A cell cycle analysis of growth-related genes expressed during T lymphocyte maturation

Elevated ribonucleotide reductase levels associate with suppressed radiochemotherapy response in human cervical cancers

OBJECTIVE

Ribonucleotide reductase (RNR) supplies deoxyribonucleotide diphosphates demanded by cells to repair radiation-induced DNA damage. Here, we investigate the impact of pretherapy RNR M1, M2, and M2b (p53R3) subunit level upon human cervical cancer radiochemosensitivity.

MATERIALS/METHODS

Immunohistochemistry was performed on a tissue array comprised of 18 paired benign uterine cervix and stage IB2 cervical cancers to evaluate the relationship between cytosolic RNR M1, M2, and M2b staining intensity and radiochemotherapy cancer response. Patients underwent surgical hysterectomy (n = 8), or daily radiation (45 Gy), coadministered once-weekly cisplatin (40 mg/m), then low-dose rate brachytherapy (30 Gy) followed by adjuvant hysterectomy (n = 10). Radiochemotherapy response was determined by Response Evaluation Criteria In Solid Tumors version 1.0 criteria during brachytherapy. Cancer relapse rates and disease-free survival were calculated.

RESULTS

M1, M2, and M2b antibody staining intensity was low (0-1+) in benign uterine cervical tissue. M1 and M2b immunoreactivity was 2+ or 3+ in most (13/18) cervical cancers. M2 immunoreactivity was 3+ in nearly all (16/18) cervical cancers. Cervical cancers overexpressing M1 and M2b had an increased hazard for incomplete radiochemotherapy response, relapse, and shortened disease-free survival.

CONCLUSIONS

Ribonucleotide reductase subunit levels may predict human cervical cancer radiochemosensitivity and subsequent posttherapy cancer outcome. Further validation testing of RNR subunits as biomarkers for radiochemotherapy response is warranted.

Regulation of ribonucleotide reductase M2 expression by the upstream AUGs

Ribonucleotide reductase catalyzes a rate-limiting reaction in DNA synthesis by converting ribonucleotides to deoxyribonucleotides. It consists of two subunits and the small one, M2 (or R2), plays an essential role in regulating the enzyme activity and its expression is finely controlled. Changes in the M2 level influence the dNTP pool and, thus, DNA synthesis and cell proliferation. M2 gene has two promoters which produce two major mRNAs with 5′-untranslated regions (5′-UTRs) of different lengths. In this study, we found that the M2 mRNAs with the short (63 nt) 5′-UTR can be translated with high efficiency whereas the mRNAs with the long (222 nt) one cannot. Examination of the long 5′-UTR revealed four upstream AUGs, which are in the same reading frame as the unique physiological translation initiation codon. Further analysis demonstrated that these upstream AUGs act as negative cis elements for initiation at the downstream translation initiation codon and their inhibitory effect on M2 translation is eIF4G dependent. Based on the findings of this study, we conclude that the expression of M2 is likely regulated by fine tuning the translation from the mRNA with a long 5′-UTR during viral infection and during the DNA replication phase of cell proliferation.

Restriction of de novo nucleotide biosynthesis interferes with clonal expansion and differentiation into effector and memory CD8 T cells

Nucleotide synthesis inhibitors are currently used in neoplastic diseases or as immunosuppressive agents for the prevention of acute rejection in organ transplantation and the treatment of autoimmune disorders. We have previously described that these inhibitors interfere with proliferation and survival of primary T cells in vitro. However, the precise effects of nucleotide restriction on effector and memory functions have not been elucidated. In this study, we investigated the impact of nucleotide synthesis inhibition on CD8 T cell differentiation by using TCR transgenic mice (F5) specific for the influenza virus nucleoprotein 68 peptide presented on the H-2Db molecule. Our results show that methotrexate and 5-fluorouracil prevent the acquisition of effector functions, such as IFN-gamma, granzyme B expression, and cytotoxic function following antigenic stimulation of naive cells. Surprisingly, in the presence of mycophenolate mofetil, activated F5 cells are still able to produce granzyme B and to kill target cells but to a lesser extent compared with control. All three inhibitors interfere with the differentiation of naive cells into memory CD8 T cells. In contrast, the drugs are unable to inhibit the development of improved cytotoxic functions displayed by memory CD8 T cells.

Polyglutamation of methotrexate with common polymorphisms in reduced folate carrier, aminoimidazole carboxamide ribonucleotide transformylase, and thymidylate synthase are associated with methotrexate effects in rheumatoid arthritis

OBJECTIVE

Methotrexate (MTX) enters cells through the reduced folate carrier (RFC-1) and exerts part of its effects through polyglutamation to MTX polyglutamates (MTXPGs) and inhibition of 5-aminoimidazole-4-carboxamide ribonucleotide transformylase (ATIC) and thymidylate synthase (TS). We investigated the contribution of common genetic polymorphisms in RFC-1 (G80A), ATIC (C347G), and TS (28-bp tandem repeats located in the TS enhancer region [TSER*2/*3]) and of MTXPGs to the effect of MTX in patients with rheumatoid arthritis.

METHODS

The study was cross-sectional. All patients received MTX for at least 3 months. The numbers of tender and swollen joints, the Visual Analog Scale (VAS) scores for the physician's global assessment of disease activity, and the modified Health Assessment Questionnaire scores were collected. Using the VAS score for the physician's assessment of patient's response to MTX, the population of patients was dichotomized into responders to MTX (VAS score < or =2 cm) and nonresponders to MTX (VAS score >2 cm). A pharmacogenetic index was calculated as the sum of homozygous variant genotypes (RFC-1 AA + ATIC 347GG + TSER *2/*2) carried by the patients. MTXPG concentrations were measured in red blood cells (RBCs) by high-performance liquid chromatography.

RESULTS

The dose of MTX was not associated with the effects of MTX (P > 0.05). In contrast, increased RBC long-chain MTXPG concentrations (median 40 nmoles/liter; range <5-131 nmoles/liter) and an increased pharmacogenetic index were associated with a lower number of tender and swollen joints (P < 0.05) and a lower score for the physician's global assessment of disease activity (P < or = 0.001). Patients with RBC MTXPG levels of >60 nmoles/liter and carriers of a homozygous variant genotype were 14.0-fold (95% confidence interval [95% CI] 3.6-53.8) and 3.7-fold (95% CI 1.7-9.1), respectively, more likely to have a good response to MTX (P <or = 0.01).

CONCLUSION

These data suggest that measuring RBC MTXPG levels and/or the common polymorphisms in the folate-purine-pyrimidine pathway may help in monitoring MTX therapy.

Overexpression of the R2 subunit of ribonucleotide reductase in human nasopharyngeal cancer cells reduces radiosensitivity

PURPOSE

Ribonucleotide reductase is the rate-limiting enzyme in the de novo synthesis of deoxyribonucleotide triphosphates, which are utilized in both DNA synthesis and DNA repair. We reported previously that RR enzyme activity and R2 (catalytic subunit of RR) protein levels were increased after exposure to ionizing radiation (IR) in growth-arrested human tumor cells, suggesting that R2 protein expression regulates RR activity to allow for IR damage repair. Using isogenic human nasopharyngeal carcinoma cells in this study, we examine the relationship of overexpression of either the R1 regulatory subunit or the R2 catalytic subunit of RR to the cellular response of IR damage.

MATERIALS AND METHODS

We used three isogenic human nasopharyngeal cancer cell lines previously derived by Zhou et al, including KB, the parental tumor cell line; KB/M1, an R1 protein-overexpressing clone stably transfected with human R1 complementary DNA; and KB/M2, a R2 protein-overexpressing clone stably transfected with human R2 complementary DNA. We initially characterized these isogenic human tumor cell lines in exponential growth for R2 protein expression, RR enzyme activity, and R2 protein changes during the cell cycle by flow cytometry. Subsequently, the IR response in these cell lines was determined by clonogenic survival, cell cycle changes occurring after IR, and an analysis of IR DNA damage determined by pulsed field gel electrophoresis. The effect of combining IR and hydroxyurea, a RR (R2) inhibitor, was also studied in KB and KB/M2 cells.

RESULTS

KB/M2 cells were found to have 4.5-fold higher R2 protein expression and a threefold higher RR enzyme activity in exponential growth than KB and KB/M1. Although R2 protein levels increased at the G1/S transition in all cell lines, KB/M2 cells also demonstrated consistently higher R2 protein levels throughout the cell cycle. Using a linear-quadratic analysis of IR clonogenic survival data, KB/M2 cells were more radioresistant than KB and KB/M1 cells, including both decreased alpha and decreased beta values, a finding that correlates with increased reparable IR damage. KB/M2 cells also show a reduced G2 cell cycle arrest and fewer DNA double strand breaks 18 hours after IR (6 Gy). Exposure of KB/M2 cells to hydroxyurea (300 microM) after exposure to IR restored in vitro radiosensitivity in a manner similar to that found in KB and KB/M1 cells.

DISCUSSION

An increase in R2 protein levels and RR activity in KB/M2 cells results in IR resistance, which appears mediated by enhanced IR damage repair during G2. R1 protein overexpression in these isogenic human tumor cells (KB/M1) did not affect RR activity or IR response.

Differential control of cell cycle, proliferation, and survival of primary T lymphocytes by purine and pyrimidine nucleotides

Purine and pyrimidine nucleotides play critical roles in DNA and RNA synthesis as well as in membrane lipid biosynthesis and protein glycosylation. They are necessary for the development and survival of mature T lymphocytes. Activation of T lymphocytes is associated with an increase of purine and pyrimidine pools. However, the question of how purine vs pyrimidine nucleotides regulate proliferation, cell cycle, and survival of primary T lymphocytes following activation has not yet been specifically addressed. This was investigated in the present study by using well-known purine (mycophenolic acid, 6-mercaptopurine) and pyrimidine (methotrexate, 5-fluorouracil) inhibitors, which are used in neoplastic diseases or as immunosuppressive agents. The effect of these inhibitors was analyzed according to their time of addition with respect to the initiation of mitogenic activation. We showed that synthesis of both purine and pyrimidine nucleotides is required for T cell proliferation. However, purine and pyrimidine nucleotides differentially regulate the cell cycle since purines control both G(1) to S phase transition and progression through the S phase, whereas pyrimidines only control progression from early to intermediate S phase. Furthermore, inhibition of pyrimidine synthesis induces apoptosis whatever the time of inhibitor addition whereas inhibition of purine nucleotides induces apoptosis only when applied to already cycling T cells, suggesting that both purine and pyrimidine nucleotides are required for survival of cells committed into S phase. These findings reveal a hitherto unknown role of purine and pyrimidine de novo synthesis in regulating cell cycle progression and maintaining survival of activated T lymphocytes.

The RAG-1/2 endonuclease causes genomic instability and controls CNS complications of lymphoblastic leukemia in p53/Prkdc-deficient mice

Double-strand DNA breaks (DSB) induce chromosomal translocations and gene amplification in cell culture, but mechanisms by which DSB cause genomic instability in vivo are poorly understood. We show that RAG-1/2-induced DSB cause IgH/c-Myc translocations in leukemic pro-B cells from p53/Prkdc-deficient mice. Strikingly, these translocations were complex, clonally heterogeneous and amplified. We observed reiterated IgH/c-Myc fusions on dicentric chromosomes, suggesting that amplification occurred by repeated cycles of bridge, breakage and fusion. Leukemogenesis was not mitigated in RAG-2/p53/Prkdc-deficient mice, but leukemic pro-B cells lacked IgH/c-Myc translocations. Thus, global genomic instability conferred by p53/Prkdc disruption efficiently transforms pro-B cells lacking RAG-1/2-induced DSB. Unexpectedly, RAG-2/p53/Prkdc-deficient mice also developed leptomeningeal leukemia, providing a novel spontaneous model for this frequent complication of human lymphoblastic malignancies.

Decreased expression of c-myc family genes in thymuses from myasthenia gravis patients

The thymus is a critical organ for the elimination of autoreactive T cells by apoptosis. We studied the expression of apoptosis-associated genes, bcl-xL, bad, caspase-3, and c-myc family genes in myasthenia gravis (MG) thymuses. We observed that the mRNA levels of myc family genes, c-myc and max, were markedly reduced in MG thymuses. These results indicate that c-myc-mediated signaling is abnormal in MG thymuses. The levels of molecules whose expressions are associated with myc, such as STAM, prothymosin-alpha, and NFkappaB, were also analyzed.

Distinct signals mediate maturation and allelic exclusion in lymphocyte progenitors

Successful in-frame rearrangement of immunoglobulin heavy chain genes or T cell antigen receptor (TCR) beta chain genes in lymphocyte progenitors results in formation of pre-BCR and pre-TCR complexes. These complexes signal progenitor cells to mature, expand in cell number, and suppress further rearrangements at the immunoglobulin heavy chain or TCRbeta chain loci, thereby ensuring allelic exclusion. We used transgenic expression of a constitutively active form of c-Raf-1 (Raf-CAAX) to demonstrate that activation of the Map kinase pathway can stimulate both maturation and expansion of B and T lymphocytes, even in the absence of pre-TCR or pre-BCR formation. However, the same Raf signal did not mediate allelic exclusion. We conclude that maturation of lymphocyte progenitors and allelic exclusion require distinct signals.

A potential role for Elf-1 in terminal transferase gene regulation

The terminal deoxynucleotidyltransferase (TdT) gene represents an attractive model for the analysis of gene regulation during an early phase of lymphocyte development. In previous studies, we identified a DNA element, termed D', which is essential for TdT promoter activity in immature lymphocytes, and two classes of D'-binding factors, Ikaros proteins and Ets proteins. Here, we report a detailed mutant analysis of the D' element which suggests that an Ets protein, rather than an Ikaros protein, activates TdT transcription. Since multiple Ets proteins are expressed in developing lymphocytes and are capable of binding to the D' element, DNA affinity chromatography was used to determine if one of the Ets proteins might bind to the D' element with a uniquely high affinity, thereby implicating that protein as a potential TdT activator. Indeed, one binding activity was greatly enriched in the high-salt eluates from a D' affinity column. Peptide microsequencing revealed that the enriched protein was Elf-1. Immunoblot analyses confirmed that in nuclear extracts, Elf-1 has a significantly higher affinity for the D' sequence than does another Ets protein, Ets-1. Transactivation and expression studies support the hypothesis that Elf-1 activates TdT transcription in immature T and B cells. Finally, a D' mutation which selectively reduces Elf-1 binding, but not the binding of other Ets proteins, was found to greatly reduce TdT promoter activity. Although Elf-1 previously had been implicated in the inducible activation of genes in mature T and B cells, our results suggest that it also plays an important role in regulating genes during an early phase of lymphocyte development.

Developmental control of the G1 to S transition in Drosophila: cyclin Eis a limiting downstream target of E2F

The E2F transcription factor is required for S phase in Drosophila. While it also triggers expression of replication genes at the G1-S transition, the relevance of this transcription is not clear because many of the induced gene products are sufficiently stable that new expression is not required for S phase. However, one unstable product could couple S phase to E2F activation. Here we show that cyclin E expression at G1-S requires E2F, that activation of E2F without cyclin E is not sufficient for S phase, and that early in G1 ectopic expression of cyclin E alone can bypass E2F and induce S phase. We conclude that cyclin E is the downstream gene that couples E2F activity to G1 control. Not all embryonic cycles are similarly coupled to E2F activation, however. The rapidly proliferating CNS cells, which exhibit no obvious G1, express cyclin E constitutively and independently to E2F. Instead, cyclin E expression activates E2F in the CNS. Thus, this tissue-specific E2F-independent transcription of cyclin E reverses the hierarchical relationship between cyclin E and E2F. Both hierarchies activate expression of the full complement of replication functions controlled by E2F; however, whereas inactivation of E2F can produce a G1 when cyclin E is downstream of E2F, we propose that an E2F-independent source of E eliminates G1.

V(D)J recombination and the cell cycle

V(D)J recombination is a major source of antigen receptor diversity and represents the only known form of site-specific DNA rearrangement in vertebrates. V(D)J recombination is initiated by specific DNA cleavage at recombinational signal sequences and requires components of the general machinery used for double-strand (DS)-break repair. The involvement of DS cleavage and repair mechanisms suggests that V(D)J recombination might be coupled to the cell cycle, as introduction or persistence of DS breaks during DNA replication or mitosis could interfere with faithful transmission of genetic information to daughter cells. Here, Weei-Chin Lin and Stephen Desiderio review recent evidence indicating that this is indeed the case and consider some biological implications of this linkage.

Inappropriate transcription from the 5' end of the murine dihydrofolate reductase gene masks transcriptional regulation

Using the nuclear run-on assay we found that in proliferating cells the transcription rate in the 5' end of the murine dihydrofolate reductase (dhfr) gene was approximately ten-fold higher than in the 3' end of the gene, suggesting transcriptional attenuation within the dhfr gene. However, when the transcription rate was measured by pulse-labeling, the rate was uniform throughout the gene, and the 5' dhfr signal was approximately ten-fold lower relative to a control gene signal than in the run-on assay. Previously, the activity of a dhfr promoter linked to a luciferase reporter gene was shown to increase about ten-fold at the G1/S-phase boundary following stimulation of serum-starved cells. To determine if the run-on procedure would detect growth regulation of the endogenous dhfr gene, serum-starved and -stimulated NIH 3T3 cells were analyzed. Using a dhfr 5' end probe no difference in transcription rate between these growth states was detected and the dhfr 3' end probe did not detect signal above background. In a cell line that was amplified at the dhfr locus, the transcription rate in the 5' end of the gene increased less than two-fold in stimulated cells, but the rate in the 3' end of the gene increased five- to seven-fold. Therefore, the dhfr gene is growth regulated at the level of transcription, but the nuclear run-on assay was only able to detect a difference in transcription rate in the 3' end of the gene in amplified cells. We suggest that isolation of nuclei may activate dhfr transcription complexes that normally are activated only at the G1/S-phase boundary.

Probing the interaction of the multidrug-resistance phenotype with the polypeptide ionophore gramicidin D via functional channel formation

It has been proposed that the multidrug resistance (MDR) transporter, P-glycoprotein (P-170), may be physiologically involved in the transport of polypeptides. As a step towards understanding the interaction of P-170 with polypeptides, we isolated various gramicidin-D-resistant mammalian cell lines. Gramicidin D is a hydrophobic pentadecapeptide ionophore that forms proton and alkali metal cation-permeable channels in lipid bilayers. Gramicidin-D-resistant cells displayed a prominent MDR gene amplification, P-170 overexpression, reduced drug accumulation, and consequent resistance to MDR-type cytotoxic agents. Modulators of the MDR phenotype, including verapamil, reserpine and quinidine, rendered these cells sensitive to gramicidin D. Using these cell lines, we established an assay that probes for the intra-membranal interaction between P-170 and gramicidin D. Gramicidin-D channel formation was followed by cellular accumulation of 86Rb+. Ionophore-resistant cells, and other MDR cells, did not show an appreciable increase in 86Rb+ influx rates, in the presence of increasing gramicidin-D concentrations. In contrast, parental cells displayed a dose-dependent increase in the 86Rb+ influx rates. Interestingly, in the absence of serum, gramicidin-D-resistant cells resumed the wild-type, ionophore-dose-dependent increase in 86Rb+ influx rates. MDR modulators caused a resumption of channel formation in ionophore-resistant cells. We conclude that acquisition of the MDR phenotype is an efficient means of cellular protection against gramicidin D. Hence, a new approach is offered in which P-170 interaction with gramicidin D is quantitatively followed by a rapid assessment of the biological activity (i.e. channel formation) of the substrate itself. Possible mechanisms of P-170 interaction with free ionophore monomers, and membrane-associated gramicidin D are discussed.

Cell cycle regulation of V(D)J recombination-activating protein RAG-2

The antigen receptors of B and T lymphocytes are encoded in multiple germ-line DNA segments that are joined during lymphocyte development. The recombination-activating proteins RAG-1 and RAG-2 are both essential for this process, termed V(D)J rearrangement. Phosphorylation of the RAG-2 protein at Thr-490 by one or more cyclin-dependent kinases is associated with its rapid degradation. In an immature B-cell line and in normal thymocytes, RAG-2 protein accumulates preferentially in the G0/G1 phases of the cell cycle and declines by at least 20-fold before cells enter S phase. The amount of RAG-2 protein remains low throughout the S, G2, and M phases. The amount of RAG-1 protein shows considerably less fluctuation. The variation in RAG-2 protein is likely to be established, at least in part, by a posttranscriptional mechanism. These observations suggest that V(D)J rearrangement occurs entirely or preferentially within G0/G1.

MCB elements and the regulation of DNA replication genes in yeast

In eukaryotic organisms, genes involved in DNA replication are often subject to some form of cell cycle control. In the yeast Saccharomyces cerevisiae, most of the DNA replication genes that have been characterized to date are regulated at the transcriptional level during G1 to S phase transition. A cis-acting element termed the MluI cell cycle box (or MCB) conveys this pattern of regulation and is common among more than 20 genes involved in DNA synthesis and repair. Recent findings indicate that the MCB element is well conserved among fungi and may play a role in controlling entry into the cell division cycle. It is evident from studies in higher systems, however, that transcriptional regulation is not the only form of control that governs the cell-cycle-dependent expression of DNA replication genes. Moreover, it is unclear why this general pattern of regulation exists for so many of these genes in various eukaryotic systems. This review summarizes recent studies of the MCB element in yeast and briefly discusses the purpose of regulating DNA replication genes in the eukaryotic cell cycle.

A rat gene with sequence homology to the Drosophila gene hairy is rapidly induced by growth factors known to influence neuronal differentiation

Several genes encoding transcription factors with a helix-loop-helix (HLH) motif are involved in the early process of neural development in Drosophila spp. We report the isolation from the rat a homolog of one of these genes, called hairy. The rat-hairy-like (RHL) gene is expressed early during embryogenesis. In contrast to the restricted expression of hairy mRNA in Drosophila spp., however, the mRNA encoded by RHL is detectable in all tissues examined. Stimulation of PC12 pheochromocytoma cells by nerve growth factor, basis fibroblast growth factor, or epidermal growth factor or of Rat-1 fibroblasts by epidermal growth factor causes a rapid and transient induction of the RHL gene. Thus, RHL acts as an immediate-early gene that can potentially transduce growth factor signals during the development of the mammalian embryo.

A rat gene with sequence homology to the Drosophila gene hairy is rapidly induced by growth factors known to influence neuronal differentiation

Several genes encoding transcription factors with a helix-loop-helix (HLH) motif are involved in the early process of neural development in Drosophila spp. We report the isolation from the rat a homolog of one of these genes, called hairy. The rat-hairy-like (RHL) gene is expressed early during embryogenesis. In contrast to the restricted expression of hairy mRNA in Drosophila spp., however, the mRNA encoded by RHL is detectable in all tissues examined. Stimulation of PC12 pheochromocytoma cells by nerve growth factor, basis fibroblast growth factor, or epidermal growth factor or of Rat-1 fibroblasts by epidermal growth factor causes a rapid and transient induction of the RHL gene. Thus, RHL acts as an immediate-early gene that can potentially transduce growth factor signals during the development of the mammalian embryo.

Ribonucleotide reductase gene expression during cyclic AMP-induced cell cycle arrest in T lymphocytes

In both 3T3 mouse fibroblasts and S49 mouse T lymphocytes the genes encoding both subunits of ribonucleotide reductase are expressed beginning in late G1 phase. In studies reported here, we compared the expression of the genes that code for the M1 and M2 subunits of ribonucleotide reductase in S49 cells, which are arrested in G1 phase by agents that increase cyclic AMP, with those from CEM human T lymphoma cells that are unaffected by exposure to dibutyryl cyclic AMP. Dibutyryl cyclic AMP treatment results in a prompt steady diminution of M2 mRNA concentration to levels at or below that of elutriated G1 cell-cycle-specific populations in S49 cells, in contrast to CEM cell M2 mRNA, which is unchanged. M1 mRNA concentration decreases more slowly than M2 mRNA in S49 cells and marginally, if at all, in CEM cells. The time course of diminution of the M2 message concentration by dibutyryl cyclic AMP in S49 cells is similar to that obtained when cells are treated with actinomycin D and to the combination of the two agents. This suggests that cyclic AMP and actinomycin D may act similarly on ribonucleotide reductase gene expression. Furthermore, cycloheximide pretreatment diminishes the effect of dibutyryl cyclic AMP, indicating that the effect might be mediated by a labile protein. Transcription runoff assays suggest a diminution of transcription rate for the M2 gene in S49 cells treated with dibutyryl cyclic AMP and a transient decline in the M1 transcription rate. These data suggest that dibutyryl cyclic AMP diminishes the transcription of ribonucleotide reductase genes in sensitive cells and that this and the short half-life of the M2 message are major factors in the disappearance of the M2 messenger RNA from dibutyryl cyclic AMP-treated cells although other mechanisms may also play a role. These events clearly precede any alteration in cell cycle distribution and thus they may contribute to G1 arrest.