Improved method for computing potential doubling time from flow cytometric data

Inhibition of human T-cell proliferation by mammalian target of rapamycin (mTOR) antagonists requires noncoding RNA growth-arrest-specific transcript 5 (GAS5)

The central importance of the serine/threonine protein kinase mTOR (mammalian Target of Rapamycin) in the control of cell growth and proliferation is well established. However, our knowledge both of the upstream pathways controlling mTOR activity and of the downstream events mediating these effects is still seriously incomplete. We report a previously unsuspected role for the nonprotein-coding RNA GAS5 in the inhibition of T-cell proliferation produced by mTOR antagonists such as rapamycin. GAS5 transcripts are up-regulated during growth arrest and after rapamycin treatment, and GAS5 has recently been shown to be necessary and sufficient for normal T-cell growth arrest. Down-regulation of GAS5 using RNA interference protects both leukemic and primary human T cells from the inhibition of proliferation produced by mTOR antagonists. The GAS5 transcript is a member of the 5' terminal oligopyrimidine class of RNAs, which is specifically controlled at the level of translation by the mTOR pathway, and the effects of GAS5 on the cell cycle provide a novel and important link to the control of proliferation. These observations point to a significant advance in our understanding of the mechanism of action of mTOR inhibitors, which is likely to lead to improvements in immunosuppressive and cancer therapy.

Growth arrest in human T-cells is controlled by the non-coding RNA growth-arrest-specific transcript 5 (GAS5)

The control of growth of lymphocyte populations is crucial to the physiological regulation of the immune system, and to the prevention of both leukaemic and autoimmune disease. This control is mediated through modulation of the cell cycle and regulation of cell death. During log-phase growth the rate of proliferation is high and there is a low rate of cell death. As the population density increases, the cell cycle is extended and apoptosis becomes more frequent as the population enters growth arrest. Here, we show that growth-arrest-specific transcript 5 (GAS5) plays an essential role in normal growth arrest in both T-cell lines and non-transformed lymphocytes. Overexpression of GAS5 causes both an increase in apoptosis and a reduction in the rate of progression through the cell-cycle. Consistent with this, downregulation of endogenous GAS5 inhibits apoptosis and maintains a more rapid cell cycle, indicating that GAS5 expression is both necessary and sufficient for normal growth arrest in T-cell lines as well as human peripheral blood T-cells. Control of apoptosis and the cell cycle by GAS5 has significant consequences for disease pathogenesis, because independent studies have already identified GAS5 as an important candidate gene in the development of autoimmune disease.

Telomerase expression is sufficient for chromosomal integrity in cells lacking p53 dependent G1 checkpoint function

Background

Secondary cultures of human fibroblasts display a finite lifespan ending at senescence. Loss of p53 function by mutation or viral oncogene expression bypasses senescence, allowing cell division to continue for an additional 10 – 20 doublings. During this time chromosomal aberrations seen in mitotic cells increase while DNA damage and decatenation checkpoint functions in G₂ cells decrease.

Methods

To explore this complex interplay between chromosomal instability and checkpoint dysfunction, human fibroblast lines were derived that expressed HPV16E6 oncoprotein or dominant-negative alleles of p53 (A143V and H179Q) with or without the catalytic subunit of telomerase.

Results

Cells with normal p53 function displayed 86 – 93% G₁ arrest after exposure to 1.5 Gy ionizing radiation (IR). Expression of HPV16E6 or p53-H179Q severely attenuated G₁ checkpoint function (3 – 20% arrest) while p53-A143V expression induced intermediate attenuation (55 – 57% arrest) irrespective of telomerase expression. All cell lines, regardless of telomerase expression or p53 status, exhibited a normal DNA damage G₂ checkpoint response following exposure to 1.5 Gy IR prior to the senescence checkpoint. As telomerase-negative cells bypassed senescence, the frequencies of chromosomal aberrations increased generally congruent with attenuation of G₂ checkpoint function. Telomerase expression allowed cells with defective p53 function to grow >175 doublings without chromosomal aberrations or attenuation of G₂ checkpoint function.

Conclusion

Thus, chromosomal instability in cells with defective p53 function appears to depend upon telomere erosion not loss of the DNA damage induced G₁ checkpoint.

A Markov model approach shows a large variation in the length of S phase in MCF-7 breast cancer cells

BACKGROUND

The potential doubling time of a tumor has been suggested to be a measurement of tumor aggressiveness; therefore, it is of interest to find reliable methods to estimate this time. Because of variability in length of the various cell cycle phases, stochastic modeling of the cell cycle might be a suitable approach.

METHODS

The relative movement curve and the DNA synthesis time were estimated by using local polynomial regression methods. Further, the rate of nucleotide incorporation was estimated by using a Markov pure birth process with one absorbing state to model the progression of the DNA distribution through S phase.

RESULTS

An estimate of the DNA synthesis time, with confidence intervals, was obtained from the relative movement curve. The Markov approach provided an estimate of the distribution of the time to complete S phase given the initial distribution. Using the Markov approach we also made an estimate of the mean number of active replicons during S phase.

CONCLUSIONS

A Markov pure birth process has shown to be useful to model the progression of cells through S phase and to increase knowledge about the variability in the length of S phase and a large variation is shown.

Outcome and human epidermal growth factor receptor (HER) 1-4 status in invasive breast carcinomas with proliferation indices evaluated by bromodeoxyuridine labelling

Background

We have shown previously that whereas overexpression of human epidermal growth factor receptor (HER)1, HER2 and HER3 is associated with poor prognosis in breast cancer, HER4 is associated with a good prognosis. Cell proliferation is a key component of aggressive cancers and is driven by growth factors. In this study, bromodeoxyuridine (BrdU)-derived proliferation indices are correlated with clinical outcome and HER1–4 status for further clarification of the differing roles for the HER family at a biological level.

Methods

Seventy-eight invasive breast cancers had BrdU labelling in vivo to determine the BrdU labelling index (BLI) and the potential tumour doubling time (T_pot). Long-term clinical follow-up was available for these patients. We used immunohistochemistry to establish the HER1–4 status in 55 patients from the BrdU cohort.

Results

We demonstrate a significant correlation between high BLI values and breast cancer-specific death (P = 0.0174). Low T_pot times were also significantly correlated with breast cancer-specific death (P = 0.0258). However, BLI did not independently predict survival in Cox's multiple regression analysis when combined with other prognostic factors such as size, grade and nodal status. Tumours found to be positive for HER1, HER2 or HER3 had significantly (P = 0.041) higher labelling indices, with HER1 also showing significantly higher indices when considered independently (P = 0.024). Conversely, HER4 positivity was significantly correlated (P = 0.013) with low BLI values, in line with previous data associating this receptor with good prognosis tumours.

Conclusions

These results support the hypothesis that HER1–3 are associated with driving tumour proliferation, whereas HER4 is involved in a non-proliferative or even protective role.

Cellular kinetics of murine lung: model system to determine basis for radioprotection with keratinocyte growth factor

PURPOSE

Normal tissue toxicity remains a dose limitation for cancer radiotherapy and chemoradiotherapy. Growth factors offer a novel means of mitigating normal tissue radiotoxicity. In particular, keratinocyte growth factor (rHuKGF), whose proliferative activity is restricted to epithelial cells, holds promise on the basis of the findings of preclinical models of epithelial cytoprotection and the clinical developments to date. We report the radioprotection of murine lung by an increase in tissue cellularity after rHuKGF-induced proliferation.

METHODS AND MATERIALS

Flow cytometric and image analysis techniques after bromodeoxyuridine labeling were used to estimate proliferative parameters. Our specialized analytical methods measure not only labeling indexes, but also the durations of S and G(2)+M phases, potential doubling times, and the net cell production rate. Image analysis techniques were used to identify the specific cell types that were proliferating (type II pneumocytes).

RESULTS

Lung labeling index control values (0.5%) rose to a maximum (5.5%) at 3 days after intratracheal rHuKGF, returning to normal by Day 7. The potential doubling time fell from 66 days to 4.4 days. The net cell production rate rose from a control value of 1%/d to >15%/d by Day 3. This resulted in a nearly twofold increase in alveolar epithelial cellularity, which remained significantly elevated on Day 7. Saline-treated control animals exhibited no significant changes in the proliferative parameter values or cellularity. On the basis of these data, mice were irradiated, solely to the thorax, with ranges of single doses of 250 kVp X-rays 7 days after either intratracheal administration of 5 mg/kg rHuKGF or phospate-buffered saline. This interval was chosen because the proliferative response of the type II cells was finished but the cellularity of the lung remained increased. Pretreatment with rHuKGF extended the latent period before onset of pneumonitis after all radiation doses. rHuKGF treatment 7 days before thoracic irradiation significantly protected against pneumonitis (median effective dose 13.7 Gy, 95% confidence limit 13.4-14.0) compared with the control pretreatment with phosphate-buffered saline (median effective dose 12.8 Gy, 95% confidence limit 12.6-13.1).

CONCLUSION

The data showed that an increase in tissue cellularity, caused by rHuKGF treatment before irradiation, protected the lung from damage due to pneumonitis.

A prospective study of pre-treatment cell kinetics and clinical outcome in nasopharyngeal carcinoma

BACKGROUND AND PURPOSE

To study pre-treatment cell kinetics and their clinical correlations in nasopharyngeal carcinoma (NPC).

MATERIALS AND METHODS

Ninety newly diagnosed NPC patients were studied using in vivo Bromodeoxyuridine (BrdU) labeling and flow cytometric analysis. Immunohistochemical staining for BrdU and Ki 67 was also performed.

RESULTS

The median S-phase duration (Ts) was 6.2 h (range 3.5-18.7 h), median flow cytometric labeling index (FCM-LI) was 7.4% (1.3-37.6%), and median potential doubling time (Tpot) was 3.6 days (0.5-19.9 days). The median histologic labeling index (H-LI) was 12.4% (1.2-43.3%), and median histologic Tpot (H-Tpot) was 2.1 days (0.5-33.3 days). FCM-LI and H-LI were both positively correlated with Ki67 whereas Tpot and H-Tpot were both negatively correlated with Ki67 and N-stage. In univariate analysis, Tpot and H-Tpot showed a trend for progression free survival. Tpot was significantly associated with local relapse free survival, but lost its significance in multivariate analysis. N-stage was the only significant prognostic factor for all radiotherapy outcomes in both univariate and multivariate analyses.

CONCLUSIONS

Tpot was the only pre-treatment cell kinetic parameter for which some evidence was found for an association with survival in NPC patients. Future studies should aim to combine cell kinetic parameters together with other biological markers and clinical parameters to provide more useful prognostic information to guide individual patient's therapy.

Simultaneous acceleration of the cell cycle and suppression of apoptosis by splice variant delta-6 of the candidate tumour suppressor LUCA-15/RBM5

BACKGROUND

The short arm of chromosome 3 is thought to include one or more tumour suppressor genes (TSGs), since carcinoma of various tissues display deletions in this region. Many genes mapping to this region have recently been identified, including the LUCA-15/RBM5 gene.

RESULTS

In this study we report the cloning from human bone marrow library of a splice variant of LUCA-15 which lacks exon 6, resulting in a frameshift and producing a truncated protein of 150 amino acids instead of 815 amino acids. This variant is widely expressed at a low level in normal tissues and is expressed at increased levels in T-leukaemic cell lines. Over-expression of this splice variant after electroporation both shortened the cell cycle and inhibited CD95-mediated apoptosis in CEM-C7 T-cells. In marked contrast, over-expression of the full length LUCA-15/RBM5 suppressed cell proliferation both by inducing apoptosis and by extending the G1 phase of the cell cycle.

CONCLUSION

These results, taken together with previous observations from ourselves and others, suggest that LUCA-15 is involved in the control of both apoptosis and the cell cycle. Since oncogenesis often relies on separate changes in molecules regulating apoptosis on the one hand, and proliferation, on the other, the discovery of a candidate tumour suppressor gene which affects both processes simultaneously is likely to be of major significance.

Inhibition of DNA replication by fish oil-treated cytoplasm is counteracted by fish oil-treated nuclear extract

We have recently noted that cells treated with fish oil and n-3-fatty acids show slower DNA replication rates than cells treated with a control emulsion or corn oil only. However, it is not clearly understood how such an effect is induced. Fish oil and its metabolites are known to have several modulating effects on signal transduction pathways. Alternatively, they may influence DNA replication by interacting directly with nuclear components. To investigate this problem in greater detail, we have studied the kinetics of DNA synthesis in a cell-free system derived from HeLa cells. Nuclei and cytosolic extract were isolated from cells synchronized in early S phase after treatment with control emulsion, corn oil, or fish oil, respectively. The nuclei were reconstituted with cytosolic extract and a reaction mixture containing bromodeoxyuridine (BrdU) triphosphate to label newly synthesized DNA. The rate of DNA synthesis was measured by bivariate DNA/BrdU analysis and flow cytometry. We show that fish oil-treated cytosol inhibits the elongation of newly synthesized DNA by ~80% in control nuclei. However, nuclei treated with fish oil escape this inhibitory effect. We also show that addition of nuclear extract from fish oil-treated cells reverses the inhibitory effect seen in the reconstitution system of control nuclei and fish oil-treated cytosol. These results indicate that polyunsaturated fatty acids can modulate DNA synthesis through cytosolic as well as soluble nuclear factors.

Fish oil slows S phase progression and may cause upstream shift of DHFR replication origin ori-beta in CHO cells

Fish oils (FOs) have been noted to reduce growth and proliferation of certain tumor cells, effects usually attributed to the content of polyunsaturated fatty acids of the n-3 family, which are thought to modulate cellular signaling pathways. We investigated the influence of FO on cell cycle kinetics of cultured Chinese hamster ovary cells. Exponentially growing cells were labeled with 5-bromo-2'-deoxyuridine (BrdU) and analyzed by flow cytometry after 5-day treatment with exogenous fat. Bivariate BrdU-DNA analysis indicated slower progression through S phase and thus longer S phase duration time in FO- but not corn oil-treated or control cells. We hypothesize that FO treatment might interfere with spatial/temporal organization of replication origins. Therefore, we mapped the well-characterized replication origin ori-beta downstream of the dihydrofolate reductase gene with the nascent strand length assay. Three DNA marker segments with known positions relative to this origin were amplified by PCR. By quantitatively assessing DNA length of the fragments in all fractions containing these markers, the location of ori-beta was established. In control or corn oil-treated cells, the location of ori-beta was consistent with previous studies. However, in FO-treated cells, DNA replication appears to start from a new site located farther upstream from ori-beta, suggesting a different replication initiation pattern. This study suggests novel mechanism(s) by which fats affect cell proliferation and DNA replication in mammalian cells.

Instantaneous evaluation of mammalian cell culture growth rates through analysis of the mitotic index

Since a culture increases in cell number when dividing cells separate into two newborn cells, the fraction of mitotic cells in a growing cell population directly reflects the overall growth behavior of a cell culture. To rapidly assess the effects of growth conditions on the fraction of mitotic cells we have employed an antibody specific for the phosphorylated form of histone H3 for the identification of mitotic cells using flow cytometry. The phosphorylation of histone H3 closely correlates with the chromosomal condensation that accompanies the onset of mitosis, and, therefore, it represents a convenient marker for dividing cells. We have optimized the protocol for the staining of mitotic cells for both Chinese hamster ovary and hybridoma cell cultures. Fluorescence micrographs taken of stained cells show that cells in the various stages of mitosis can be detected based on the morphological characteristics of the chromosomes. The variation in the mitotic cell fraction has been determined throughout the batch growth phases of cultures under different growth conditions. The dynamics of the mitotic index show that balanced growth was never truly reached and that the growth rate is in fact quite variable for these cultures since large variations in the mitotic index are observed. In addition, a large increase in the fraction of mitotic cells just prior to the exponential growth phase for all cultures indicates that they are partially synchronized at the exit from the lag phase. According to a two-staged, age structured population balance model, the mitotic index is directly proportional to the growth rate of a culture. The proportionality constant for this case is shown to be the time required for cells to progress through mitosis. This time is believed to be constant for a particular cell line, as shown by experimental data. Thus, growth rates can be determined solely by measurement of the fraction of cells in mitosis. The mitotic index measurements were then used to calculate the growth in cell number of the cultures, and these simulations accurately reflect observed cell counts. Other simulations also show that changes in cell growth can be predicted before they are reflected in the cell count data. This technique can be used as a sensitive indicator of cell growth and could be useful as a process monitoring technique and for developing better feeding strategies for animal cell cultures.

Modelling complex populations formed by proliferating, quiescent and quasi-quiescent cells: application to plant root meristems

A proliferating population of cells may be considered complex when its proliferative or growth fraction P is lower than 1 and/or when it is formed by subpopulations with different mean cycle times. The present paper shows that in such complex populations exponential growth is consistent with a steady-state distribution of cells. Obviously, when P=1 then cell distribution is only a function of cell age. An analytical model has been developed to study complex populations including both quiescent fractions formed by cells with unreplicated genome (G(0) cells) and cells with fully duplicated chromosomes (Q(2) cells). The model also considers those quasi-quiescent cells in their last transit through G(1) and S (Q(1) and Q(s) cells) before becoming quiescent. In order to solve the difficulties of a direct analysis of the whole population, its kinetic parameters have been obtained by studying the negative exponential distribution of two subpopulations: one formed by the proliferating cells and another formed by the quasi-quiescent cells. Additionally, the model could be applied when quiescence is initiated at any other cycle phase different from G(1) and G(2), for instance, cells in the process of replicating their DNA or being at any other mitotic phases. The utility of the method was illustrated in populations which constitute the root meristems of both Allium cepa L. and Pisum sativum L. Three facts should be stressed: (1) the method seems to be rather powerful because it can be carried out from different sets of experimentally measured parameters; (2) the rate of division and, therefore, the population doubling time can be easily estimated by this method; and (3) it also allows the determination of the amount of cells that had become quiescent either before they had replicated their DNA (G(0)) or after having completed their replication (Q(2)), as well as those quasi-quiescent cells which are progressing throughout their last pre-replicative and replicative periods (thus Q(1) and Q(s), respectively).

Expression of truncated FHIT transcripts in cervical cancers and in normal human cells

To analyse FHIT transcription patterns in cervical cancer, a series of primary cervical tumors and normal control samples were studied using RT-PCR. Full length and truncated FHIT transcripts were detectable in all samples tested. Interestingly, the expression of truncated FHIT transcripts by primary epithelial cells in vitro was associated with confluency. The breakpoints of most transcript deletions coincided with genuine splice site sequences, suggesting that they resulted from alternative splicing. These findings demonstrate that truncated FHIT transcripts are commonly detected in both normal and tumor tissues, and suggest that these altered transcripts are not causally related to tumorigenesis in cervical cancer.

Plasma hyaluronidase (Hyal-1) promotes tumor cell cycling

Paradoxically, both hyaluronan (HA) and hyaluronidase are involved in malignant transformation and cancer progression. Their mechanisms of action, given the apparent disparities, are not understood. In many malignancies, levels of HA correlate with metastatic behavior while hyaluronidases suppress malignant progression. Hyal-1, product of one of six paralogous hyaluronidase-like sequences, is the predominant circulating hyaluronidase. HYAL1, the gene that codes for Hyal-1, is located on chromosome 3p21.3, a region containing a tumor suppressor gene. Loss of HYAL1 often correlates with tumor progression, particularly in tobacco-related cancers. In other malignancies, however, hyaluronidase functions as a tumor promoter. Testicular hyaluronidase (PH-20), used as an adjuvant in chemotherapy, is assumed to enhance drug permeability. By an unknown mechanism, hyaluronidases recruit tumor cells back into the cycling pool, making these malignancies more sensitive to chemotherapeutic drugs. Such contradictory observations might be resolved by assuming that HA and hyaluronidase are required at different times in the multiple steps that lead to malignant transformation. We have undertaken a systematic investigation of their roles in cancer progression. Here, we investigate the effect of Hyal-1 expression on cell cycle kinetics. A tumor cell line was constructed with an ecdysone-inducible promoter located upstream from the cDNA of HYAL1. Fluorescent-activated cell sorting was used to monitor cell cycle kinetics following Hyal-1 induction. Enhanced cell cycling was observed, with a 13.6% increase in S phase and 9.6% decrease in G(1)/G(0) phase cells.

Is there a future for cell kinetic measurements using IdUrd or BdUrd?

PURPOSE

The analysis of causes of radiation failure both in retrospective series of patients with head and neck cancer and in several randomized clinical trials suggests a loss of local control as the overall treatment time increases for the same total dose. This is attributed to tumor cell proliferation during fractionated radiotherapy. As longer treatment times lead to loss of local control, it has been suggested that shorter treatment times could lead to an increase in local control. For this reason accelerated treatment regimens have been and are being designed. However, these treatments may cause severe acute reactions. Due to this, lower total doses are sometimes given. Slowly proliferating tumors, therefore, may do worse when treated with accelerated schedules compared with conventional schedules. In addition, it is not desirable to subject all patients to the more intense acute reactions of accelerated schedules. It would thus be useful to predict which tumors will rapidly proliferate during treatment and are likely to benefit from accelerated radiotherapy. The potential doubling time (Tpot) is defined as the time within which the cell population of a tumor would double if there were no cell loss. The hypothesis is that the median Tpot measured before treatment might correlate with the effective doubling time (Tp) during treatment.

CONCLUSION

Tpot can be calculated knowing the labeling index (LI; proportion of cells incorporating the DNA precursor IdUrd or BdUrd) and Ts (the DNA synthesis time) measured by flow cytometry. A recent multicenter analysis has shown that the only pretreatment kinetic parameter for which some evidence is found for an association with local control is LI, not Tpot. Pitfalls associated with cell kinetic measurements such as assay variability, intratumor and intertumor variability, interlaboratory variability and the problem of an admixture of normal and malignant cells make Tpot not accurate and reproducible enough for a robust predictive assay. It therefore appears that pretreatment Tpot measurements using flow cytometry, provide only a relatively weak predictor of outcome after radiotherapy in head and neck cancer. Immunohistochemistry allows a simple measure of LI and may give additional independent information from labeling patterns, suggesting that this method is the (short term) future for clinical cell kinetic measurements using BdUrd or IdUrd.

The nuclear death domain protein p84N5 activates a G2/M cell cycle checkpoint prior to the onset of apoptosis

In contrast to extracellular signals, the mechanisms utilized to transduce nuclear apoptotic signals are not well understood. Characterizing these mechanisms is important for predicting how tumors will respond to genotoxic radiation or chemotherapy. The retinoblastoma (Rb) tumor suppressor protein can regulate apoptosis triggered by DNA damage through an unknown mechanism. The nuclear death domain-containing protein p84N5 can induce apoptosis that is inhibited by association with Rb. The pattern of caspase and NF-kappaB activation during p84N5-induced apoptosis is similar to p53-independent cellular responses to DNA damage. One hallmark of this response is the activation of a G(2)/M cell cycle checkpoint. In this report, we characterize the effects of p84N5 on the cell cycle. Expression of p84N5 induces changes in cell cycle distribution and kinetics that are consistent with the activation of a G(2)/M cell cycle checkpoint. Like the radiation-induced checkpoint, caffeine blocks p84N5-induced G(2)/M arrest but not subsequent apoptotic cell death. The p84N5-induced checkpoint is functional in ataxia telangiectasia-mutated kinase-deficient cells. We conclude that p84N5 induces an ataxia telangiectasia-mutated kinase (ATM)-independent, caffeine-sensitive G(2)/M cell cycle arrest prior to the onset of apoptosis. This conclusion is consistent with the hypotheses that p84N5 functions in an Rb-regulated cellular response that is similar to that triggered by DNA damage.

G2-phase delays after irradiation and/or heat treatment as assessed by two-parameter flow cytometry

Similar to what has been observed after irradiation, the fraction of G(2)-phase cells increases as a consequence of heat treatment. On the basis of cell cycle distributions alone, however, it is difficult to say whether the two results are related. In particular, comparison is complicated by the fact that the accompanying changes in the S-phase transition are different. These changes play a minor role after irradiation but constitute by far the most important cell cycle effect after heat treatment. Two-parameter flow cytometry was used here to study the proliferation of human melanoma cells in vitro. Cultures were pulse-labeled with BrdU after irradiation and/or heat treatment and were fixed either immediately or after a delay of up to 36 h. DNA-synthesizing cells were identified with the help of an FITC-conjugated antibody against BrdU; DNA was quantified after staining with propidium iodide. In this way, the cell cycle distribution could be determined and the progression through the cell cycle could be analyzed. From the movement of labeled cells through the cycle, in particular the appearance of labeled cells in the G(1) compartment (after they had gone through mitosis), the delay in G(2) phase could be determined. The duration of the G(2)/M phase in control cells was about 6 h. This was increased to 12, 13 and 16 h after irradiation (4 Gy X rays), heat treatment (1 h at 43 degrees C), and a combination of the two, respectively. In all these cases, the G(2)-phase block was completely overcome within 48 h after treatment, whereas changes in the S phase were still observable at this time. As expected, the radiation-induced G(2)-phase block was almost completely removed by incubating the cells with 5 or 10 mM caffeine. In the case of hyperthermia alone or in combination with radiation, however, caffeine was somewhat less effective. This does not mean, however, that the mechanisms involved are necessarily different. It can also be seen as a result of the differences in the time course of events. The long delay in S phase after heat treatment may lead to a loss of susceptibility to caffeine by the time the cells move into the G(2) phase.

Mechanisms of G2 arrest in response to overexpression of p53

Overexpression of p53 causes G2 arrest, attributable in part to the loss of CDC2 activity. Transcription of cdc2 and cyclin B1, determined using reporter constructs driven by the two promoters, was suppressed in response to the induction of p53. Suppression requires the regions -287 to -123 of the cyclin B1 promoter and -104 to -74 of the cdc2 promoter. p53 did not affect the inhibitory phosphorylations of CDC2 at threonine 14 or tyrosine 15 or the activity of the cyclin-dependent kinase that activates CDC2 by phosphorylating it at threonine 161. Overexpression of p53 may also interfere with the accumulation of CDC2/cyclin B1 in the nucleus, required for cells to enter mitosis. Constitutive expression of cyclin B1, alone or in combination with the constitutively active CDC2 protein T14A Y15F, did not reverse p53-dependent G2 arrest. However, targeting cyclin B1 to the nucleus in cells also expressing CDC2 T14A Y15F did overcome this arrest. It is likely that several distinct pathways contribute to p53-dependent G2 arrest.

Determination of tumor kinetics: strategies for the delivery of radiotherapy and chemotherapy

Both clinical and laboratory evidence indicates that local control rates for many experimental and clinical human tumors decrease with protraction of the overall duration of radiation therapy and that a likely basis for this decrease is tumor cell repopulation during treatment. Such observations have stimulated interest in tumor kinetics, and a number of techniques have been developed that increase the potential for meaningful clinical study of the proliferative behavior of tumors. This review discusses the clinical and experimental evidence for proliferation during treatment, describes two potential approaches-accelerated fractionation and concurrent chemotherapy and radiotherapy-that can be employed to counteract such intratreatment proliferation, explores methods available for measuring tumor cell kinetics, and discusses how kinetics information may be used in the future to tailor therapy to a tumor's individual characteristics.

Cell kinetics and repopulation parameters of irradiated xenograft tumours in SCID mice: comparison of two dose-fractionation regimens

The extent and mechanism(s) of repopulation were assessed in SiHa (human cervical squamous cell carcinoma) xenografts in SCID mice for two fractionated irradiation regimens. Mice in one arm of the study received 50 Gy in 20 fractions over 23 days with a 14 day split between 10 fraction, 5 day courses. The other tumours were treated with 50 Gy in 20 fractions over 10 consecutive days. Cell kinetics and tumour regrowth parameters were monitored during and after treatment by measuring tumour volume and analysing cellular DNA content and proliferation parameters with flow cytometry. Repopulation occurred rapidly, beginning during irradiation and largely attributable to an increased growth fraction and decreased potential doubling time, apparently triggered by increased cell loss. Cell cycle time, in contrast, remained relatively constant throughout. Extrapolation of these results to humans suggests that treatment times should be minimised whenever possible, since regrowth rates exceeded those predicted from pretreatment Tpot measurements.

Repopulation characteristics and cell kinetic parameters resulting from multi-fraction irradiation of xenograft tumors in SCID mice

PURPOSE

Cell kinetics and repopulation rates during multifraction irradiation have previously been measured in SiHa human cervical carcinoma cells grown as spheroids. The current study applied similar techniques to SiHa tumor xenografts with the ultimate goal of assessing the clinical prognostic value of in situ cell kinetics.

METHODS AND MATERIALS

SiHa (human squamous cell cervical tumor) cells were inoculated subcutaneously in the flank or back of SCID mice. When tumors reached a size of 200-300 mg, they received 25 Gy in 10 fractions over 5 days. Tumor regrowth and cell kinetics parameters were followed during treatment, and for 10 days after completion by measuring tumor volume and analyzing cellular BrdUrd and IdUrd incorporation with flow cytometry.

RESULTS

Tumor volume was of limited use in assessing response to irradiation. The fraction of proliferating cells increased early during irradiation as did the labeling index; potential doubling time (Tpot) decreased during treatment and returned to the pre-irradiation value after treatment. Cell cycle time remained relatively constant throughout the experiments.

CONCLUSION

These results confirm the feasibility of evaluating cell cycle kinetics and repopulation parameters in a murine tumor model undergoing a fractionated course of irradiation. Repopulation of clonogenic tumor cells occurred more rapidly than predicted by pretreatment measurements, primarily due to an increased growth fraction and consequent decrease in Tpot.

Reproducibility of measurements of potential doubling time of tumour cells in the multicentre National Cancer Institute protocol T92-0045

We compared the flow cytometric measurement and analysis of the potential doubling time (Tpot) between three centres involved in the National Cancer Institute (NCI) protocol T92-0045. The primary purpose was to understand and minimize the variation within the measurement. A total of 102 specimens were selected at random from patients entered into the trial. Samples were prepared, stained, run and analysed in each centre and a single set of data analysed by all three centres. Analysis of the disc data set revealed that the measurement of labelling index (LI) was robust and reproducible. The estimation of duration of S-phase (T(S)) was subject to errors of profile interpretation, particularly DNA ploidy status, and analysis. The LI dominated the variation in Tpot such that the level of final agreement, after removal of outliers and ploidy agreement, reached correlation coefficients of 0.9. The sample data showed poor agreement within each of the components of the measurement. There was some improvement when ploidy was in agreement, but correlation coefficients failed to exceed values of 0.5 for Tpot. The data suggest that observer-associated analysis of T(S) and tissue processing and tumour heterogeneity were the major causes of variability in the Tpot measurement. The first two aspects can be standardized and minimized, but heterogeneity will remain a problem with biopsy techniques.

Potential tumour doubling time: determination of Tpot for various canine and feline tumours

Spontaneous tumours in dogs and cats are an excellent model for clinical human research, such as in developing proton conformation radiotherapy for humans. The kinetics of tumour cells can be used effectively to predict prognosis and response to therapy in patients with tumours. Knowledge of the kinetic parameters in these tumours is therefore important. In the present study the kinetic parameters evaluated included the labelling index (LI), relative movement (RM), mitotic index (MI), and potential doubling time (Tpot). These parameters were determined using in vivo labelling with bromodeoxyuridine, flow cytometry and histological preparation. Samples were obtained and evaluated from 72 dogs and 20 cats, presenting as patients in our clinic. Within the groups of epithelial and mesenchymal tumours from dogs and cats, the kinetic parameters LI, RM and MI were compared with Tpot. Significant correlations were observed for the comparison Tpot and LI. No correlation was found between Tpot and RM.

Bcl-2 slows in vitro breast cancer growth despite its antiapoptotic effect

BACKGROUND

Although the Bcl-2 protein promotes tumor cell survival by blocking programmed cell death (apoptosis), Bcl-2 expression has been associated with favorable prognostic indicators in breast cancer. We hypothesize that despite its antiapoptotic effects, Bcl-2 slows tumor cell proliferation.

MATERIALS AND METHODS

Bcl-2-negative breast cancer cells (SKBr3) were transfected with the bcl-2 gene (Bcl2-1 clone, low expression; Bcl2-2 clone, high expression) or plasmid control (Neo). Cell cycle distribution and kinetics were analyzed using bivariate flow cytometry (PI staining and pulse BrdU uptake). Cells were treated for 72 h with doxorubicin (100 ng/ml) or vehicle (0.01% DMSO) and assayed for cytosolic DNA with diphenylamine to measure apoptosis.

RESULTS

Cell counting showed increased doubling time in the Bcl-2-expressing clones Bcl2-1 and Bcl2-2 (Bcl-2(+)) relative to the Bcl-2-nonexpressing lines SKBr3 and Neo (Bcl-2(-)). Cell cycle analysis showed a decreased S phase fraction in Bcl-2(+) cells. Pulse BrdU uptake showed an increased G1/G0 fraction in Bcl-2(+) cells. Doxorubicin-induced apoptosis occurred in Bcl-2(-) but not in Bcl-2(+) cell lines.

CONCLUSIONS

Despite antiapoptotic effects favoring tumor survival, Bcl-2 prolongs cell cycle. Decreased tumor proliferation may account for the association of Bcl-2 expression with a favorable outcome in breast cancer, even though Bcl-2 may mediate chemoresistance in some patients.

Atm selectively regulates distinct p53-dependent cell-cycle checkpoint and apoptotic pathways

Atm is part of a pathway that responds to DNA damage from ionizing radiation (IR). This pathway involves p53, as Atm-deficient cell lines and mice are defective in p53 induction after IR. p53 is a multi-functional protein that simultaneously regulates distinct downstream pathways controlling cell-cycle progression and apoptosis. However, the mechanisms by which p53 differentially activates downstream pathways are unknown. To determine the relationship between Atm and p53, we examined cell-cycle and apoptotic responses in Atm-, p53-(ref. 8) and p21-deficient mice after IR in the whole animal. As expected, p53 protein levels were not induced by IR in thymus of Atm-deficient mice. IR-induced cell-cycle checkpoint function was also defective, and induction of p21 was attenuated in thymus from Atm-deficient mice. However, IR-induced apoptosis and Bax induction were completely normal; both of which are mediated by p53. IR-induced thymic apoptosis was suppressed in Atm/p53 double-mutant mice but not in Atm/p21 double mutants, demonstrating p53 dependence and Atm independence. Thus, Atm deficiency results in lack of p53 induction by IR, but only selective disruption of p53-dependent functions. Our results support a model in which upstream effectors such as Atm selectively activate p53 to regulate specific downstream pathways, providing a mechanism for controlling distinct cell-cycle and apoptotic responses.

Human cytomegalovirus IE2 86-kilodalton protein binds p53 but does not abrogate G1 checkpoint function

Physical interactions between human cytomegalovirus (HCMV) immediate-early (IE) proteins and key cell cycle regulatory proteins have been suggested as a mechanism whereby this herpesvirus modifies cellular control of proliferation. Observed similarities to interactions of other DNA virus proteins (human papillomavirus type 16 E6 and E7, simian virus 40 large T antigen, and adenovirus type 5 E1A and E1B) with cell cycle modulatory proteins such as p53 and Rb have suggested that HCMV IE proteins may likewise alter the G1-to-S phase transition. The IE2 region gene product IE86 has been shown to specifically bind p53, potentially modifying p53 G1 checkpoint function. To examine this possibility, p53-mediated G1 arrest in the presence of IE86 was assessed. Retroviral constructs were created to facilitate the stable expression of IE86 and IE72, another IE protein implicated in HCMV-mediated alteration of cell cycle progression. Western analysis and immunoprecipitation confirmed IE protein expression and binding of IE86 to p53, respectively. Chloramphenicol acetyltransferase assays examining the ability of IE86 to repress activity from the HCMV major IE promoter or activate the HCMV early promoter for the 2.2-kb class of RNAs demonstrated the functional integrity of the IE86 protein. Induction of DNA damage in normal, uninfected fibroblasts (FB) or FB expressing IE86 by actinomycin D (Act D) resulted in increased p53 levels, a predominance of the hypophosphorylated form of Rb, and increased expression of both p21(CIP1/WAF1) and mdm-2. Fluorescence-activated cell sorting revealed that both uninfected and IE86-expressing FB experienced dramatic G1 arrest following exposure to Act D. The clear demonstration of these p53-dependent responses in the presence of IE86 indicates that binding to this viral protein does not compromise the ability of p53 to elicit growth arrest following DNA damage.

Steel's potential doubling time and its estimation in cell populations affected by nonuniform cell loss

The in vivo infusion of the thymidine analogue bromodeoxyuridine (BrdUrd). followed by delayed biopsy and bivariate DNA-BrdUrd flow cytometry, makes it possible to estimate Steel's potential doubling time (Tpot) of human tumors. In the present paper, the expression of Steel's Tpot for a rather general cell population model, in which the distribution of cell loss is assumed to be nonuniform, is derived in terms of the model parameters. We show that Steel's Tpot of a population can be markedly different for the doubling time that would be exhibited by the population in the absence of cell loss. These doubling times, on the contrary, are equal when loss is uniform. Moreover, we studied the influence of modes of cell loss different from the uniform random loss on the estimation of Tpot by using the labeling index or the nu-function, quantities that can be determined from the bivariate DNA-BrdUrd distribution.

Comparison of mathematical formulas used for estimation of DNA synthesis time of bromodeoxyuridine-labelled cell populations with different proliferative characteristics

Growth kinetic data of human tumours, obtained by flow cytometric analysis of cells labelled with bromodeoxyuridine (BrdUrd) might provide prognostic information and allow prediction of response to radio- and chemotherapy. However, the theoretical models applied for calculation of growth kinetic data are not fully evaluated. The purpose of this study was to investigate the dependence of the estimation of DNA synthesis time (Ts) on sampling time after BrdUrd labelling, using four different mathematical formulas (Begg et al., White & Meistrich, White et al. and Johansson et al.) which have been developed for the evaluation of flow cytometry-derived data of BrdUrd-labelled cells. In addition, we have investigated the influence of the growth kinetic properties of the cell populations using two cultured cell lines (one slow and one fast growing), and two hetero-transplanted human tumours. The dependence of the estimation of Ts on sampling time was more or less pronounced, depending on the cell population examined and on the formula used. In the fast growing cell line, the estimates of Ts did not vary significantly with sampling time when using the formulas by White et al., whereas in the slow growing cell line, the estimates of Ts did not show any significant dependence on sampling time when using the formula by Johansson et al. In the tumours, the estimation of Ts depended on sampling time with all formulas used, although to different degrees. In one of the tumours, this was mainly caused by the influence of mouse cells, as we demonstrate. Our results indicate that the proliferative characteristics of a cell population should be taken into consideration when choosing a mathematical formula in order to attain Ts values that are independent of sampling time.

Accelerated repopulation during fractionated irradiation of a murine ovarian carcinoma: downregulation of apoptosis as a possible mechanism

PURPOSE

To test whether accelerated tumor clonogen repopulation occurs during continuous fractionated radiotherapy of a slow-growing mouse ovarian tumor, and if so whether the accelerated rate of repopulation is predicted by the pretreatment potential doubling time, and whether changes in apoptotic response are a possible mechanism for this change.

METHODS AND MATERIALS

The rate of clonogen production during fractionated radiotherapy was followed using the tumor-control assay, with an independent determination of the sensitivity to repeated dose fractions in vivo in the absence of repopulation. The pretreatment potential doubling time was measured by bromodeoxyuridine (BrdUrd) labeling and fluorescence measurements. The apoptotic and mitotic indices at various times during treatment were scored histologically.

RESULTS

The slow-growing (pretreatment volume doubling time 6 days) ovarian tumor OCA responds to daily irradiation with 6 Gy under hypoxia by negligible tumor clonogen production in the first few days, followed by a change at about 9 days to accelerated repopulation, after which the effective clonogen doubling time Tclon was about 2 days, near the pretreatment Tpot of 1.7 days. Alternative interpretations of the data, such as a change in radiosensitivity vs. a change in the repopulation rate or acceleration at 3 days as opposed to 9 days, were shown to be unlikely. This change was accompanied by a reduced apoptotic response (measured morphometrically).

CONCLUSIONS

When sensitivity to fractionated doses has been corrected for in vivo, this slow-growing mouse tumor exhibits a change to accelerated clonogen production during a continuous radiotherapy regimen that is accompanied or preceded by a reduced histologic apoptotic response. Tclon during accelerated repopulation was slightly longer than the pretreatment Tpot.

Growth patterns and cell kinetics of human osteosarcoma xenografts in serial passages in nude mice analyzed by in vivo labelling with iododeoxyuridine

A human osteoblastic osteosarcoma was transplanted in nude mice and followed in seven serial passages. Tumor cell kinetics was analyzed by in vivo labelling with the thymidine analogue iododeoxyuridine (IdUrd). Immunohistochemistry was used to measure the IdUrd labelling index. Duration of S phase (ts) was estimated by flow cytometry. From these two parameters potential doubling time (tpot) was calculated. Cell kinetic parameters showed low variations between passages and also between xenografts in same passage. Smaller variations of ts compared to labelling index and tpot were found. tpot was generally short with an interpassageal mean of 1.3 days and CV=14.8%. All xenografts showed DNA aneuploidy (mean DNA index --1.6). Homogeneous tumor growth was indicated by low variations of volume doubling time and lag time. There was no correlation between tumor growth and cell proliferation. Histopathological characteristics of the donor patients tumor were retained during serial transplantation.

Cell kinetics in vivo of human breast cancer

Rates of cell proliferation within tumours may provide prognostic information and help in the rational administration of chemotherapy and radiotherapy. In vivo labelling with 5-bromo-2'-deoxyuridine (BrdU) was used in 89 women with breast cancer to determine the labelling index for BrdU (BLI), the length of S phase (TS) and the potential doubling time of the tumour (Tpot). Kinetic data were obtained in 84 patients. The median BLI was 3.2 per cent, TS 12 h, and Tpot 12.5 days. There was no systematic difference in the labelling index determined by flow cytometry and by manual counting on sections. Labelling indices were significantly higher in aneuploid tumours, and in tumours not expressing oestrogen receptors, but were not correlated with tumour size, nodal status, or expression of c-erbB2. In vivo measurement of tumour cell kinetics can be made rapidly and reliably in the majority of human breast cancers, and may have a role to play in planning therapy.

Heterogeneous cell kinetics in tumors analyzed with a simulation model for bromodeoxyuridine single and multiple labeling

Bromodeoxyuridine (BrdUrd) labeling of DNA and flow cytometry measurement of bivariate BrdUrd-DNA content distributions yield proportions of cells in the cycle phases. After application of BrdUrd, with time, these proportions change according to the cell kinetic parameters of the investigated cell line or tumor. In a previous study of S-phase transit time using the relative movement method, we obtained better fits with S-duration distributions rather than constant values (Baisch and Otto: Cell Prolif 26:439-448, 1993). Now, we have developed a simulation model using asymmetric phase duration distributions in all phases of the cell cycle to fit the experimental data after single or multiple BrdUrd labeling. The model includes transit of cells from proliferating to quiescent compartments in all phases. The results yield the phase duration distributions, mean and median percentages of quiescent cells in all phases, growth fraction, and potential doubling time. The model was used to fit data of five renal cell carcinomas xenotransplanted into nude mice that were obtained after single and multiple labeling up to 93 hours. The estimated phase duration distributions varied from narrow to extremely asymmetric. In particular, TG2M duration and asymmetry were nearly as large as those of G1 phase in some tumors. The contribution of inter- and intratumoral heterogeneity cannot be separated by the simulation model, but evidence of intratumoral heterogeneity is provided by DNA content distributions at extended time spans after BrdUrd labeling.

Cellular kinetics in rectal cancer

Measurements of dynamic tumour cell kinetic parameters, particularly the potential doubling time (Tpot) may have potential as predictive assays for treatment outcome after radiotherapy. This paper details the distributions of Tpot and other kinetic and DNA content parameters measured in rectal cancers. Biopsies were taken from 119 patients approximately 6 h after infusion of 200 mg m-2 bromodeoxyuridine (BrdUrd). The samples were analysed by bivariate DNA/BrdUrd flow cytometry. The primary purpose of the study was to measure the kinetic parameters of labelling index (LI), duration of S-phase (TS) and Tpot. Secondarily, tumour DNA ploidy (DNA index) and S-phase fractions (SPFs) were also estimated from the univariate DNA histograms. The 101 evaluable patients were classified according to clinical stage as T2 (n = 12), T3 (n = 53), T4 (n = 28) or recurrent tumours (n = 8). Of the evaluable tumours, 73 were DNA aneuploid. The median LI, TS, and Tpot of the aneuploid tumours were 21%, 20 h and 3.3 days respectively. The calculated LI, TS, and Tpot of diploid tumours were subject to uncertainties because of the contribution of normal cells. The LI and SPF of all tumours were, however, significantly (P < 0.001) correlated, having a correlation coefficient of only 0.76. The wide distributions of values for LI (quartiles 13.5%, 26.9%) and Tpot (quartiles 2.4, 5.6 days) that were found are necessary baseline information if these parameters are to be useful in individual treatment selection or as predictors of treatment outcome.

Specific staining of iododeoxyuridine and bromodeoxyuridine in tumors double labelled in vivo: a cell kinetic analysis

The simultaneous and specific staining of iododeoxyuridine (IdUrd) and chlorodeoxyuridine (CldUrd) allows for more accurate estimates of potential doubling time (Tpot). Because CldUrd is not approved for human use, the procedure was adapted for the staining of IdUrd and bromodeoxy-uridine (BrdUrd). The fluorescein isothiocyanate-conjugated B44 antibody (B44-FITC) stained both IdUrd and BrdUrd in tumor nuclei labelled singly with one or the other pyrimidine analogue. However, when MCaK tumors in exponential growth in vivo were pulse labelled with both IdUrd and BrdUrd, the staining of BrdUrd was not seen, and the labelling pattern reflected specificity to IdUrd. These observations were confirmed using tumors pulse labelled with IdUrd and/or BrdUrd at 6 h and/or 0.3 h prior to tumor removal in all possible combinations. Simultaneous specific staining of BrdUrd by Br3 and of IdUrd by B44-FITC was documented by quantification of labelling indices (LIs) from double-labelled tumors. The specificity of B44-FITC for IdUrd in double-labelled tumors was due to a greater affinity of this antibody for IdUrd than for BrdUrd. This technique allowed for two independent estimates of LI and Tpot when tumors were double labelled for 3.0 and 5.5 h. Both IdUrd and BrdUrd are approved for clinical use, and this double-labelling technique should prove to be valuable for measuring the cell kinetics of solid tumors in vivo.

Upstream stimulatory factor regulates expression of the cell cycle-dependent cyclin B1 gene promoter

Progression through the somatic cell cycle requires the temporal regulation of cyclin gene expression and cyclin protein turnover. One of the best-characterized examples of this regulation is seen for the B-type cyclins. These cyclins and their catalytic component, cdc2, have been shown to mediate both the entry into and maintenance of mitosis. The cyclin B1 gene has been shown to be expressed between the late S and G2 phases of the cell cycle, while the protein is degraded specifically at interphase via ubiquitination. To understand the molecular basis for transcriptional regulation of the cyclin B1 gene, we cloned the human cyclin B1 gene promoter region. Using a chloramphenicol acetyltransferase reporter system and both stable and transient assays, we have shown that the cyclin B1 gene promoter (extending to -3800 bp relative to the cap site) can confer G2-enhanced promoter activity. Further analysis revealed that an upstream stimulatory factor (USF)-binding site and its cognate transcription factor(s) are critical for expression from the cyclin B1 promoter in cycling HeLa cells. Interestingly, USF DNA-binding activity appears to be regulated in a G2-specific fashion, supporting the idea that USF may play some role in cyclin B1 gene activation. These studies suggest an important link between USF and the cyclin B1 gene, which in part explains how maturation promoting factor complex formation is regulated.

S-phase arrest of nutrient deprived lung cancer cells

The human small cell lung cancer cell line NCI-H82 was used to study the effect of nutritional status on cell proliferative parameters. Incorporation of bromodeoxyuridine (BrdUrd) was used to characterize actively proliferating cells and to obtain information on cell cycle dynamics. During several days, in which the culture medium was not changed, a gradual decrease in overall cell growth, labeling index, and vitality was observed. Simultaneously, an increase in the number of S-phase cells that did not incorporate BrdUrd was noticed. From a more detailed kinetic study on d 6 of nutrient depletion, it appeared that, although the cells incorporated BrdUrd, they stopped cycling. When the same cells were regrown in fresh culture medium, a delay of 10 h in G1-phase entry and exit was measured. After this delay the cells resumed the cell cycle at normal phase transit rates. In addition, BrdUrd unlabeled S-phase cells were gradually lost from the culture. Bivariate flow cytometric DNA/proliferating cell nuclear antigen (PCNA) and DNA/Ki67-antigen analyses confirmed a delay in G1 phase entry and exit. In this paper we show that nutrient depletion can cause cell cycle arrest as indicated by the occurrence of BrdUrd unlabeled S-phase cells. This arrest could lead to overestimation of kinetic parameters such as S-phase transit time (Ts) and potential time (Tpot) as determined after in vivo labeling of tumors.

Repopulation kinetics during fractionated irradiation and the relationship to the potential doubling time, Tpot

PURPOSE

Therapeutic outcome may be adversely affected by repopulation in solid tumors during fractionated irradiation. It has been proposed that the repopulation rate of the surviving cells may be reflected by the pretreatment potential doubling time (Tpot). This concept has been examined by comparing pretreatment Tpot measurements to repopulation monitored in five transplantable murine tumors during fractionated radiation treatment.

METHODS AND MATERIALS

Up to nine fractions of 2 Gy were given to clamped tumors on a 6 h schedule, which allowed adequate time for repair, or on a 24 h schedule, which incorporated more time for repopulation. Tumors were removed from treatment at various times and tumor cell survival was analyzed using an excision assay. The ratio of the cell survival in tumors treated with the same total dose on the two different fractionation schedules (24 h/6 h) was used to calculate an effective doubling time for repopulation during the treatment (Teff). Potential doubling time was assessed in untreated tumors by giving the tumor-bearing animals 5-bromodeoxyuridine (BrdUrd) and, at various times later, removing the tumors for flow cytometric analysis. Tpot values were calculated by two different widely used methods.

RESULTS

For four tumors (RIF-1, KHT-C, KHT-LP1, and B16-F1), the Teff was greater than Tpot indicating that repopulation was not as rapid as suggested by Tpot. For SCC-VII, the only carcinoma tested, Teff was smaller than Tpot indicating that repopulation was more rapid than predicted by Tpot. Individual estimates of Tpot made from single tumor samples taken at different times after BrdUrd administration varied by factors of 2 to 7 for the different tumors.

CONCLUSION

These findings indicate a need for caution in applying measurements of Tpot for prediction of regrowth rates in individual patients' tumors.

Insulin-like growth factor-1 is not mitogenic for the Walker-256 carcinosarcoma

This study was designed to determine whether intravenous infusion of recombinant human insulin-like growth factor 1 (IGF-1) stimulates tumor growth. In order to determine the potential interaction between nutrition and IGF-1 administration the study was conducted in fasting rats and during continuous feeding by total parenteral nutrition. Tumor cell cycle kinetics including labeling index, DNA synthesis time, cell cycle time in Go/G1, and G2/M in the total cell cycle, and potential doubling time were determined by flow cytometry after in vivo pulse labeling the rats bearing the Walker-256 Carcinosarcoma with 5'-bromo-2'-deoxyuridine (BrdUrd). The results show that IGF-1 treatment has no significant effects on the proliferative characteristics of the tumor model regardless of the feeding status of the animal. This study provides preliminary cell-cycle kinetics data on the short-term effect of IGF-1 on tumor growth. Failure to show a significant effect of IGF-1 on the proliferative characteristics of the tumor suggests that IGF-1 may be given to cancer patients in amounts sufficient to promote weight gain without deleterious stimulation of tumor proliferation.

DNA replication time accounts for tumor growth variation induced by dietary fat in a breast carcinoma model

Female Fischer rats were pair-fed on diets containing either safflower oil (SO) or fish oil (FO) for 6 weeks. Implanted breast 13762 MAT tumors had a doubling times of 35.4 and 55.5 h in SO and FO rats, respectively (P < 0.001). Proliferation kinetics were measured in vivo by bromedeoxyuridine (BrdUrd) labeling and bivariate DNA/BrdUrd analysis by flow cytometry. After 1 h of pulsing, the labeling index was similar in both groups. However, 6 h later, tumor cells from FO rats had significantly lower relative movement of BrdUrd-labeled cells (0.78 vs. 0.91, P < 0.001). These results reflected a significantly longer S phase duration (15.0 vs. 9.1 h, P < 0.001) in FO rats and accounted for all the difference in tumor growth rates. This mechanism, which has not previously been reported, implies a significant role for fatty acids in DNA replication.

The ability of human papillomavirus E6 proteins to target p53 for degradation in vivo correlates with their ability to abrogate actinomycin D-induced growth arrest

Functional p53 protein is associated with the ability of cells to arrest in G1 after DNA damage. The E6 protein of cancer-associated human papillomavirus type 16 (HPV-16) binds to p53 and targets its degradation through the ubiquitin pathway. To determine whether the ability of E6 to interact with p53 leads to a disruption of cell cycle control, mutated E6 proteins were tested for p53 binding and p53 degradation targeting in vitro, the ability to reduce intracellular p53 levels in vivo, and the ability to abrogate actinomycin D-induced growth arrest in human keratinocytes. Mutations scattered throughout the amino terminus, either zinc finger or the central region but not the carboxy terminus, severely reduced the ability of E6 to interact with p53. Expression of HPV-16 E6 or mutated E6 proteins that bound and targeted p53 for degradation in vitro sharply reduced the level of intracellular p53 induced by actinomycin D in human keratinocytes. A perfect correlation between the ability of E6 proteins to reduce the level of intracellular p53 and their ability to block actinomycin D-induced cellular growth arrest was observed. These results suggest that interaction with p53 is important for the ability of HPV E6 proteins to circumvent growth arrest.

Influence of sampling time on assessment of potential doubling time

Potential doubling time (Tpot), S-phase transit time (Ts), and labeling index were determined in three experimental rodent tumors by in vivo incorporation of bromodeoxyuridine and flow cytometry. The kinetic parameters were derived from the relative movement method at varying sampling times (time between injection of bromodeoxyuridine and tumor excision). Ts and Tpot were close to the expected (extrapolated) values of the two parameters with sampling time within the range 60-100% of the expected Ts. With short sampling time, Ts and Tpot were considerably over- or underestimated. By use of the original method by (Begg et al., Cytometry 6:620-626, 1985), the Tpot came out with a twofold overestimation with a short sampling time. Generally, modified methods for calculation of Ts did not improve the results. When Tpot is measured in human tumors, there is no a priori knowledge of the kinetic parameters. Consequently, the sampling time must be based on the general experience of tumor cell kinetics in the specific tumor type measured. A relatively long sampling time should be aimed for when measuring Tpot in human tumors. With proper sampling time, Tpot assessed by flow cytometry was found to be a precise and reproducible parameter.