Exposure to caffeine and suppression of DNA replication combine to stabilize the proteins and RNA required for premature mitotic events

Extended access methamphetamine decreases immature neurons in the hippocampus which results from loss and altered development of neural progenitors without altered dynamics of the S-phase of the cell cycle

Methamphetamine addicts demonstrate impaired hippocampal-dependent cognitive function that could result from methamphetamine-induced maladaptive plasticity in the hippocampus. Reduced adult hippocampal neurogenesis observed in a rodent model of compulsive methamphetamine self-administration partially contributes to the maladaptive plasticity in the hippocampus. The potential mechanisms underlying methamphetamine-induced inhibition of hippocampal neurogenesis were identified in the present study. Key aspects of the cell cycle dynamics of hippocampal progenitors, including proliferation and neuronal development, were studied in rats that intravenously self-administered methamphetamine in a limited access (1h/day: short access (ShA)-4 days and ShA-13 days) or extended access (6h/day: long access (LgA)-4 days and LgA-13 days) paradigm. Immunohistochemical analysis of Ki-67 cells with 5-chloro-2'-deoxyuridine (CldU) demonstrated that LgA methamphetamine inhibited hippocampal proliferation by decreasing the proliferating pool of progenitors that are in the synthesis (S)-phase of the cell cycle. Double S-phase labeling with CldU and 5-iodo-2'-deoxyuridine (IdU) revealed that reduced S-phase cells were not due to alterations in the length of the S-phase. Further systematic analysis of Ki-67 cells with GFAP, Sox2, and DCX revealed that LgA methamphetamine-induced inhibition of hippocampal neurogenesis was attributable to impairment in the development of neuronal progenitors from preneuronal progenitors to immature neurons. Methamphetamine concomitantly increased hippocampal apoptosis, changes that were evident during the earlier days of self-administration. These findings demonstrate that methamphetamine self-administration initiates allostatic changes in adult neuroplasticity maintained by the hippocampus, including increased apoptosis, and altered dynamics of hippocampal neural progenitors. These data suggest that altered hippocampal plasticity by methamphetamine could partially contribute to methamphetamine-induced impairments in hippocampal function.

Effects of Caffeine on Protein Phosphorylation and Cell Cycle Progression in X-irradiated Two-cell Mouse Embryos

The G2 phase/mitosis transition in cleavage-stage mouse embryos is correlated with an increased phosphorylation of a defined set of proteins at 46, 35, 30, and 29 kDa. Cell cleavage and the associated changes in protein phosphorylation are delayed after X-irradiation. To understand the mechanism of the caffeine-induced uncoupling of mitosis and the cellular reactions to DNA-damaging agents, we have studied the effects of caffeine treatment on cell cycle progression and protein phosphorylation in two-cell mouse embryos after X-irradiation. Caffeine alone had no effect on timing of and changes in phosphorylation associated with the embryonic cell cycle. In combination with X-rays, however, caffeine was able to override the radiation induced G2 block and restored the normal timing of these phosphorylation changes after X-irradiation. However, new additional changes in protein phosphorylation appeared after the combined treatment. Isobutylmethylxanthine (IBMX), a substance chemically related to caffeine but a more specific inhibitor of the phosphodiesterase that breaks down cyclic AMP, reduced the radiation induced G2 block from 4 to 5 h to about 1 h and restored the cell cycle associated changes in protein phosphorylation. However, the same new changes which appeared after the combined treatment of caffeine and X-rays were observed after the combination of IBMX and X-irradiation. IBMX specific changes in protein phosphorylation were detected in both the single and the combined treatment. These results indicate a similar action of caffeine and IBMX in overriding the radiation induced G2 block in two-cell mouse embryos.

Radiomimetic Cell Cycle Delay Induced by Tetranodecanoyl Phorbol Acetate is Enhanced by Caffeine and by the Protein Kinase Inhibitor 2-aminopurine

The tumour promoter and protein kinase C agonist, 12-O-tetranodecanoyl-phorbol-13-acetate (TPA), has been reported to show a radiomimetic action because it transiently delays the passage of HeLa cells through the G2 phase, as do ionizing radiation and other DNA damaging agents. Caffeine is known to override the G2 delay imposed by DNA damage; it is shown here that caffeine does not override the radiomimetic delay imposed by TPA in HeLa, but instead enhances it, without affecting G2 progression in control cells. Most of the other agents which more specifically affect some of the diverse range of caffeine targets either do not affect G2 progression after TPA, or delay G2 progression in control cells and exert a further delay in the presence of TPA. The exception is 2-aminopurine, a protein kinase inhibitor which has been shown to have an action similar to that of caffeine is allowing progression of the cell cycle to mitosis after the inhibition of DNA synthesis, without affecting normal cycle progression through G2. This agent, like caffeine, also has the contrary action of retarding cycle progression after TPA. It is concluded that the G2 delays induced by ionizing radiation and by TPA operate by different mechanisms, which are modulated in opposite senses by mechanisms involving protein kinase inhibition.

DNA replication and progression through the cell cycle

Somatic cells possess control mechanisms which monitor DNA replication and assure that it is complete before mitosis is initiated. We have been investigating these mechanisms in Xenopus egg extracts. Using in vitro cycling extracts, which spontaneously alternate between interphase and mitosis, we found that the onset of mitosis is inhibited by the presence of unreplicated DNA, demonstrating that the completion of DNA replication and the initiation of mitosis are coupled in these extracts. As in somatic cells, this coupling is sensitive to caffeine and to okadaic acid. In Xenopus extracts unreplicated DNA increases the tyrosine phosphorylation of p34cdc2, thereby maintaining MPF (mitosis-promoting factor) in an inactive state and preventing the onset of mitosis. The block to mitosis in the presence of unreplicated DNA can be reversed by the addition of bacterially expressed cdc25 protein. The extent of MPF activation by cdc25 protein under these conditions depends on the number of nuclei present. We have developed an assay to examine the rate of tyrosine phosphorylation on p34cdc2. It is increased by unreplicated DNA, in a manner consistent with unreplicated DNA up-regulating the kinase that phosphorylates p34cdc2. We have begun to examine how unreplicated DNA generates the signal that inhibits MPF activation by testing the ability of naked single- and double-stranded DNA templates to inhibit mitosis, and by investigating the role of RCC1, a chromatin-associated protein required for the coupling of DNA replication and mitosis.

A novel strategy for identifying mutations that sensitize Drosophila eye development to caffeine and hydroxyurea

This report describes a novel strategy for isolating Drosophila mutants with conditional eye phenotypes that should be generally applicable for identifying genes required for cellular responses to specific drugs. To test the strategy, we screened 3 of the 5 major chromosome arms for hydroxyurea- and (or) caffeine-sensitive (huc) mutants, and isolated mutations affecting 5 different complementation groups. Most of these were represented by single alleles; however, we also isolated multiple alleles of huc(29DE) gene, an essential gene that is also associated with a nonconditional pupal lethal phenotype. We also identified huc(95E) mutants, which are extremely sensitive to caffeine. Although huc(95E) is a nonessential gene, mutant imaginal disc cells undergo caffeine-dependent apoptosis, and huc(95E) gene function is required for the viability of the organism when mutant larvae are exposed to levels of caffeine that controls can easily tolerate. We have mapped the cytological positions of huc(29D) and huc(95E) as a first step toward molecularly characterizing the relevant genes.

Mornings with art, lessons learned: Feedback regulation, restriction threshold biology, and redundancy govern molecular stress responses

Work from the laboratory of Dr. Arthur B. Pardee has highlighted basic principles that govern cellular and molecular biological processes in living cells. Among the most important governing principles in cellular and molecular responses are: (i) threshold "restriction" responses, wherein a level of response is reached and a "point of no return" is achieved; (ii) feedback regulation; and (iii) redundancy. Lessons learned from the molecular biology of cellular stress responses in mammalian cancer versus normal cells after ionizing radiation (IR) or chemotherapeutic agent exposures reveal similar instances of these guiding principles in mammalian cells. Among these are the: (i) induction of cell death responses by beta-lapachone (beta-lap), a naphthoquinone anti-tumor agent that kills cancer cells via an NQO1 (i.e., X-ray-inducible protein-3, xip3)-dependent mechanism; (ii) induction of secretory clusterin (sCLU) in response to TGF-beta1 exposure, and the ability of induced sCLU protein to down-regulate TGF-beta1 signaling; and (iii) induction of DNA mismatch repair-dependent G(2) cell cycle checkpoint responses after exposure to alkylating agents. We have learned these lessons and now adopted strategies to exploit them for improved therapy. These examples will be discussed and compared to the pioneering findings of researchers in the Pardee laboratory over the years.

Chronic morphine induces premature mitosis of proliferating cells in the adult mouse subgranular zone

The birth of cells with neurogenic potential in the adult brain is assessed commonly by detection of exogenous S phase markers, such as bromodeoxyuridine (BrdU). Analysis of other phases of the cell cycle, however, can provide insight into how external factors, such as opiates, influence the cycling of newly born cells. To this end, we examined the expression of two endogenous cell cycle markers in relation to BrdU: proliferating cell nuclear antigen (PCNA) and phosphorylated histone H3 (pHisH3). Two hours after one intraperitoneal BrdU injection, BrdU-, PCNA-, and pHisH3-immunoreactive (IR) cells exhibited similar distribution in the adult mouse subgranular zone (SGZ). Quantitative analysis within the SGZ revealed a relative abundance of cells labeled for PCNA > BrdU >> pHisH3. Similar to our reports in rat SGZ, chronic morphine treatment decreased BrdU- and PCNA-IR cells in mouse SGZ by 28 and 38%, respectively. We also show that pHisH3-IR cells are influenced by chronic morphine to a greater extent (58% decrease) than are BrdU- or PCNA-IR cells. Cell cycle phase analysis of SGZ BrdU-IR cells using triple labeling for BrdU, PCNA, and pHisH3 revealed premature mitosis in chronic morphine-treated mice. These results suggest that morphine-treated mice have a shorter Gap2/mitosis (G(2)/M) phase when compared to sham-treated mice. These findings demonstrate the power of using a combination of exogenous and endogenous cell cycle markers and nuclear morphology to track proliferating cells through different phases of the cell cycle and to reveal the regulation of cell cycle phase by chronic morphine.

Caffeine mimics adenine and 2'-deoxyadenosine, both of which inhibit the guanine-nucleotide exchange activity of RCC1 and the kinase activity of ATR

BACKGROUND

Both caffeine and the inactivation of RCC1, the guanine-nucleotide exchange factor (GEF) of Ran, induce premature chromatin condensation (PCC) in hamster BHK21 cells arrested in the S-phase, suggesting that RCC1 is a target for caffeine.

RESULTS

Caffeine inhibited the Ran-GEF activity of RCC1 by preventing the binary complex formation of Ran-RCC1. Inhibition of the Ran-GEF activity of RCC1 by caffeine and its derivatives was correlated with their ability to induce PCC. Since caffeine is a derivative of xanthine, the bases and nucleosides were screened for their ability to inhibit RCC1. Adenine, adenosine, and all of the 2'-deoxynucleosides inhibited the Ran-GEF activity of RCC1; however, only adenine and 2'-deoxyadenosine (2'-dA) induced PCC. A factor(s) other than RCC1, should therefore be involved in PCC-induction. We found that both adenine and 2'-dA, but none of the other 2'-deoxynucleosides, inhibited the kinase activity of ATR, similar to that of caffeine. The ATR pathway was also abrogated by the inactivation of RCC1 in tsBN2 cells.

CONCLUSION

The effect of caffeine on cell-cycle control mimics the biological effect of adenine and 2'-dA, both of which inhibit ATR. dATP, a final metabolite of adenine and 2'-dA, is suggested to inhibit ATR, resulting in PCC.

Caffeine does not cause override of the G2/M block induced by UVc or gamma radiation in normal human skin fibroblasts

Caffeine has for many years been known to be involved in the sensitization of DNA to damage. One potential mechanism recently put forward is an override of the G2/M block induced by irradiation, which would leave the cells less time for DNA repair prior to mitosis. However, different cell types display a variety of responses and no clear pathway has yet emerged, especially as little is known about the capacity of this agent to enhance DNA damage in normal, untransformed cells. Continuous exposure to commonly used caffeine concentrations (1-5 mM) inhibited the proliferation of normal human fibroblasts (NHFs) in a dose-dependent manner to up to 80% at 5 mM. Exposure of exponentially growing NHFs to UVc radiation (20 J m(-2)) or gamma radiation (2.5-8 Gy) led to a 45-60% inhibition of proliferation and protracted accumulation of cells in the G2/M phase. Addition of 2 mM caffeine after irradiation induced slowing of the S phase passage, with a resultant delay in G2/M accumulation mimicking a G2/M block override. These results were confirmed by stathmokinetic studies, which showed delayed entry of the cells into mitosis in the presence of caffeine. Our data demonstrate that caffeine primarily inhibits replicative DNA synthesis and suggest that, at least in normal cells, caffeine potentiates the cytotoxicity of radiation by intervening in DNA repair rather than by overriding the G2/M block.

The role of the Ran GTPase pathway in cell cycle control and interphase nuclear functions

Ran is a small, highly abundant, nuclear GTPase. Mutants in Ran and in proteins that interact with it disrupt the normal checkpoint control of mitosis with respect to the completion of DNA synthesis. Ran and other components of this pathway are also required for numerous nuclear functions such as RNA export, protein import, RNA processing and DNA replication. It will be important to understand how these facets of Ran's activities are linked and how they promote correct control of the cell cycle. This review examines recent progress in discovering other components of the Ran GTPase pathway and considers how this pathway may be required for the control of the cell cycle.

Microtubule dependence of chromosome cycles in Xenopus laevis blastomeres under the influence of a DNA synthesis inhibitor, aphidicolin

The spindle-assembly checkpoint of the cell cycle develops in Xenopus laevis embryos at the midblastula transition (MBT). Our previous experiments using animal-cap blastomeres indicate that the checkpoint is regulated by a mechanism that depends on age, but not on the nucleocytoplasmic (N/C) ratio (Clute and Masui, 1995). In the present study, the time of appearance of the spindle-assembly checkpoint is examined in animal-cap blastomeres whose N/C ratio is reduced by treatment with aphidicolin. Animal-cap blastomeres treated with aphidicolin from the 2-cell stage cleave more slowly after 4th cleavage, in a dose-dependent manner, but cleavage and chromosome cycles continue up to the 11th to 13th cleavage and then arrest. Blastomeres treated with aphidicolin have a reduced DNA content and N/C ratio compared to control blastomeres of the same age. Nevertheless, nocodazole-sensitive chromosome cycles appear at the same time as in control blastomeres, at 3 to 5 hr after 5th cleavage, regardless of the N/C ratio. The arrest in interphase caused by treating blastula stage animals caps with aphidicolin can be reversed by treatment with caffeine. The caffeine-induced mitosis becomes sensitive to nocodazole after the MBT, but not before. Therefore, the same mechanism which stabilizes maturation-promoting factor activity in the absence of a mitotic spindle also operates after the MBT in blastomeres that are treated with aphidicolin, if mitosis is induced by caffeine. This mechanism may involve the translation of a maternal mRNA at the time of the MBT, as suggested previously.

Caffeine-resistance in fission yeast is caused by mutations in a single essential gene, crm1+

Caffeine is a base analogue and is known to affect a wide variety of cellular processes. In order to dissect genetically molecules which mediate the biological effects of caffeine, temperature-sensitive (ts) and caffeine-resistant mutants were isolated from fission yeast, Schizosaccharomyces pombe. Surprisingly, all twelve ts isolates contained a mutation in the same locus, crm1. Cells of the ts crm1 mutant showed an abnormal chromosome structure at the restrictive temperature, an elevated expression of Pap1-dependent transcription, and cross-resistance to an unrelated drug such as staurosporine. Overproduction of pap1+ also conferred caffeine resistance, whilst the resistance of the crm1 mutant is abolished in the pap1- background. These results show that the crm1+ gene is a major locus for caffeine resistance, which arises from Pap1-dependent transcriptional activation.

A mutant form of the Ran/TC4 protein disrupts nuclear function in Xenopus laevis egg extracts by inhibiting the RCC1 protein, a regulator of chromosome condensation

The Ran protein is a small GTPase that has been implicated in a large number of nuclear processes including transport. RNA processing and cell cycle checkpoint control. A similar spectrum of nuclear activities has been shown to require RCC1, the guanine nucleotide exchange factor (GEF) for Ran. We have used the Xenopus laevis egg extract system and in vitro assays of purified proteins to examine how Ran or RCC1 could be involved in these numerous processes. In these studies, we employed mutant Ran proteins to perturb nuclear assembly and function. The addition of a bacterially expressed mutant form of Ran (T24N-Ran), which was predicted to be primarily in the GDP-bound state, profoundly disrupted nuclear assembly and DNA replication in extracts. We further examined the molecular mechanism by which T24N-Ran disrupts normal nuclear activity and found that T24N-Ran binds tightly to the RCC1 protein within the extract, resulting in its inactivation as a GEF. The capacity of T24N-Ran-blocked interphase extracts to assemble nuclei from de-membranated sperm chromatin and to replicate their DNA could be restored by supplementing the extract with excess RCC1 and thereby providing excess GEF activity. Conversely, nuclear assembly and DNA replication were both rescued in extracts lacking RCC1 by the addition of high levels of wild-type GTP-bound Ran protein, indicating that RCC1 does not have an essential function beyond its role as a GEF in interphase Xenopus extracts.

Potential genotoxic, mutagenic and antimutagenic effects of coffee: a review

Coffee and caffeine are mutagenic to bacteria and fungi, and in high concentrations they are also mutagenic to mammalian cells in culture. However, the mutagenic effects of coffee disappear when bacteria or mammalian cells are cultured in the presence of liver extracts which contain detoxifying enzymes. In vivo, coffee and caffeine are devoid of mutagenic effects. Coffee and caffeine are able to interact with many other mutagens and their effects are synergistic with X-rays, ultraviolet light and some chemical agents. Caffeine seems to potentiate rather than to induce chromosomal aberrations and also to transform sublethal damage of mutagenic agents into lethal damage. Conversely, coffee and caffeine are also able to inhibit the mutagenic effects of numerous chemicals. These antimutagenic effects depend on the time of administration of coffee as compared to the acting time of the mutagenic agent. In that case, caffeine seems to be able to restore the normal cycle of mitosis and phosphorylation in irradiated cells. Finally, the potential genotoxic and mutagenic effects of the most important constituents of coffee are reviewed. Mutagenicity of caffeine is mainly attributed to chemically reactive components such as aliphatic dicarbonyls. The latter compounds, formed during the roasting process, are mutagenic to bacteria but less to mammalian cells. Hydrogen peroxide is not very active but seems to considerably enhance mutagenic properties of methylglyoxal. Phenolic compounds are not mutagenic but rather anticarcinogenic. Benzopyrene and mutagens formed during pyrolysis are not mutagenic whereas roasting of coffee beans at high temperature generates mutagenic heterocyclic amines. In conclusion, the mutagenic potential of coffee and caffeine has been demonstrated in lower organisms, but usually at doses several orders of magnitude greater than the estimated lethal dose for caffeine in humans. Therefore, the chances of coffee and caffeine consumption in moderate to normal amounts to induce mutagenic effects in humans are almost nonexistent.

The rad3+ gene of Schizosaccharomyces pombe is involved in multiple checkpoint functions and in DNA repair

A number of important molecular checkpoints are believed to control the orderly progression of cell cycle events. We have found that the radiation-sensitive Schizosaccharomyces pombe mutant rad3-136 is deficient in two molecular checkpoint functions. Unlike wild-type cells, the mutant cells are unable to arrest in the G2 phase of the cell cycle after DNA damage by gamma-irradiation and are also incapable of maintaining the dependence of mitosis upon the completion of DNA synthesis. An S. pombe genomic clone that complements the UV sensitivity of the rad3-136 mutant completely restores the missing checkpoint functions. The rad3+ gene is also likely to play a role in DNA repair.

Loosening of cell cycle controls of human lymphocytes under the action of tumour promoter TPA

The effect of tumour promoter TPA (12-O-tetradecanoylphorbol-13-acetate) on the cell cycle of human peripheral blood lymphocytes stimulated by phytohaemagglutinin (PHA) in vitro was studied and it was found that TPA caused cells to accumulate in all the cell cycle phases. This accumulation took place preferentially at later culture passages, when lymphocytes stimulated by PHA alone stopped mainly in G0/G1 phases. Other effects of TPA were cell induction to enter higher DNA ploidy and to survive and even synthesize DNA under colchicine block of mitosis or under cytochalasin block of cytokinesis. In addition, in experiments in which a transitory block through the G1 phase of cell cycle was applied with use of aminopterin, we could show that a fraction of TPA-treated cells still entered the active phase of DNA synthesis. These findings suggest that TPA causes cell cycle controls to become loose, thereby enhancing adaptability of human lymphocytes to various hindrances in the course of cell cycle and eventually causing them to acquire characteristics known to be common for tumour cells.

Feedback control of mitosis in budding yeast

We have investigated the feedback control that prevents cells with incompletely assembled spindles from leaving mitosis. We isolated budding yeast mutants sensitive to the anti-microtubule drug benomyl. Mitotic arrest-deficient (mad) mutants are the subclass of benomyl-sensitive mutants in which the completion of mitosis is not delayed in the presence of benomyl and that die as a consequence of their premature exit from mitosis. A number of properties of the mad mutants indicate that they are defective in the feedback control over the exit from mitosis: their killing by benomyl requires passage through mitosis; their benomyl sensitivity can be suppressed by an independent method for delaying the exit from mitosis; they have normal microtubules; and they have increased frequencies of chromosome loss. We cloned MAD2, which encodes a putative calcium-binding protein whose disruption is lethal. We discuss the role of feedback controls in coordinating events in the cell cycle.

Caffeine overcomes a restriction point associated with DNA replication, but does not accelerate mitosis

Mitotic chromosome condensation is normally dependent on the previous completion of replication. Caffeine spectacularly deranges cell cycle controls after DNA polymerase inhibition or DNA damage; it induces the condensation, in cells that have not completed replication, of fragmented nuclear structures, analogous to the S-phase prematurely condensed chromosomes seen when replicating cells are fused with mitotic cells. Caffeine has been reported to induce S-phase condensation in cells where replication is arrested, by accelerating cell cycle progression as well as by uncoupling it from replication; for, in BHK or CHO hamster cells arrested in early S-phase and given caffeine, condensed chromosomes appear well before the normal time at which mitosis occurs in cells released from arrest. However, we have found that this apparent acceleration depends on the technique of synchrony and cell line employed. In other cells, and in synchronized hamster cells where the cycle has not been subjected to prolonged continual arrest, condensation in replication-arrested cells given caffeine occurs at the same time as normal mitosis in parallel populations where replication is allowed to proceed. This caffeine-induced condensation is therefore "premature" with respect to the chromatin structure of the S-phase nucleus, but not with respect to the timing of the normal cycle. Caffeine in replication-arrested cells thus overcomes the restriction on the formation of mitotic condensing factors that is normally imposed during DNA replication, but does not accelerate the timing of condensation unless cycle controls have previously been disturbed by synchronization procedures.

Completion of DNA replication is monitored by a feedback system that controls the initiation of mitosis in vitro: studies in Xenopus

During cell division complete DNA replication must occur before mitosis is initiated. Using a cell-free extract derived from Xenopus eggs that oscillates between S phase and mitosis, we have investigated how completion of DNA synthesis is coupled to the initiation of mitosis. We find that Xenopus eggs contain a feedback pathway which suppresses mitosis until replication is completed and that activation of this inhibitory system is dependent on the presence of a threshold concentration of unreplicated DNA. We demonstrate that in the presence of unreplicated DNA the active feedback system inhibits initiation of mitosis by blocking the activation of MPF, a regulator of mitosis found in all eukaryotic cells. Our results demonstrate that the feedback system does not inhibit MPF activation by blocking the synthesis or accumulation of cyclin protein, a subunit of MPF, or by blocking association of cyclin with the cdc2 subunit of MPF. We propose that the feedback system blocks mitosis by maintaining MPF in an inactive state by modulating posttranslational modifications critical for MPF activation.

Action of caffeine on DNA replication after ultraviolet irradiation in Indian muntjac cells: no connection with action on cell cycle delay

Indian muntjac fibroblasts of the SV40-transformed line SVM are hypersensitivity to UV, and after UV irradiation have defective post-replication recovery and a high level of sister chromatid exchanges and chromosome aberrations. The lethal and clastogenic effects of UV on SVM have elsewhere been shown to be aggravated by caffeine, which overcomes the block to cycle traverse imposed by DNA damage; however, in DM cells, an Indian muntjac line of normal UV sensitivity, caffeine has no effect on cycle traverse, but nevertheless enhances UV killing and sister chromatid exchanges. In this paper, the effects of caffeine on irradiated DM cells are shown to be due to its inhibition of post-replication recovery, with subsequent formation of DNA double-strand breaks at the strand gaps thus produced. By contrast, in SVM cells the limited capacity for post-replication recovery is relatively insensitive to caffeine after UV fluences which permit significant cell survival; however, caffeine still strongly induces DNA double-strand breaks and chromosome aberrations, apparently by an alternative mechanism. The SVM and DM cell lines therefore exemplify separate actions of caffeine on mammalian cells, deficient in the caffeine effects on post-replication recovery and cell cycle progression, respectively.

Caffeine increases sensitivity of DNA to denaturation in chromatin of L1210 cells

Exposure of exponentially growing L1210 cells to 5 mM and higher concentrations of caffeine perturbs their progression through the cell cycle and results in increased sensitivity of DNA in situ to denaturation. The latter is detected by the increased metachromatic stainability of DNA with acridine orange (AO) and sensitivity to S1 nuclease, measured by flow cytometry. Decreased DNA stability is generally characteristic of chromatin condensation and in untreated cells is observed in mitosis or quiescence (G0). The caffeine-induced decrease in DNA stability affects the interphase cells regardless of their position in the cycle and the changes are stochastic, concentration- and time-dependent. Populations of cells responding to caffeine are very heterogenous with respect to the degree of destabilization of DNA; sensitivity of DNA to denaturation of the maximally affected cells is similar to that of untreated cells in mitosis. The present method allows one to quantitatively express effects of caffeine on nuclear chromatin in individual cells of large cell populations and may be employed in studies correlating chromatin changes induced by this agent with its effects in modulation of cell sensitivity to radiation or antitumour drugs.

Anticarcinogenic potential of DNA-repair modulators

Effects of compounds that inhibit repair of DNA lesions in cells have been reported frequently. The consequences include altered incidence of carcinogenicity in vivo, tumorigenic transformation of cultured cells, mutations, and increased lethality as well as sister-chromatid exchanges and chromosome aberrations. This literature is reviewed here, with major emphasis on methylxanthines (caffeine in particular) and nicotinamide analogs. Existing information is also summarized on a novel potent repair inhibitor, beta-lapachone. Compounds that inhibit both DNA replication and repair are not discussed in detail since they have been reviewed often, but miscellaneous inhibitors of repair are summarized in a table. The relatively small number of experiments performed on the anticarcinogenic effects of methyl-xanthines and nicotinamide analogs gave very conflicting results. Some investigators report decreased carcinogenicity of DNA-damaging agents when caffeine was provided, but others obtained the opposite effect. The three studies with nicotinamide analogs all reported enhanced tumorigenicity of carcinogens. The data are too few to enable firm conclusions to be drawn regarding the possibility of using repair inhibitors to prevent cancer in humans. Variations of experimental conditions, carcinogens, cells, etc. have provided conflicting results. The possibility of cancer prevention is, nevertheless, so important that further investigations with DNA-repair inhibitors, particularly with human cells, seem very well justified.

Periodic mitotic events induced in the absence of DNA replication

We have discovered and report here a means of separating a mitotic "subcycle" from the G1- and S-phase events of the mammalian cell cycle. Time-lapse videomicroscopy of Syrian hamster fibroblast (BHK) cells revealed that caffeine could induce multiple entries into mitosis while cells were blocked in DNA synthesis. As with normal mitoses, the abundance of mitosis-specific phosphoproteins was coupled with the condensation of chromatin. The BHK temperature-sensitive mutant tsBN2 also completed multiple entries into mitosis while arrested during DNA replication and raised to the restrictive temperature. Periodic mitotic events occurred even when BHK cells were exposed to low concentrations of serum or cycloheximide, conditions that prevent the cycling of BHK cells by blocking their entry into S phase. These results suggest that an oscillation governing the activation and inactivation of mitotic factors can be generated in mammalian cells and uncoupled from the G1 and DNA replication events of the normal cell cycle. This system will be useful for examining the molecular nature of mitotic factors.

Isolation and characterization of the active cDNA of the human cell cycle gene (RCC1) involved in the regulation of onset of chromosome condensation

The human RCC1 gene was cloned after DNA-mediated gene transfer into the tsBN2 cell line, which shows premature chromosome condensation at nonpermissive temperatures (39.5-40 degrees C). This gene codes for a 2.5-kb poly(A)+ RNA that is well conserved in hamsters and humans. We isolated 15 cDNA clones from the Okayama-Berg human cDNA library, and found two that can complement the tsBN2 mutation with an efficiency comparable to that of the genomic DNA clone. The base sequences of these two active cDNA clones differ at the 5' proximal end, yet both have a common open reading frame, encoding a protein of 421 amino acids with a calculated molecular weight of 44,847 and with seven homologous repeated domains of about 60 amino acids. This human RCC1 gene was located to human chromosome 1 using sorted chromosomal fractions.