An in vivo study on the synchronizing effect of hydroxyurea

Enhancing [177Lu]Lu-DOTA-TATE therapeutic efficacy in vitro by combining it with metronomic chemotherapeutics

BACKGROUND

Peptide receptor radionuclide therapy (PRRT) uses [177Lu]Lu-[DOTA0-Tyr3]octreotate ([177Lu]Lu-DOTA-TATE) to treat patients with neuroendocrine tumours (NETs) overexpressing the somatostatin receptor 2A (SSTR2A). It has shown significant short-term improvements in survival and symptom alleviation, but there remains room for improvement. Here, we investigated whether combining [177Lu]Lu-DOTA-TATE with chemotherapeutics enhanced the in vitro therapeutic efficacy of [177Lu]Lu-DOTA-TATE.

RESULTS

Transfected human osteosarcoma (U2OS + SSTR2A, high SSTR2A expression) and pancreatic NET (BON1 + STTR2A, medium SSTR2A expression) cells were subjected to hydroxyurea, gemcitabine or triapine for 24 h at 37oC and 5% CO2. Cells were then recovered for 4 h prior to a 24-hour incubation with 0.7-1.03 MBq [177Lu]Lu-DOTA-TATE (25 nM) for uptake and metabolic viability studies. Incubation of U2OS + SSTR2A cells with hydroxyurea, gemcitabine, and triapine enhanced uptake of [177Lu]Lu-DOTA-TATE from 0.2 ± 0.1 in untreated cells to 0.4 ± 0.1, 1.1 ± 0.2, and 0.9 ± 0.2 Bq/cell in U2OS + SSTR2A cells, respectively. Cell viability post treatment with [177Lu]Lu-DOTA-TATE in cells pre-treated with chemotherapeutics was decreased compared to cells treated with [177Lu]Lu-DOTA-TATE monotherapy. For example, the viability of U2OS + SSTR2A cells incubated with [177Lu]Lu-DOTA-TATE decreased from 59.5 ± 22.3% to 18.8 ± 5.2% when pre-treated with hydroxyurea. Control conditions showed no reduced metabolic viability. Cells were also harvested to assess cell cycle progression, SSTR2A expression, and cell size by flow cytometry. Chemotherapeutics increased SSTR2A expression and cell size in U2OS + SSTR2A and BON1 + STTR2A cells. The S-phase sub-population of asynchronous U2OS + SSTR2A cell cultures was increased from 45.5 ± 3.3% to 84.8 ± 2.5%, 85.9 ± 1.9%, and 86.6 ± 2.2% when treated with hydroxyurea, gemcitabine, and triapine, respectively.

CONCLUSIONS

Hydroxyurea, gemcitabine and triapine all increased cell size, SSTR2A expression, and [177Lu]Lu-DOTA-TATE uptake, whilst reducing cell metabolic viability in U2OS + SSTR2A cells when compared to [177Lu]Lu-DOTA-TATE monotherapy. Further investigations could transform patient care and positively increase outcomes for patients treated with [177Lu]Lu-DOTA-TATE.

Fludarabine increases nuclease-free AAV- and CRISPR/Cas9-mediated homologous recombination in mice

Homologous recombination (HR)-based gene therapy using adeno-associated viruses (AAV-HR) without nucleases has several advantages over classic gene therapy, especially the potential for permanent transgene expression. However, the low efficiency of AAV-HR remains a major limitation. Here, we tested a series of small-molecule compounds and found that ribonucleotide reductase (RNR) inhibitors substantially enhance AAV-HR efficiency in mouse and human liver cell lines approximately threefold. Short-term administration of the RNR inhibitor fludarabine increased the in vivo efficiency of both non-nuclease- and CRISPR/Cas9-mediated AAV-HR two- to sevenfold in the murine liver, without causing overt toxicity. Fludarabine administration induced transient DNA damage signaling in both proliferating and quiescent hepatocytes. Notably, the majority of AAV-HR events occurred in non-proliferating hepatocytes in both fludarabine-treated and control mice, suggesting that the induction of transient DNA repair signaling in non-dividing hepatocytes was responsible for enhancing AAV-HR efficiency in mice. These results suggest that use of a clinically approved RNR inhibitor can potentiate AAV-HR-based genome-editing therapeutics.

A systemic cell cycle block impacts stage-specific histone modification profiles during Xenopus embryogenesis

Forming an embryo from a zygote poses an apparent conflict for epigenetic regulation. On the one hand, the de novo induction of cell fate identities requires the establishment and subsequent maintenance of epigenetic information to harness developmental gene expression. On the other hand, the embryo depends on cell proliferation, and every round of DNA replication dilutes preexisting histone modifications by incorporation of new unmodified histones into chromatin. Here, we investigated the possible relationship between the propagation of epigenetic information and the developmental cell proliferation during Xenopus embryogenesis. We systemically inhibited cell proliferation during the G1/S transition in gastrula embryos and followed their development until the tadpole stage. Comparing wild-type and cell cycle-arrested embryos, we show that the inhibition of cell proliferation is principally compatible with embryo survival and cellular differentiation. In parallel, we quantified by mass spectrometry the abundance of a large set of histone modification states, which reflects the developmental maturation of the embryonic epigenome. The arrested embryos developed abnormal stage-specific histone modification profiles (HMPs), in which transcriptionally repressive histone marks were overrepresented. Embryos released from the cell cycle block during neurulation reverted toward normality on morphological, molecular, and epigenetic levels. These results suggest that the cell cycle block by HUA alters stage-specific HMPs. We propose that this influence is strong enough to control developmental decisions, specifically in cell populations that switch between resting and proliferating states such as stem cells.

Method to Synchronize Cell Cycle of Human Pluripotent Stem Cells without Affecting Their Fundamental Characteristics

Cell cycle progression and cell fate decisions are closely linked in human pluripotent stem cells (hPSCs). However, the study of these interplays at the molecular level remains challenging due to the lack of efficient methods allowing cell cycle synchronization of large quantities of cells. Here, we screened inhibitors of cell cycle progression and identified nocodazole as the most efficient small molecule to synchronize hPSCs in the G2/M phase. Following nocodazole treatment, hPSCs remain pluripotent, retain a normal karyotype and can successfully differentiate into the three germ layers and functional cell types. Moreover, genome-wide transcriptomic analyses on single cells synchronized for their cell cycle and differentiated toward the endoderm lineage validated our findings and showed that nocodazole treatment has no effect on gene expression during the differentiation process. Thus, our synchronization method provides a robust approach to study cell cycle mechanisms in hPSCs.

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Nocodazole can enrich cells in the G2/M, G1, and S phases of the cell cycle

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Treatment with nocodazole does not affect pluripotency maintenance

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hPSCs can efficiently form functional cell types after nocodazole treatment

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Nocodazole treatment allows genome-wide analyses of synchronous populations

Stem cell factor and NSC87877 synergism enhances c-Kit mediated proliferation of human erythroid cells

The biological mechanisms underlying the effects of stem cell factor (SCF) and an inhibitor, NSC87877 (N) of the c-Kit negative regulator (SHP-1 and SHP-2) on cell proliferation are different. Therefore, we compared the cell's response to these two either alone or in combination in K562 cells. Binding of SCF (S) to c-Kit induces dimerization that activates its kinase activity. The activated c-Kit undergoes autophosphorylation at tyrosine residues that serve as a docking site for signal transduction molecules containing SH2 domains. Predominantly, the phosphotyrosine 568 (pY568) in Juxtamembrane (JM) region of c-Kit interacts with adaptor protein APS, Src family kinase, and SHP-2, while phosphotyrosine 570 (pY570) interacts with the SHP-1 and the adaptor protein Shc. The dephosphorylation of phosphotyrosine residues by SHP-1/SHP-2 leads to inhibition of c-Kit proliferative signaling. A chemical molecule, N is reported to inhibit the enzymatic activity of SHP-1/SHP-2, but its effect on c-Kit-mediated proliferation has not been studied yet. Thus, this work aims at examining the effect of the combination of S and N on cells growth as compared to individual treatment. The present study is performed with erythroleukemic K562 cells, chosen for its mRNA expression concerning the c-Kit, and SHP-1/SHP-2. Interestingly, proliferation assay showed that combination significantly increased proliferation when G₁ sorted K562 cells were used. These changes were significantly higher when K562 cells were initially treated with N followed by S treatment. Collectively, these results give mechanistic insight into the proliferation enhancement of bone marrow transplantation through the synergistic effect of S and N by inhibiting SHP-1/SHP-2. The study gives solid evidence that S and N combination can be used to enhance cell proliferation/growth.

Sustained Regression of Hydroxycarbamide Induced Actinic Keratoses after Switching to Anagrelide

Hydroxycarbamide (HC) is the first-line treatment for certain myeloproliferative neoplasms, such as polycythemia vera and essential thrombocytosis (ET). In a subset of these patients long-term treatment with HC can result in the development of confluent actinic keratoses (AK) followed by invasive keratinocytic carcinomas (“squamous dysplasia”), preferentially on sun-exposed skin. Discontinuation or dose reduction of HC may result in partial improvement. A 59-year-old farmer after 14 years on HC (2 gr/d) and acetylsalicylic acid (100 mg/d) for ET, was referred for numerous, hyperkeratotic AK on face, scalp, and hands that could not be controlled with repeated (N=15) cryosurgery sessions in the previous 3 years. Acitretin (0.32 mg/kg daily) and topical treatments (cryosurgery with ingenol mebutate) were initiated with only marginal improvement after 3 months. Acitretin dose was doubled and HC was switched to anagrelide (0.5 mg twice daily). Within a month the AK load regressed significantly and, at 3 months follow-up, complete clinical remission was achieved and acitretin was discontinued. Twenty months later the patient is clear from AK. In conclusion, the impressive and sustainable AK remission under anagrelide draws attention to a possible role of the phosphodiesterase 3 pathway, the major pharmacological target of anagrelide, as a potential therapeutic target for keratinocytic cancers.

Digitoxin and its synthetic analog MonoD have potent antiproliferative effects on lung cancer cells and potentiate the effects of hydroxyurea and paclitaxel

Despite significant advances in the understanding of lung cancer biology, the prognosis of cancer patients remains poor. Part of the failure of anticancer therapy is due to intratumoral heterogeneity in these patients that limits the efficacy of single agents. Therefore, there is an urgent need for new anticancer drugs or drug combination regimens that possess increased activity against all cellular subtypes found within the tumor. In this study, we evaluated the in vitro antiproliferative activity of the cardiac glycosides (CGs) digitoxin and its synthetic analog MonoD on H460 lung cancer cells grown under different culture conditions. The CGs were tested alone in H460 cells under routine culture as well as in cells growing under short (24–72 h) and prolonged serum starvation (7 days) in order to evaluate the activity of drugs on cancer cells under varied degrees of proliferation. Our results showed that both CGs, and MonoD in particular, have potent antiproliferative activity at clinically relevant concentrations against cells in all the tested culture conditions. In contrast, paclitaxel, hydroxyurea and colchicine were only active in cells growing in routine culture conditions, and relatively inactive in serum-starved conditions. Importantly, both CGs were able to potentiate the effect of clinically relevant concentrations of hydroxyurea or paclitaxel in serum-starved conditions. When paclitaxel was used in combination with CGs, the highest antiproliferative effect was obtained when paclitaxel was administered first, followed by either digitoxin or MonoD. Our results indicate that CGs have potential clinical applications in translational oncology especially in combination with other drugs, and warrants further investigation of CGs in more advanced preclinical models of lung cancer.

A novel role for non-ubiquitinated FANCD2 in response to hydroxyurea-induced DNA damage

Fanconi anemia (FA) is a genetic disease of bone marrow failure, cancer susceptibility, and sensitivity to DNA crosslinking agents. FANCD2, the central protein of the FA pathway, is monoubiquitinated upon DNA damage, such as crosslinkers and replication blockers such as hydroxyurea (HU). Even though FA cells demonstrate unequivocal sensitivity to crosslinkers, such as mitomycin C (MMC), we find that they are largely resistant to HU, except for cells absent for expression of FANCD2. FANCD2, RAD51 and RAD18 form a complex, which is enhanced upon HU exposure. Surprisingly, although FANCD2 is required for this enhanced interaction, its monoubiquitination is not. Similarly, non-ubiquitinated FANCD2 can still support proliferation cell nuclear antigen (PCNA) monoubiquitination. RAD51, but not BRCA2, is also required for PCNA monoubiquitination in response to HU, suggesting that this function is independent of homologous recombination (HR). We further show that translesion (TLS) polymerase PolH chromatin localization is decreased in FANCD2 deficient cells, FANCD2 siRNA knockdown cells and RAD51 siRNA knockdown cells, and PolH knockdown results in HU sensitivity only. Our data suggest that FANCD2 and RAD51 have an important role in PCNA monoubiquitination and TLS in a FANCD2 monoubiquitination and HR-independent manner in response to HU. This effect is not observed with MMC treatment, suggesting a non-canonical function for the FA pathway in response to different types of DNA damage.

Synthesis, anticancer, and antibacterial activities of piplartine derivatives on cell cycle regulation and growth inhibition

A series of piplartine derivatives were synthesized via Baylis-Hillman reaction and evaluated for anticancer and antibacterial activities. The cytotoxicity of these compounds was examined in two different human tumor cell lines, IMR-32 and HeLa. The antibacterial activity was examined in Staphylococcus aureus and Pseudomonas aeruginosa. The results showed that compounds 2b, 2e, and 2j were found to be the most active compounds, which displayed line no cytotoxicity, but G2-M cell cycle arrest in tumor cells, and showed cytostatic effects in bacteria.

A phase 2 consortium (P2C) trial of 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP) for advanced adenocarcinoma of the pancreas

3-Aminopyridine-2-carboxaldehyde thiosemicarbazone (3-AP, Triapine) is a novel small molecule inhibitor of ribonucleotide reductase (RR) with clinical signs of activity in pancreatic cancer. Therefore, the Phase 2 Consortium (P2C) initiated a trial (two single stage studies with planned interim analysis) of 3-AP at 96 mg/m(2) intravenously days 1-4 and 15-18 of a 28-day cycle in both chemotherapy-naive and gemcitabine-refractory (GR) patients with advanced pancreatic cancer. The primary endpoint was survival at six months (chemotherapy-naive) and four months (GR). Secondary endpoints were toxicity, response, overall survival, time to progression and mechanistic studies. Fifteen patients were enrolled including one chemotherapy-naïve and 14 GR. The chemotherapy-naïve patient progressed during cycle 1 with grade 3 and 4 toxicities. Of 14 GR patients, seven received two cycles, six received one cycle and one received eight cycles. Progression precluded further treatment in 11 GR patients. Additionally, one died of an ileus in cycle 1 considered related to treatment and two stopped treatment due to toxicity. Five GR patients had grade 4 toxicities possibly related to 3-AP and six GR patients had grade 3 fatigue. Toxicities and lack of meaningful clinical benefit prompted early study closure. Four-month survival in GR patients was 21% (95% CI: 8-58%). Correlative studies confirmed that 3-AP increased the percentage of S-phase buccal mucosal cells, the presence of multidrug resistance gene polymorphisms appeared to predict leukopenia, and baseline pancreatic tumor RR M2 expression was low relative to other tumors treated with 3-AP. In conclusion, this regimen appears inactive against predominantly GR pancreatic cancer. RR M2 protein may not have a critical role in the malignant potential of pancreatic cancer.

Measles virus inhibits human immunodeficiency virus type 1 reverse transcription and replication by blocking cell-cycle progression of CD4+ T lymphocytes

Acute measles virus (MV) infection results in a decrease in plasma human immunodeficiency virus type 1 (HIV-1) RNA levels in co-infected children. An in vitro peripheral blood mononuclear cell (PBMC) culture system was used to assess the mechanisms by which MV blocks HIV-1 replication. MV inhibited proliferation of CD4(+) T lymphocytes, the target cell for HIV-1 replication. In the presence of MV, cells did not progress to G(1b) and S phases, steps critical for the completion of HIV-1 reverse transcription and productive replication. This block in cell-cycle progression was characterized by an increased proportion of CD4(+) and HIV-1-infected cells retained in the parental generation in PBMCs co-cultured with MV and HIV-1, and decreased levels of cyclins and RNA synthesis. Early HIV-1 replication was also inhibited in the presence of MV, as measured by reduced expression of a luciferase reporter gene and lower levels of both early (LTR) and late (LTR-gag) DNA intermediates of HIV-1 reverse transcription in the presence of CCR5-tropic HIV-1. The effects of MV on lymphoproliferation and p24 antigen production were reproduced by n-butyrate and hydroxyurea, drugs that block the cell cycle in G(1a) and G(1)/S, respectively. It was concluded that MV inhibits HIV-1 productive replication in part by blocking the proliferation of CD4(+) T lymphocytes.

Calcium Channel Antagonists Augment Hydroxyurea- And Ru486-Induced Inhibition Of Meningioma Growth In Vivo And In Vitro

OBJECTIVE

Although the chemotherapy drug hydroxyurea (HU) and the antiprogesterone mifepristone (RU486) have been used to treat meningiomas for which surgical and radiation therapies have failed, results have been disappointing. The addition of calcium channel antagonists (CCAs) to chemotherapeutic drugs enhances tumor growth inhibition in other tumor types, and the authors demonstrated that CCAs can block meningioma growth in vitro and in vivo. The purpose of this study was to test the effects of the addition of a CCA to HU or RU486 on meningioma growth.

METHODS

Primary and malignant (IOMM-Lee) meningioma cell lines were treated with HU, RU486, or either of these plus diltiazem or verapamil. Assays for cell growth, apoptosis, and fluorescent-activated cell sorting were performed on in vitro cultures. Similar cell lines were implanted into nude mice and were treated with HU or RU486, in combination with a CCA. Tumors were analyzed by light microscopy, MIB-1, and factor VIII immunohistochemical staining studies.

RESULTS

The addition of diltiazem or verapamil to HU or RU486 augmented meningioma growth inhibition by 20 to 60% in vitro. In vivo, tumors treated with combination drugs were smaller; and immunohistochemical analysis of the IOMM-Lee tumors showed a 10% decrease in the MIB-1 ratio (from 0.41 to 0.30) and an approximate 75% decrease in microvascular density.

CONCLUSION

The addition of diltiazem or verapamil to HU or RU486 augments meningioma growth inhibition in vitro by inducing apoptosis and G1 cell-cycle arrest. The combination of HU and diltiazem inhibited the growth of meningiomas in vivo by decreasing proliferation and microvascular density. These results suggest a possible role for these drugs as an additional adjuvant therapy for recurrent or unresectable meningiomas.

Decreased origin usage and initiation of DNA replication in haploinsufficient HCT116 Ku80+/- cells

One of the functions of the abundant heterodimeric nuclear protein, Ku (Ku70/Ku80), is its involvement in the initiation of DNA replication through its ability to bind to chromosomal replication origins in a sequence-specific and cell cycle dependent manner. Here, using HCT116 Ku80+/- cells, the effect of Ku80 deficiency on cell cycle progression and origin activation was examined. Western blot analyses revealed a 75% and 36% decrease in the nuclear expression of Ku80 and Ku70, respectively. This was concomitant with a 33% and 40% decrease in chromatin binding of both proteins, respectively. Cell cycle analysis of asynchronous and late G1 synchronized Ku80+/- cells revealed a prolonged G1 phase. Furthermore, these Ku-deficient cells had a 4.5-, 3.4- and 4.3-fold decrease in nascent strand DNA abundance at the lamin B2, beta-globin and c-myc replication origins, respectively. Chromatin immunoprecipitation (ChIP) assays showed that the association of Ku80 with the lamin B2, beta-globin and c-myc origins was decreased by 1.5-, 2.3- and 2.5-fold, respectively, whereas that of Ku70 was similarly decreased (by 2.1-, 1.5- and 1.7-fold, respectively) in Ku80+/- cells. The results indicate that a deficiency of Ku80 resulted in a prolonged G1 phase, as well as decreased Ku binding to and activation of origins of DNA replication.

Human progesterone receptor displays cell cycle-dependent changes in transcriptional activity

The human progesterone receptor (PR) contains multiple Ser-Pro phosphorylation sites that are potential substrates for cyclin-dependent kinases, suggesting that PR activity might be regulated during the cell cycle. Using T47D breast cancer cells stably transfected with an mouse mammary tumor virus (MMTV) chloramphenicol acetyltransferase reporter (Cat0) synchronized in different phases of the cell cycle, we found that PR function and phosphorylation is remarkably cell cycle dependent, with the highest activity in S phase. Although PR expression was reduced in the G2/M phase, the activity per molecule of receptor was markedly reduced in both G1 and G2/M phases compared to the results seen with the S phase of the cell cycle. Although PR is recruited to the MMTV promoter equivalently in the G1 and S phases, recruitment of SRC-1, SRC-3, and, consequently, CBP is reduced in G1 phase despite comparable expression levels of SRC-1 and SRC-3. In G2/M phase, site-specific phosphorylation of PR at Ser162 and at Ser294, a site previously reported to be critical for transcriptional activity and receptor turnover, was abolished. Treatment with the histone deacetylase inhibitor trichostatin A elevated G1 and G2/M activity to that of the S phase, indicating that the failure to recruit sufficient levels of active histone acetyltransferase is the primary defect in PR-mediated transactivation.

Hydroxyurea exerts a cytostatic but not immunosuppressive effect on T lymphocytes

OBJECTIVE

To demonstrate that, despite a dose-dependent cytostatic effect, hydroxyurea (HU) does not have immunosuppressive effects.

METHODS

The effects of HU on T lymphocyte proliferation parameters, activation phenotype and cytokine production were examined in vitro after exposure to clinically relevant concentrations of HU (10, 50, and 100 micromol/l). The effects of HU in vivo on CD4 T cell counts, viral load, activation phenotype and virus-specific response were examined in 17 Rhesus macaques infected with SIV(mac251) and randomized into three groups: untreated controls; treated with (R)-9-(2-phosphonylmethoxypropyl)adenine (PMPA) and didanosine (ddI) only; and treated with PMPA, didanosine, and HU.

RESULTS

The in vitro inhibition of T lymphocyte proliferation confirmed the cytostatic effect of HU, with a linear dose-dependent effect; however, no relevant differences were found in the expression of activation markers between treated and untreated controls. Both T helper type 1 and type 2 cytokine production were enhanced by HU. Consistent with the in vitro results, a blunted increase of peripheral CD4 T cells was observed in vivo in the HU group, without relevant effects on the expression of activation markers, and SIV-specific T cell responses were not affected by HU.

CONCLUSIONS

Hyper-proliferation of T-lymphocytes is a major factor contributing to HIV pathogenesis. HU exerts a cytostatic effect on T lymphocytes, without altering their activation and apparently without having an immunosuppressive effect. The increase in cytokine production at the single cell level might compensate for the decrease in the percentage of activated CD4 T lymphocytes, without overall impairment of HIV-specific immune responses.

Synchronization of Plodia interpunctella lepidopteran cells and effects of 20-hydroxyecdysone

We have investigated the molecular and cellular mechanisms involved in the control of insect cell cycle by 20-hydroxyecdysone (20E) using the IAL-PID2 cell line established from imaginal wing discs of Plodia interpunctella. We first defined conditions for use of hydroxyurea, a reversible inhibitor of DNA synthesis, in order to synchronize the IAL-PID2 cells in their division cycle. A high degree of synchrony was reached when cells were exposed to two consecutive hydroxyurea treatments at 1 mm for 36 h spaced 16 h apart. Under these conditions, flow cytometry analysis demonstrated that 20E at 10(-6) m induced an inhibition of cell growth by an arrest of 90% of the cells in G2/M phase. Using cDNA probes specifically designed from E75 and HR3 nuclear receptors of Plodia interpunctella, we showed that PiE75 and PHR3 were highly induced by 20E through S and G2 phases with maximal enhancement just before the G2/M arrest of cells. These findings suggest that PiE75 and PHR3 could be involved in a 20E-induced genetic cascade leading to G2/M arrest.

Cell kinetics-dependent antitumor effect of irinotecan hydrochloride induced by the synchronizing effect of hydroxyurea: cell kinetics and dosing time

Influence of hydroxyurea (HU) on the antitumor effect of irinotecan hydrochloride (CPT-11) was investigated in ICR male mice transplanted with sarcoma 180 cells (S-180). A single dose of CPT-11 (100 mg/kg) was injected at various times after a single dose of HU (300 mg/kg). The relative tumor weight varied significantly depending on the timing of CPT-11 injection after HU injection (P < 0.01). The higher antitumor effect of CPT-11 was observed when DNA synthesis of S-180 cells increased (20 hr), and the lower effect was observed when the DNA synthesis decreased (0 hr). The loss of body weight also varied significantly depending on the timing of CPT-11 injection after HU injection (P < 0.01). The toxicity of CPT-11 was higher when the inhibitory effect of HU on DNA synthesis of bone marrow cells was stronger (15 hr), and the lower toxicity was observed when the inhibitory effect was not observed (0 hr). The plasma SN-38 concentration at 2 hr after CPT-11 injection was higher at 20 hr after HU injection than at 0 hr after HU injection. The difference in plasma esterase activity between 0 hr and 20 hr after HU injection was regarded as the mechanism underlying the dosing time-dependent difference of the SN-38 concentration. These experiments suggest that HU can produce a different phase of cell cycle between tumor cells and normal cells. This leads to increase the antitumor effect of CPT-11 without increasing the adverse effect of the drug. It is essential to consider the dosing time in the two-drug combination therapy.

A cytostatic drug improves control of HIV-1 replication during structured treatment interruptions: a randomized study

OBJECTIVE

To study the effect of highly active antiretroviral therapy (HAART) with and without hydroxyurea (HU) on changes in plasma viral load (VL) set-point, and on HIV-1-specific responses, after five cycles of structured treatment interruptions (STI).

METHODS

A group of 20 patients taking HAART for chronic HIV infection with VL < 20 copies/ml were randomized to continue HAART or HAART plus HU for 24 weeks followed by five STI cycles. HU was also stopped in cycles 1-3 but continued in cycles 4 and 5. The number of individuals maintaining a VL set-point < 5000 copies/ml during the fifth interruption were determined.

RESULTS

VL remained < 5000 copies/ml in eight out of nine patients in the HU group and in four out of ten patients in the HAART group after a median 48 weeks of follow-up after the fifth interruption ( P=0.039). By STI cycle 5, there was a significant increase in the neutralizing activity (NA), in both magnitude and breadth of the total cytotoxic T lymphocyte (CTL) response and in lymphoproliferative response (LPR) from baseline. No significant differences were observed between HAART and HU groups in NA, CTL and LPR at any time-point. There were no differences in the NA titers at any time-point between responder and non-responder patients. There was a trend for higher CTL and LPR levels in responder patients (P= 0.10).

CONCLUSIONS

In this randomized, controlled study of STI with cycles of HAART or HAART plus HU, a lower peak VL rebound and a lower VL set-point was achieved in patients continuing HU while other drugs were discontinued. HU did not blunt anti-HIV-1-specific responses; however, control of VL did not correlate with anti-HIV-1-specific cellular immune responses.

Hydroxyurea and periodicity in myeloproliferative disease

Three patients, one with polycythaemia vera (PV) and two with chronic myeloid leukaemia (CML), are described who had cycling of blood counts which became apparent whilst receiving hydroxyurea therapy. Significant periodicity was confirmed with the use of the Lomb periodogram. This is Fourier power spectral analysis tailored for unevenly sampled data. The patient with PV had marked oscillations of platelet counts with a periodicity of 29 d and an amplitude of (202-588)x10(9)/L. Smaller oscillations of neutrophil, monocyte and lymphocyte numbers and Hb levels occurred with a similar periodicity. Anticipatory changes in hydroxyurea dosage or the maintenance of a constant dose did not abolish periodicity, but a change in therapy to the non-cycle-specific drug anagrelide dampened and abolished the cycling. One of the patients with CML had tremendous and clear oscillations in white cell, platelet and Hb levels, with a mean periodicity of 74 d. The other had erratic counts which were confirmed to be significantly periodic (64 d), on spectral analysis. A change in therapy to busulphan in both these patients again dampened and abolished the cycling. Hydroxyurea, which is a cell-cycle-specific agent, probably exacerbates the periodicity which may be present in some patients with myeloproliferative disease. A change in therapy to non-cycle-acting compounds such as busulphan or anagrelide results in much more stable counts in such patients.

Proliferating status of peripheral blood progenitor cells from patients with BCR/ABL-positive chronic myelogenous leukemia

To investigate the mechanisms behind the leukemic expansion of BCR/ABL-positive chronic myelogenous leukemia (CML), we examined the cell cycle status of hematopoietic progenitor cells from peripheral blood (PB) and bone marrow (BM) of 37 patients with newly diagnosed BCR/ABL-positive CML. We found a high proportion of 12.51 +/- 1.19% of CD34+ peripheral blood progenitor cells (PBPC) in S/G2M phase. Comparison of PB and BM from 19 cases revealed similar proliferation rates (10.74 +/- 1.41% vs 15.97 +/- 1.95%). Furthermore, even primitive CD34+/CD38- PBPC displayed high proliferation rates (17.45 +/- 2.98%) in 10 cases examined. In contrast, PBPC from 11 patients with BCR/ABL-negative myeloproliferative disorders were almost noncycling (S/G2M 1.46 +/- 0.47%). When matched pairs of PB and BM from six patients with BCR/ABL-negative myeloproliferative disorders were examined, only 0.89 +/- 0.41% of the CD34+ PBPC, but 8.29 +/- 3.13% CD34+ cells from BM were in S/G2M phase. Consistently, as compared to 19 patients with newly diagnosed BCR/ABL-positive CML, a significantly lower PB/BM ratio of CD34+ cells in S/G2M phase was found in these six patients with BCR/ABL-negative myeloprolifrative disorders. Administration of the tyrosine kinase inhibitor STI571 to 13 patients with CML in chronic phase, accelerated phase, or blast crisis lead to an inhibition of PBPC proliferation within a few days. Interestingly, CD34+ hematopoietic progenitor cells from BM remained proliferating in five cases examined, indicating that CML PBPC are more easily inhibited by STI571 as compared to CD34+ CML hematopoietic progenitor cells from BM. These data suggest that BCR/ABL leads to an enhanced cell cycle activation of CD34+ cells, which seems to be, at least in part, independent of additional factors provided by the bone marrow microenvironment.

The expression of MHC class II genes in macrophages is cell cycle dependent

Using different drugs, we stopped the cell cycle of bone marrow-derived macrophages at different points. After IFN-gamma stimulation, macrophages arrested at the G(1) phase of the cell cycle did not increase cell surface expression of the MHC class II IA. This inhibition is specific, because, under the same conditions, IFN-gamma induces the expression of Fcgamma receptors and the inducible NO synthase mRNA. Treatments that inhibit macrophage proliferation by blocking the cell cycle at the G(1) phase, such as adenosine, forskolin, or LPS, blocked the IFN-gamma induction of IA. Under IFN-gamma treatment, the steady-state levels of IAalpha and IAss mRNA did not increase in cells arrested at the G(1) phase and the half-life of the MHC mRNA was not modified. These data suggest that the cell cycle modulation of IFN-gamma-induced MHC II gene expression occurs at the transcriptional level. The expression of the class II transactivator mRNA induced by IFN-gamma was also blocked when macrophages were arrested at the G(1) phase of the cell cycle, suggesting that the lack of IFN-gamma response occurs at the early steps of MHC class II expression. Finally, macrophages arrested at the G(1) phase showed increased basal levels of cell surface IA due to an increase of the translational efficiency. These data show that the expression of MHC class II genes is regulated by the cell cycle.

Hydroxyurea Can Be Used to Increase Mouse c-kit+Thy-1.1loLin−/loSca-1+ Hematopoietic Cell Number and Frequency in Cell Cycle In Vivo

The DNA synthesis inhibitor hydroxyurea (HU) was administered to determine whether it induces changes in the cell-cycle status of primitive hematopoietic stem cells (HSCs)/progenitors. Administration of HU to mice leads to bone marrow accumulation of c-kit+Thy-1.1loLin−/loSca-1+ (KTLS) cells in S/G2/M phases of the cell cycle. HU is a relatively nontoxic, reversible cell-cycle agent that can lead to approximately a threefold expansion of KTLS cells in vivo and approximately an eightfold increase in the number of KTLS cells in S/G2/M. HSCs in HU-treated mice have undiminished multilineage long-term and short-term clonal reconstitution activity.

Hydroxyurea Can Be Used to Increase Mouse c-kit+Thy-1.1loLin−/loSca-1+ Hematopoietic Cell Number and Frequency in Cell Cycle In Vivo

The DNA synthesis inhibitor hydroxyurea (HU) was administered to determine whether it induces changes in the cell-cycle status of primitive hematopoietic stem cells (HSCs)/progenitors. Administration of HU to mice leads to bone marrow accumulation of c-kit+Thy-1.1loLin−/loSca-1+ (KTLS) cells in S/G2/M phases of the cell cycle. HU is a relatively nontoxic, reversible cell-cycle agent that can lead to approximately a threefold expansion of KTLS cells in vivo and approximately an eightfold increase in the number of KTLS cells in S/G2/M. HSCs in HU-treated mice have undiminished multilineage long-term and short-term clonal reconstitution activity.

The MDR1 (P-glycoprotein) and MRP (P-190) transporters do not play a major role in the intrinsic multiple drug resistance of Jurkat T lymphocytes

The response of T cells in relation to the cell cycle has not been extensively studied. We have attempted to address this question using Jurkat T cells treated with cytostatic drugs known to arrest cells at various transition points of their cycle. We tested several concentrations of drugs that act at G1/S (hydroxyurea, lovastatin, thymidine), early S (aphidicolin, cyclosporin A, rapamycin) or G2+M (colchicine, nocodazole) in 24 h cultures. Cytofluorimetric analyses showed that cycling Jurkat cells were equally distributed between the G1 (44.9 +/- 6.5%) and S (42.3 +/- 8.0%) phases. Cell distribution in G2+M was 12.7 +/- 2.8%. Hydroxyurea but not lovastatin increased the percentage of cells in S phase to approximately 60-70% and both drugs decreased it to approximately 30% in G1. Thymidine had no effects. Aphidicolin increased the distribution in S phase to approximately 70% with a decrease in G1 to approximately 30%. Cyclosporin A and rapamycin increased the percentage of the cells in G1 to approximately 70% and decreased it to approximately 25% in S phase. Nocodazole increased cell distribution in G2+M to approximately 60% and induced a decrease in G1 to approximately 10%. The effects of the drugs were not related to their toxicity and their limited efficiency raised the possibility that Jurkat cells possessed an intrinsic resistance to these xenobiotics. Time-course analysis showed (scanning electron microscopy) that the early morphological changes induced by colchicine were reversible. Drug efflux experiments (vinblastine) suggested that an ATP-dependent process could be involved. However, Northern blot analyses showed a weak signal for MDR1 (P-glycoprotein). In contrast, a probe for MRP (P-190) showed a strong signal in Jurkat and peripheral lymphocytes. The presence of drugs (cyclosporin A, nocodazole, thymidine) (24 h) did not upregulate its message and cell treatment with DL-butathione (S,R)-sulfoximine only moderately affected the efficiency of the glutathione S-conjugate MRP transporter. Our data suggest that the intrinsic multidrug resistance of leukemic Jurkat T cells does not appear to involve the MDR1 and MRP members of the ABC family of reverse drug transporters and these observations raise the possibility of the involvement of multifaceted mechanisms.

The MRD1 (P-glycoprotein) and MRP (P-190) transporters do not play a major role in the intrinsic multiple drug resistance of Jurkat T lymphocytes

The response of T cells in relation to the cell cycle has not been extensively studied. We have attempted to address this question using Jurkat T cells treated with cytostatic drugs known to arrest cells at various transition points of their cycle. We tested various concentrations of drugs that act at G1/S (hydroxyurea, lovastatin, thymidine), early S [aphidicolin, cyclosporin A (CsA), rapamycin] or G2 + M (colchicine, nocodazole) in 24 h cultures. Cytofluorimetric analyses showed that cycling Jurkat cells were equally distributed between the G1 (44.9 +/- 6.5%) and S (42.3 +/- 8.0%) phases. Cell distribution in G2 + M was 12.7 +/- 2.8%. Hydroxyurea but not lovastatin increased the percentage of cells in S phase to ca 60-70% and both drugs decreased it to ca 30% in G1. Thymidine had no effects. Aphidicolin increased the distribution in S phase to ca 70% with a decrease in G1 to ca 30%. CsA and rapamycin increased the percentage of the cells in G1 to ca 70% and decreased it to ca 25% in S phase. Nocodazole increased cell distribution in G2 + M to ca 60% and induced a decrease in G1 to ca 10%. The effects of the drugs were not related to their toxicity and their limited efficiency raised the possibility that Jurkat cells possessed an intrinsic resistance to these xenobiotics. Time-course analysis showed (scanning electron microscopy) that the early morphological changes induced by colchicine were reversible. Drug efflux experiments (vinblastine) suggested that an ATP-dependent process could be involved. However, Northern blot analyses showed a weak signal for MDR1 (MDR, multiple drug resistance). In contrast, a probe for multidrug resistance-associated protein (P-190; MRP) showed a strong signal in Jurkat and peripheral lymphocytes. The presence of drugs (CsA, nocodazole, thymidine) (24 h) did not up-regulate its message and cell treatment with BSO only moderately affected the efficiency of the glutathione S-conjugate MRP transporter. Our data suggest that the intrinsic multidrug resistance of leukemic Jurkat T cells does not appear to involve the MDR1 and MRP members of the ABC family of reverse drug transporters and these observations raise the possibility of the involvement of multi-faceted mechanisms.

Synchronization of Aedes albopictus mosquito cells using hydroxyurea

We have established conditions for use of hydroxyurea, a reversible inhibitor of DNA synthesis, to synchronize the division cycle of a continuous cell line from the mosquito, Aedes albopictus. In the range of 0.15-0.25 mM hydroxyurea, an 18 h treatment, followed by removal of the drug, results in effective synchronization. When combined with the partial synchronization that occurs within 10 h of dilution and plating, more than 80% of cells treated with hydroxyurea could be recovered in the synthesis (S) phase of the cell cycle during the 4 h period after removal of the drug. The degree of synchrony was enhanced when cells were exposed to two consecutive hydroxyurea treatments spaced 10 h apart. Synchronized cells expressed maximal levels of a reporter gene when transfected immediately after removal of hydroxyurea. This is the first description of effective chemical synchronization of an insect cell line using hydroxyurea.

Hydroxyurea for treatment of unresectable and recurrent meningiomas. I. Inhibition of primary human meningioma cells in culture and in meningioma transplants by induction of the apoptotic pathway

Meningiomas, which invade intracranial bone structures and the adjacent connective tissue, are frequently unresectable because of their aggressive and recalcitrant growth behavior. They have a high recurrence rate, and in approximately 10% of these tumors there is an increased risk of malignancy. Significant morbidity and mortality rates associated with recurrent meningiomas demand nonsurgical approaches. To date, adjuvant hormonal treatment has not proven beneficial. The anticancer drug hydroxyurea was therefore tested for its potential use in the treatment of meningiomas. Early-passaged cell cultures were established from 20 different meningiomas. The addition of 5 x 10(-4) and 10(-3) M hydroxyurea over a period of 5 to 9 days resulted in a remarkable decrease in cell proliferation and even blocked tumor cell growth when compared with untreated cells. A significant arrest of meningioma cell growth in the S phase of the cell cycle was revealed on DNA flow cytometry. Electron micrographs of hydroxyurea-treated tumor cells showed ultrastructural features consistent with apoptosis, and light microscopy demonstrated DNA fragmentation by in situ DNA strand break labeling. Short-term treatment of meningioma cell cultures with hydroxyurea for 24 to 48 hours resulted in discrete oligonucleosomal fragments (DNA ladder), another characteristic sign of apoptosis. In addition to the in vitro studies, tissue from five different meningiomas was transplanted into nude mice followed by treatment with 0.5 mg/g body weight hydroxyurea over 15 days. In situ DNA strand break labeling demonstrated DNA fragmentation in distinct regions with different tumor cell densities in all hydroxyurea-treated meningioma transplants. These data provide evidence that hydroxyurea is a powerful inhibitor of meningioma cell growth, most likely by causing apoptosis in the tumor cells. Thus, hydroxyurea may be a suitable chemotherapeutic agent for the long-term treatment of unresectable or semi- to malignant meningiomas, or for preventing recurrent growth of meningiomas after resection.

Hydroxyurea for treatment of unresectable and recurrent meningiomas. I. Inhibition of primary human meningioma cells in culture and in meningioma transplants by induction of the apoptotic pathway

Meningiomas, which invade intracranial bone structures and the adjacent connective tissue, are frequently unresectable because of their aggressive and recalcitrant growth behavior. They have a high recurrence rate, and in approximately 10% of these tumors there is an increased risk of malignancy. Significant morbidity and mortality rates associated with recurrent meningiomas demand nonsurgical approaches. To date, adjuvant hormonal treatment has not proven beneficial. The anticancer drug hydroxyurea was therefore tested for its potential use in the treatment of meningiomas.

Early-passaged cell cultures were established from 20 different meningiomas. The addition of 5 X 10−4 and 10−3 M hydroxyurea over a period of 5 to 9 days resulted in a remarkable decrease in cell proliferation and even blocked tumor cell growth when compared with untreated cells. A significant arrest of meningioma cell growth in the S phase of the cell cycle was revealed on DNA flow cytometry.

Electron micrographs of hydroxyurea-treated tumor cells showed ultrastructural features consistent with apoptosis, and light microscopy demonstrated DNA fragmentation by in situ DNA strand break labeling. Short-term treatment of meningioma cell cultures with hydroxyurea for 24 to 48 hours resulted in discrete oligonucleosomal fragments (DNA ladder), another characteristic sign of apoptosis. In addition to the in vitro studies, tissue from five different meningiomas was transplanted into nude mice followed by treatment with 0.5 mg/g body weight hydroxyurea over 15 days. In situ DNA strand break labeling demonstrated DNA fragmentation in distinct regions with different tumor cell densities in all hydroxyurea-treated meningioma transplants.

These data provide evidence that hydroxyurea is a powerful inhibitor of meningioma cell growth, most likely by causing apoptosis in the tumor cells. Thus, hydroxyurea may be a suitable chemotherapeutic agent for the long-term treatment of unresectable or semi- to malignant meningiomas, or for preventing recurrent growth of meningiomas after resection.

Seminal plasmin, a bovine seminal plasma protein, lyses dividing but not resting mammalian cells

Seminal plasmin, an antimicrobial and transcription-inhibitory protein of bovine seminal plasma, is shown to lyse dividing mammalian cells in vitro. It lyses cells in culture such as CHO, Vero, HeLa and L929. It also lyses regenerating rat liver parenchymal cells and cells of two ascitic tumours of rat--the Zajdela ascitic hepatoma and the AK-5. However, it does not lyse resting cells such as adult liver parenchymal cells, erythrocytes, or resting lymphocytes, though it binds to their cell surface. It can be used, therefore, to distinguish cells that are in the division cycle from cells that are in the resting phase. The cell-lytic activity of seminal plasmin is inhibited by Ca2+.

Hydroxyurea exposure alters mouse testicular kinetics and sperm chromatin structure

The effects of hydroxyurea (HU) on testicular cell kinetics and sperm chromatin differentiation were investigated in mice. Whole testis, minced testicular cell suspensions and caudal epididymal sperm cells were obtained at 8 and 29 days after i.p. injections containing 0, 25, 50, 100, 200, 400 and 500 mg/kg HU x 5 days. Testis weights were unaffected by 25 mg/kg HU while 500 mg/kg caused up to a 50% loss of testicular weight by 29 days. Flow cytometrically measured acridine-orange (AO) stained testicular cells revealed altered population ratios at the highest dosages at 8 days and for all dosages except 25 mg/kg HU at 29 days. At 8 days, 400-500 mg/kg HU caused a near depletion of tetraploid cells. Flow cytometry of AO stained sperm, previously treated with acid to potentially induce DNA denaturation, was used to follow the shift from normal chromatin structure to an abnormal form with increased sensitivity to DNA denaturation in situ. The extent of DNA denaturation was quantitated for each cell by the computer-derived value alpha t, alpha t = [red/(red+green) fluorescence]. The flow cytometry measures, standard deviation of alpha t (SD alpha t), mean of alpha t (X alpha t) and cells outside the main peak of alpha t (COMP alpha t), gave similar dose response curves to the sperm head morphology assay. SD alpha t was more sensitive than the X alpha t as a measure of HU-induced alteration of chromatin structure. The major conclusions reached are that HU inhibits DNA synthesis, probably by inhibiting ribonucleotide reductase, causing maturation depletion of pachytene spermatocytes and, subsequently, depletion of meiotic daughter cells and differentiated cell types leading to mature sperm. This inhibition of DNA synthesis is related to an alteration of sperm chromatin structure and abnormal sperm head morphology.

Two alleles of a developmentally regulated alpha-tubulin locus in Physarum polycephalum replicate on different schedules

The replication timing of a pair of natural alleles was compared at two alpha-tubulin loci of the Physarum plasmodium. Taking advantage of the naturally synchronous cell cycle of nuclei within the syncytial plasmodium, we analyzed the replication schedule of specific DNA fragments to a resolution of 10-min intervals within a 3-h S phase. At this level of resolution, differences in replication timing between polymorphic alleles at the same locus can be detected in a heterozygote. Specifically, the 3' region of the altA1 allele completes replication at between 20 and 40 min of S phase. The same region of the altA2 allele completes replication at between 40 and 80 min of S phase. In contrast, both alleles at the altB locus replicate concurrently within the first 10 to 15 min of S phase. Previous studies showed that both altA and altB are expressed in the plasmodium, their message levels peaking at mitosis, just minutes before the onset of S phase. However, altB message is detected at substantially higher levels than altA message on Northern (RNA) blots. The temporal windows over which the altA alleles each replicate are very broad in comparison with the levels of mitotic synchrony and altB replication synchrony in a single plasmodium. The allele-specific replication schedule of the altA locus demonstrates that the temporal organization of replicons is not strictly conserved between homologous chromosomes.

Two alleles of a developmentally regulated alpha-tubulin locus in Physarum polycephalum replicate on different schedules

The replication timing of a pair of natural alleles was compared at two alpha-tubulin loci of the Physarum plasmodium. Taking advantage of the naturally synchronous cell cycle of nuclei within the syncytial plasmodium, we analyzed the replication schedule of specific DNA fragments to a resolution of 10-min intervals within a 3-h S phase. At this level of resolution, differences in replication timing between polymorphic alleles at the same locus can be detected in a heterozygote. Specifically, the 3' region of the altA1 allele completes replication at between 20 and 40 min of S phase. The same region of the altA2 allele completes replication at between 40 and 80 min of S phase. In contrast, both alleles at the altB locus replicate concurrently within the first 10 to 15 min of S phase. Previous studies showed that both altA and altB are expressed in the plasmodium, their message levels peaking at mitosis, just minutes before the onset of S phase. However, altB message is detected at substantially higher levels than altA message on Northern (RNA) blots. The temporal windows over which the altA alleles each replicate are very broad in comparison with the levels of mitotic synchrony and altB replication synchrony in a single plasmodium. The allele-specific replication schedule of the altA locus demonstrates that the temporal organization of replicons is not strictly conserved between homologous chromosomes.

Observations on a proposed measure of genotoxicity in rat gastric mucosa

Current methods for the study of the toxicological effects of antisecretory medications on the gastric mucosa possess disadvantages or limitations. A novel assay has been proposed to assess gastric mucosal genotoxicity in which the proton-pump inhibitor omeprazole has been reported to induce direct damage to cellular DNA, raising questions about the safety of this drug. To define the applicability of this proposed measure of genotoxicity and to examine the effects of omeprazole in this assay, control agents, known carcinogens, and omeprazole in various doses and formulations were administered to rats by gavage, followed by [3H]thymidine labeling of DNA in vivo approximately 14 hours later. The incorporation of the [3H]thymidine label into DNA of gastric mucosal cells liberated by limited pronase digestion was in close agreement with published results for negative and positive controls. Omeprazole, administered in doses ranging from 10 mg/kg to 300 mg/kg, did not increase [3H]thymidine incorporation into cellular DNA in this assay. The gastric carcinogen 1-methyl-2-nitro-1-nitrosoguanidine at 20 and 50 mg/kg increased [3H]thymidine incorporation. Pretreatment in vivo with hydroxyurea before [3H]thymidine labeling to inhibit replicative DNA synthesis suppressed [3H]thymidine incorporation more than 97% in negative controls and MNNG and more than 93% in omeprazole treatments. This indicates that replicative DNA synthesis was almost totally responsible for the [3H]thymidine incorporation and the contribution of unscheduled DNA synthesis to the total [3H]thymidine incorporation is minor. Flow cytometric analysis of the cell cycle of the gastric mucosal cells liberated by the limited pronase digestion indicated significant contamination of the preparation with dividing cells (4% in negative controls and 14% in MNNG-treated positive controls). These findings indicate that the proposed screening assay for genotoxicity in rat gastric mucosa is not a reliable measure of unscheduled DNA synthesis in its present form, and conclusions about genotoxic effects of any drug using this assay as initially proposed appear questionable.

Neuronal determination without cell division in Xenopus embryos

Cell division in the Xenopus CNS was blocked by incubating embryos in a mixture of the DNA synthesis inhibitors hydroxyurea and aphidicolin. Surprisingly, embryos treated at the beginning of gastrulation proceeded normally through neurulation, neural tube closure, and CNS subdivision. Thus, cell division is not critical for neural induction or early morphogenetic events in the CNS. Neuroblasts in treated embryos differentiated into neurons of many classes, indicating that cellular determination in the CNS can be dissociated from lineage and birth date. Axonal tracts and embryonic reflexes also developed. The remarkable amount of normal CNS development that occurs in these animals may be explained by a series of sequential inductions that are largely independent of cell proliferation.

Cell cycle arrest and cellular differentiation mediated by a cell surface sialoglycopeptide

Cell cycling by a relatively wide variety of cell lines was shown to be reversibly inhibited by a cell surface sialoglycopeptide (SGP) isolated and purified from intact bovine cerebral cortex cells. Cell cycle arrest, mediated by the bovine SGP inhibitor, was shown to be completely reversible with mouse Swiss 3T3, mouse 1316 fibrosarcoma, mouse N2a neuroblastoma, bovine MDBK and monkey BSC-1 cells. These cell lines represented both fibroblast and epithelial-like cells, transformed and nontransformed cells, as well as their being derived from a broad array of species. In contrast to the others tested, human HL-60 leukemic cells were sensitive to the inhibitory effects of the SGP but did not reenter the mitotic cycle after the removal of the inhibitor. Instead, the mitotic arrest of HL-60 cells appeared to enhance entry into a terminal and irreversible state of cellular differentiation.

The cell cycle dependence of c-sis gene expression: artifactual conclusions in cells prepared by chemical but not physical techniques

The c-sis oncogene encoding the B-chain of platelet-derived growth factor (PDGF) may be involved in an autocrine growth stimulation of tumours expressing the PDGF receptor, such as glioblastomas and sarcomas. To investigate whether expression of c-sis RNA is regulated in a cell cycle dependent manner, human A172 glioblastoma cells were synchronized by either centrifugal elutriation or chemical blockage with the DNA synthesis inhibitors hydroxyurea or aphidicolin. In non-perturbed elutriated cells, c-sis RNA levels were lower in the S phase of the cell cycle than in the G1 phase. In contrast, the chemically synchronized cells revealed a transient rise in c-sis RNA shortly after drug release, in early S phase. The RNA changes occurring after release from drug inhibition represent cell recovery from drug induced metabolic disturbances rather than true cell cycle dependent effects.

Inhibition of DNA synthesis and cell division by a cell surface sialoglycopeptide

We have isolated and purified a cell surface sialoglycopeptide (SGP) from bovine cerebral cortex cells that previously was shown to be a potent inhibitor of cellular protein synthesis. The following studies were carried out to characterize the potential ability of the SGP to inhibit DNA synthesis and to arrest cell division. Treatment of exponentially proliferating Swiss 3T3 cells with the SGP inhibitor resulted in a marked inhibition of thymidine incorporation within 24 h. When the SGP was removed from inhibited cultures, a sharp rise in 3H-thymidine incorporation followed within 3-4 h that peaked well above that measured in exponentially growing cultures, suggesting that the inhibitory action of the SGP was reversible and that a significant proportion of the arrested cells was synchronized in the mitotic cycle. In addition to DNA synthesis, the inhibitory action of the SGP was monitored by direct measurement of cell number. Consistent with the thymidine incorporation data, the SGP completely inhibited 3T3 cell division 20 h after its addition to exponentially growing cultures. Upon reversal there was a delay of 15 h before cell division resumed, when the arrested cells quickly doubled. Most, if not all, of the growth-arrested cells appeared to have been synchronized by the SGP. The SGP inhibited DNA synthesis in a surprisingly wide variety of target cells, and the relative degree of their sensitivity to the inhibitor was remarkably similar. Cells sensitive to the SGP ranged from vertebrate to invertebrate cells, fibroblast and epitheliallike cells, primary cells and established cell cultures, as well as a wide range of transformed cell lines.