by inhibition of de novo protein synthesis. In contrast, inhibition of caspase activity blocked BubR1 degradation and prolonged mitosis. To confirm a direct link between caspase activity and BubR1 protein expression, we identified by site-directed mutagenesis the specific caspase cleavage sites cleaved after exposure to paclitaxel. Surprisingly, BubR1 has two sites of cleavage: primarily at Asp607/Asp610 and secondarily at Asp576/Asp579. BubR1 mutated at both locations (BubR1Delta579Delta610) was resistant to paclitaxel-induced degradation. Expression of BubR1Delta579Delta610 augmented the mitotic delay induced by spindle disruption in transfected cells as well as in clones engineered to inducibly express the mutant protein upon exposure to doxycycline and ultimately led to increased aneuploidy. Underscoring the importance of these caspase cleavage sites, both tetrapeptide motifs are identified in the amino acid sequences of human, mouse, chicken, and Xenopus BubR1. These results are potentially the first to link the control of the stability of a key mitotic checkpoint protein to caspase activation, a regulatory pathway that may be involved in killing defective cells and that has been evolutionarily conserved.

Amino Acid Motifs↗

PubMed2004

Pictures, progress, and perplexities: the immediate cell biological effects of ionizing radiation.

An unexpected image made by an invisible beam precipitated the discovery of X-rays in the waning days of the 19th century. The usefulness and dangers of this new discovery soon became clear, emphasizing the importance of DNA damage recognition and repair after exposure to ionizing radiation (IR). One hundred years later, new images of the immediate effects of ionizing radiation have once again ignited excitement among investigators. The combination of advanced imaging techniques with new reagents targeting molecular components of the cellular DNA damage response have enabled visualization of sites of DNA damage, resulting in fresh insights into the immediate cell biological effects of IR. This review will review these advances, highlighting recent progress as well as critical questions that remain unanswered.

Apoptosis↗

PubMed2004

Caspase-mediated specific cleavage of human histone deacetylase 4.

Histone deacetylase 4 (HDAC4) is a class II HDAC implicated in controlling gene expression important for diverse cellular functions, but little is known about how its expression and stability are regulated. We report here that this deacetylase is unusually unstable, with a half-life of less than 8 h. Consistent with the instability of HDAC4 protein, its mRNA was also highly unstable (with a half-life of less than 4 h). The degradation of HDAC4 could be accelerated by exposure of cells to ultraviolet irradiation. HDAC4 degradation was not dependent on proteasome or CRM1-mediated export activity but instead was caspase-dependent and was detectable in diverse human cancer lines. Of two potential caspase consensus motifs in HDAC4, both lying within a region containing proline-, glutamic acid-, serine-, and threonine-rich (PEST) sequences, we identified, by site-directed mutagenesis, Asp-289 as the prime cleavage site. Notably, this residue is not conserved among other class IIa members, HDAC5, -7, and -9. Finally, the induced expression of caspase-cleavable HDAC4 led to markedly increased apoptosis. These results therefore unexpectedly link the regulation of HDAC4 protein stability to caspases, enzymes that are important for controlling cell death and differentiation.

Amino Acid Motifs↗

PubMed2004

Inactivation of the mitotic checkpoint as a determinant of the efficacy of microtubule-targeted drugs in killing human cancer cells.

Drugs that disrupt microtubule dynamics include some of the most important of cancer chemotherapies. While these drugs, which include paclitaxel (Taxol), are known to invoke the mitotic checkpoint, the factors that determine cancer cell killing remain incompletely characterized. Cells that are relatively resistant to killing by these drugs block robustly in mitosis, whereas cells sensitive to killing block only transiently in mitosis before undergoing nuclear fragmentation and death. Passage through mitosis was an absolute requirement of drug-induced death, because death was markedly reduced in cells blocked at both G(1)-S and G(2). Cell killing was at least in part linked to the absence or inactivation of BubR1, a kinetochore-associated phosphoprotein that mediates the mitotic checkpoint. Sensitivity to paclitaxel correlated with decreased BubR1 protein expression in human cancer cell lines, including those derived from breast and ovarian cancers. Silencing of BubR1 via RNA interference inactivated the mitotic checkpoint in drug-resistant cells, and reversed resistance to paclitaxel and nocodazole. Together, these results suggest that the mitotic checkpoint is an important determinant of the efficacy of microtubule-targeting drugs in killing cancer cells, potentially providing novel targets for increasing treatment efficacy.

Antineoplastic Agents, Phytogenic↗

PubMed2003

Histone deacetylase 4 interacts with 53BP1 to mediate the DNA damage response.

Anumber of proteins are recruited to nuclear foci upon exposure to double-strand DNA damage, including 53BP1 and Rad51, but the precise role of these DNA damage-induced foci remain unclear. Here we show in a variety of human cell lines that histone deacetylase (HDAC) 4 is recruited to foci with kinetics similar to, and colocalizes with, 53BP1 after exposure to agents causing double-stranded DNA breaks. HDAC4 foci gradually disappeared in repair-proficient cells but persisted in repair-deficient cell lines or cells irradiated with a lethal dose, suggesting that resolution of HDAC4 foci is linked to repair. Silencing of HDAC4 via RNA interference surprisingly also decreased levels of 53BP1 protein, abrogated the DNA damage-induced G2 delay, and radiosensitized HeLa cells. Our combined results suggest that HDAC4 is a critical component of the DNA damage response pathway that acts through 53BP1 and perhaps contributes in maintaining the G2 cell cycle checkpoint.

Carrier Proteins↗

PubMed2002

Assembly of the SMRT-histone deacetylase 3 repression complex requires the TCP-1 ring complex.

The acetylation of histone tails is a primary determinant of gene activity. Histone deacetylase 3 (HDAC3) requires the nuclear receptor corepressor SMRT for HDAC enzyme activity. Here we report that HDAC3 interacts with SMRT only after priming by cellular chaperones including the TCP-1 ring complex (TRiC), which is required for proper folding of HDAC3 in an ATP-dependent process. SMRT displaces TRiC from HDAC3, yielding an active HDAC enzyme. The SMRT-HDAC3 repression complex thus joins the VHL-elongin BC tumor suppression complex and the cyclin E-Cdk2 cell cycle regulation complex as critical cellular machines requiring TRiC for proper assembly and function. The strict control of HDAC3 activity underscores the cellular imperative that histone deacetylation occur only in targeted regions of the genome.

Adenosine Triphosphate↗

PubMed2002

Coupling of DNA synthesis and histone synthesis in S phase independent of cyclin/cdk2 activity.

DNA and histone synthesis are both triggered at the beginning of S phase by cyclin/cdk2 activity. Previous studies showed that inhibition of DNA synthesis with hydroxyurea or cytosine arabinoside (AraC) triggers a concerted repression of histone synthesis, indicating that sustained histone synthesis depends on continued DNA synthesis. Here we show that ectopic expression of HIRA, the likely human ortholog of two cell cycle-regulated repressors of histone gene transcription in yeast (Hir1p and Hir2p), represses transcription of histones and that this, in turn, triggers a concerted block of DNA synthesis. Thus, in mammalian cells sustained DNA synthesis and histone synthesis are mutually dependent on each other during S phase. Although cyclin/cdk2 activity drives activation of both DNA and histone synthesis at the G1/S transition of cycling cells, concerted repression of DNA or histone synthesis in response to inhibition of either one of these is not accompanied by prolonged inhibition of cyclin A/cdk2 or E/cdk2 activity. Therefore, during S phase coupling of DNA and histone synthesis occurs, at least in part, through a mechanism that is independent of cyclin/cdk2 activity. Coupling of DNA and histone synthesis in S phase presumably contributes to the prompt and orderly assembly of newly replicated DNA into chromatin.

Antigens, CD19↗