Analysis of the intra- and intertumoral heterogeneity of hypoxia in pancreatic cancer patients receiving the nitroimidazole tracer pimonidazole

Background:

Hypoxia is thought to be an adverse feature of pancreatic cancer, but direct measurement in patients is technically challenging. To address this, we characterised the intra/interpatient heterogeneity of hypoxia in surgical specimens from patients who received the 2-nitroimidazole tracer pimonidazole pre-operatively.

Methods:

Pimondazole was given intravenously 16–20 h before pancreatectomy, and the extent and intratumoral heterogeneity of hypoxia determined by image analysis applied to multiple tissue blocks stained by immunohistochemistry. Intra/interpatient heterogeneity was estimated by variance component analysis.

Results:

Pimonidazole staining was analysed in 10 tumours. The extent of labelling varied amongst patients (0–26%), with a broader range of hypoxia in the epithelial (1–39%) compared with the stromal (1–13%) compartments. Variance component analysis demonstrated greater inter- than intrapatient variability of hypoxia, and that multiple (4–5) tumour sections are required to provide a consistent evaluation of its extent in individual tumours.

Conclusions:

There is significant intra- and intertumoral heterogeneity of hypoxia in pancreatic cancers, and these do not appear to be generally more hypoxic than other cancer types. This study establishes the feasibility to assess hypoxia in pancreatic cancer patients using pimonidazole, but questions the reliability of measurements made using a single tissue section.

A phase I/II study of imatinib plus reinduction therapy for c-kit-positive relapsed/refractory acute myeloid leukemia: inhibition of Akt activation correlates with complete response

This phase I/II study evaluated imatinib as a c-kit inhibitor combined with mitoxantrone, etoposide and cytarabine therapy for patients with primary refractory or relapsed c-kit+ acute myeloid leukemia (AML). Imatinib was escalated through three dose levels in successive six patient cohorts. The combination was well tolerated up to 400 mg/day imatinib. Of 21 patients treated at this dose, 13 (62%) achieved complete response (CR), 7 (33%) were non-responders and one died during induction. The CR rate was 80% in patients with standard-risk karyotype versus 33% in patients with adverse karyotype. The CR rate for primary non-responders was 6/14 (43%) versus 7/7 (100%) for relapsed patients. AML blasts from peripheral blood were assayed for phosphorylated Akt (pAkt) and phosphorylated ERK (pERK) by flow cytometry before to and after imatinib dosing. Of eight patients achieving CR with reinduction, seven demonstrated marked (≥60%) pAkt inhibition with imatinib therapy. In contrast, all the six non-responders to reinduction demonstrated <60% pAkt inhibition (P=0.005). There was no correlation between pERK inhibition and response to therapy. These results indicate that lack of pAkt inhibition in vivo is associated with resistance to reinduction therapy using this regimen. Further studies using agents that are able to inhibit Akt more effectively are warranted.

Activity of a novel, dual PI3-kinase/mTor inhibitor NVP-BEZ235 against primary human pancreatic cancers grown as orthotopic xenografts

The phosphatidylinositol-3-kinase (PI3K)/Akt signalling pathway is frequently deregulated in pancreatic cancers, and is believed to be an important determinant of their biological aggression and drug resistance. NVP-BEZ235 is a novel, dual class I PI3K/mammalian target of rapamycin (mTor) inhibitor undergoing phase I human clinical trials. To simulate clinical testing, the effects of NVP-BEZ235 were studied in five early passage primary pancreatic cancer xenografts, grown orthotopically. These tumours showed activated PKB/Akt, and increased levels of at least one of the receptor tyrosine kinases that are commonly activated in pancreatic cancers. Pharmacodynamic effects were measured following acute single doses, and anticancer effects were determined in separate groups following chronic drug exposure. Acute oral dosing with NVP-BEZ235 strongly suppressed the phosphorylation of PKB/Akt, followed by recovery over 24 h. There was also inhibition of Ser235/236 S6 ribosomal protein and Thr37/46 4E-BP1, consistent with the effects of NVP-BEZ235 as a dual PI3K/mTor inhibitor. Chronic dosing with 45 mg kg⁻¹ of NVP-BEZ235 was well tolerated, and produced significant tumour growth inhibition in three models. These results predict that agents targeting the PI3K/Akt/mTor pathway might have anticancer activity in pancreatic cancer patients, and support the testing of combination studies involving chemotherapy or other molecular targeted agents.

Effects of gemcitabine on APE/ref-1 endonuclease activity in pancreatic cancer cells, and the therapeutic potential of antisense oligonucleotides

Apurinic/apyrimidinic endonuclease (APE) is a key enzyme involved in DNA base excision repair (BER) that is often expressed at elevated levels in human cancers. Pancreatic cancer cells treated with the nucleoside analogue gemcitabine (2′, 2′-difluoro-2′deoxycytidine) showed increases in APE/redox effector factor (ref-1) protein levels (approximately two-fold for Panc-1 and six-fold for MiaPaCa-2), with corresponding increases in endonuclease activity. These results suggested that the activation of APE/ref-1 might be an adaptive response that contributes to gemcitabine resistance by facilitating BER. To test this hypothesis, we examined the effects of disrupting APE/ref-1 using antisense on gemcitabine toxicity. Antisense oligonucleotides decreased protein levels three-fold in MiaPaCa-2 and five-fold in Panc-1 in comparison to controls, associated with reduced endonuclease activity. Combination treatments with antisense oligonucleotides and gemcitabine partially suppressed the increase in APE/ref-1 activity seen in cells exposed to gemcitabine alone. While clonogenic assays showed only slight decreases in colony formation in cells treated with either antisense oligonucleotides or gemcitabine alone, the combination with APE/ref-1 antisense resulted in a 2-log enhancement of gemcitabine toxicity in Panc-1 cells. Overall these findings suggest that APE/ref-1 plays a significant role in gemcitabine resistance in some pancreatic cancer cells, and support the further investigation of novel treatments that target this protein.

Inhibition of gamma-glutamyl transpeptidase activity decreases intracellular cysteine levels in cervical carcinoma

PURPOSE

To determine whether gamma-glutamyl transpeptidase (gamma-GT) is involved in the maintenance of elevated cysteine levels in cervical carcinoma.

METHODS

Four cervical carcinoma cell lines were tested in vitro for cysteine accumulation and gamma-GT levels. The highest and lowest gamma-GT-expressing cell lines were used in in vivo experiments to determine the effect of gamma-GT inhibition on cysteine levels.

RESULTS

Treatment of a series of cervical carcinoma cell lines with acivicin decreased intracellular cysteine concentrations. Cysteine depletion was evident in Me180 cells which had the greatest levels of gamma-GT activity, and had a more pronounced cysteine decrease in medium with glutathione and cysteine concentrations simulating the in vivo situation. Also investigated were the effects of inhibition of gamma-GT activity on intracellular cysteine levels in xenografts grown in severe combined immunodeficient (SCID) mice. With the use of 35 mg/kg of acivicin, gamma-GT activity decreased to basal levels of detection in both tumour types and significant decreases in cysteine levels were seen in the high gamma-GT-expressing tumours (Me180). Thus, inhibition of gamma-GT activity may have therapeutic potential in high-expressing cancers.

CONCLUSIONS

In tumours and cell lines with elevated levels of gamma-GT activity, inhibition of this enzyme led to decreases of cysteine levels.

Assessment of tumor cell repopulation after chemotherapy for advanced ovarian cancer: pilot study

BACKGROUND

Repopulation of clonogenic tumor cells appears to increase during fractionated radiation treatment and is recognized as an important factor affecting local control. Given the longer intervals between cycles and longer total duration of treatment, the impact of repopulation is likely to be greater after chemotherapy.

METHODS

We assessed tumor cell repopulation with the proliferative marker Ki-67 in 21 patients with ovarian carcinoma who received initial chemotherapy. Paraffin slides were evaluated from the diagnostic biopsy and from tumor obtained at debulking surgery after chemotherapy. Immunohistochemistry using the MIB-1 antibody was performed on the paired samples and analyzed with a digital imaging device linked to a color camera mounted on a transmitted-light microscope. The ratio of Ki-67 positive to all nuclei was used as a proliferative index and compared for pre- and postchemotherapy specimens.

RESULTS

All patients received platinum-based chemotherapy and most showed a response to treatment. The median duration between last chemotherapy and debulking surgery was 33 days (range, 22-50 days). Four (19%) of 21 patients showed an increased proliferative index after chemotherapy, and the remainder showed a decrease (n = 12) or no significant change (n = 5).

CONCLUSIONS

Our results did not suggest an increase in proliferation of tumor cells after this type of chemotherapy in the majority of patients with ovarian cancer.

Hypoxia-inducible factor-1alpha is an intrinsic marker for hypoxia in cervical cancer xenografts

The hypoxia-inducible factor 1 (HIF-1) is known to induce the expression of several proteins linked to the maintenance of oxygen homeostasis, cellular energy metabolism, and tumor progression. Its alpha subunit (HIF-1alpha) is stabilized under hypoxic conditions and, therefore, might represent an intrinsic marker for tissue hypoxia. Here we report on the spatial relationship between HIF-1alpha and the nitroimidazole hypoxia marker EF5 in cervical carcinoma xenografts, and on their spatial relationship to tumor blood vessels. EF5 was administered to mice bearing ME180 and SiHa cervical cancer xenografts. Frozen tumor tissue sections, triple-stained for HIF-1alpha, the endothelial cell marker CD31, and EF5, were imaged using wide-field multiparameter immunofluorescence microscopy. Expression levels of EF5 and HIF-1alpha were similar in ME180 xenografts, but the percentage of tumor area stained with EF5 was significantly smaller than the percentage of HIF-1alpha-positive area in SiHa tumors. In both tumor types the EF5-HIF-1alpha overlap was statistically significant, thus confirming their spatial and temporal colocalization. Spatial distribution analysis of EF5 and HIF-1alpha is consistent with different pO2 value "thresholds" for EF5 binding and HIF-1alpha expression. Summarized, our results indicate that HIF-1alpha is a useful intrinsic marker for hypoxia in cervical carcinoma xenografts.

Wortmannin inhibits pkb/akt phosphorylation and promotes gemcitabine antitumor activity in orthotopic human pancreatic cancer xenografts in immunodeficient mice

Pancreatic cancer is resistant to almost all classes of cytotoxic agents. Gemcitabine seems to be the current drug of choice. We have recently reported that inhibition of the phosphatidylinositide 3-kinase-protein kinase B (PKB/Akt) cell survival pathway by wortmannin enhances gemcitabine-induced apoptosis in cultured human pancreatic cancer cells (1). The present study investigated the effects of wortmannin on orthotopic human pancreatic cancer xenografts implanted in severe combined immunodeficient mice. Animals were given single i.v. bolus injections of 0.175, 0.35, or 0.7 mg/kg of wortmannin and killed at 0.5, 1, 2, or 4 h after treatment. Phosphorylated PKB/Akt levels in tumor tissues were measured by fluorescence image analysis. Wortmannin was found to inhibit PKB/Akt phosphorylation in a time- and dose-dependent manner, reaching a plateau at 4 h and at 0.7 mg/kg. The levels of phosphorylated PKB/Akt were maximally decreased by approximately 50% relative to the vehicle control. Subsequently, the extent of apoptosis in tumors treated with gemcitabine or wortmannin alone or in combination was determined using terminal deoxynucleotidyl transferase-mediated nick end labeling assay and computerized image analysis. Orthotopic tumors exposed to 80 mg/kg gemcitabine for 48 h and then 0.7 mg/kg wortmannin for 4 h showed a 5-fold increase (P = 0.002) in apoptosis compared with those treated with each agent alone and with the vehicle control. The combination treatment also significantly (P < 0.001) inhibited tumor growth. Taken together, our findings support the potential of phosphatidylinositide 3-kinase inhibitors as adjuncts to conventional chemotherapy in the treatment of pancreatic cancer.

The BH3 domain of BAD fused to the Antennapedia peptide induces apoptosis via its alpha helical structure and independent of Bcl-2

Since the over-expression of Bcl-2 is a common cause of multi-drug resistance, cytotoxic peptides that overcome the effects of Bcl-2 may be clinically useful. We harnessed the death-promoting alpha helical properties of the BH3 domain of BAD by fusing it to the Antennapedia (ANT) domain, which allows for cell entry (ANTBH3BAD). Treatment of 32D cells with the ANTBH3BAD peptide results in a 99% inhibition of colony formation. No significant toxicity is observed after treatment with ANT or BH3BAD alone. A mutant fusion peptide unable to bind Bcl-2 induces cell death as effectively as the wild-type ANTBH3BAD. Furthermore, 32D cells over-expressing Bcl-2 show no resistance to the ANTBH3BAD peptide. Therefore, the toxicity of the peptide was independent of the Bcl-2 pathway. We demonstrate that the toxicity of the peptide is due to its alpha helicity that disrupts mitochondrial function. Since this peptide overcomes major forms of drug resistance, it may be therapeutically useful if appropriately targeted to malignant cells.

BAD induces apoptosis in cells over-expressing Bcl-2 or Bcl-xL without loss of mitochondrial membrane potential

Inhibitors of Bcl-2 may be useful therapeutic agents for the treatment of a wide variety of malignancies including leukemia. A potential prototype of such a compound is the endogenous Bcl-2 and Bcl-xL binding protein BAD. Previous reports indicate that BAD can overcome the anti-apoptotic effect of Bcl-xL but not Bcl-2. If BAD cannot induce apoptosis in cells over-expressing Bcl-2, it would limit the application of molecules like BAD as novel anti-tumor agents. We report that transient transfection of BAD induced cell death in cells with and without over-expression of Bcl-2 or Bcl-xL. Forty-eight hours after transfection, BAD increased cell death in COS, COS Bcl-2, and COS Bcl-xL cells as demonstrated by decreased GFP expression, and an increase in the number of number of floating cells. In addition, BAD induced cell death in leukemic cell lines over-expressing Bcl-2 and Bcl-xL as determined by changes in luciferase activity. BAD-induced apoptosis was not accompanied by loss of mitochondrial membrane potential. Therefore, we conclude that transient transfection of BAD directly induces apoptosis in cells over-expressing Bcl-2 or Bcl-xL and validates the pursuit of molecules like BAD as novel therapeutic agents.

Differential effects of buthionine sulphoximine in hypoxic and non-hypoxic regions of human cervical carcinoma xenografts

BACKGROUND AND PURPOSE

Recently we reported increased glutathione (GSH) levels in hypoxic regions of ME 180 and SiHa cervical cancer xenografts. Since this association might act synergistically to protect from radiotherapy, we examined the differential effects of the GSH depleting agent buthionine suiphoximine (BSO) in relation to tumor oxygenation.

MATERIALS AND METHODS

The nitroimidazole EF5 was used to label tumor hypoxia. GSH levels were determined in cryostat sections using a sensitive HPLC assay and in parallel sections using fluorescence image analysis. Using a dual-labeling method, GSH levels were determined selectively in hypoxic and non-hypoxic tumor regions.

RESULTS

GSH levels were higher in hypoxic than in non-hypoxic regions of cervical carcinoma xenografts. Treatment with BSO produced a more pronounced GSH depletion in regions of hypoxia, resulting in similar post-treatment levels in hypoxic and non-hypoxic areas.

CONCLUSIONS

BSO effectively depletes GSH in hypoxic microregions of tumors. These findings suggest a potential role for BSO as an adjunct to radiotherapy in cervical cancer patients.

Imaging of whole tumor cut sections using a novel scanning beam confocal fluorescence MACROscope

Hypoxia caused by inadequate structure and function of the tumor vasculature has been found to negatively determine the prognosis of cancer patients. Hence, understanding the biological basis of tumor hypoxia is of significant clinical interest. To study solid tumor microenvironments in sufficient detail, large areas (several mm in diameter) need to be imaged at microm resolutions. We have used a novel confocal scanning laser MACROscope (CSLM) capable of acquiring images over fields of view up to 2cm x 2cm. To demonstrate its performance, frozen sections from a cervical carcinoma xenograft were triple labeled for tissue hypoxia, blood vessels and hypoxia-inducible transcription factor 1 alpha (HIF-1alpha), imaged using the CSLM and compared to images obtained using a standard epifluorescence microscope imaging system. The results indicate that the CSLM is a useful instrument for imaging tissue-based fluorescence at resolutions comparable to standard low-power microscope objectives.

Measurement of MAP kinase activation by flow cytometry using phospho-specific antibodies to MEK and ERK: potential for pharmacodynamic monitoring of signal transduction inhibitors

Cancer cells frequently show abnormal signaling via the mitogen activated protein kinase (MAP kinase) pathway due to increased activity of surface receptors for growth factors, or as a result of ras mutations. The development of potent anti-cancer agents that target this pathway prompts the need for analytical methods that allow pharmacodynamic monitoring of drug effects in patients during early phase clinical trial. We describe such a method, based on the activation of T-lymphocytes in undiluted peripheral blood using phorbol myristate acetate (PMA). Following rapid hypotonic lysis and formaldehyde fixation, activation of the MAP kinase pathway can then be demonstrated using phospho-specific antibodies that recognize the activated mediators MEK or ERK, followed by surface labeling with anti-CD3 to identify T-lymphocytes. This method was used to investigate the effects of a MEK inhibitor, U0126, and a new raf kinase inhibitor BAY 37-9751 in blood samples from normal donors. Dose-dependent inhibition of pERK activation was demonstrated for both agents. Furthermore, differential effects on pMEK activation allowed the molecular targets of the two inhibitors to be distinguished. In addition to monitoring drug effects in patients during treatment with inhibitors of the MAP kinase pathway, the general methodology described in this paper has the potential for wide application to the study of signal transduction at the single cell level using flow cytometry.

Respiratory chain-generated oxidative stress following treatment of leukemic blasts with DNA-damaging agents

Oxidative stress occurs in diverse life forms during programmed cell death and appears to be a significant mediator since a wide range of manipulations that enhance cellular antioxidant systems are protective. Using a recently developed flow cytometry technique to assess respiratory chain function, we have investigated the mechanism of reactive oxygen generation in OCI/AML-2 leukemic blasts following treatment with cytosine arabinoside, etoposide, and gamma-irradiation. Increases in mitochondrially generated reactive oxygen were seen using all three agents, in association with hyperpolarization of the mitochondrial inner membrane. Increased reactive oxygen occurred when mitochondria were energized using substrates for either complex I or complex II, indicating that the likely source is complex III (cytochrome c reductase). These findings are consistent with impaired adenine nucleotide exchange across the mitochondrial membrane, recently proposed to be an important event during the early stages of apoptosis induction (M. G. Vander Heiden et al., 1999, Mol. Cell 3, 159-167). Elevations of the antioxidants glutathione and thioredoxin occurred in association with this oxidative stress, likely the result of feedback mechanisms based on redox-sensitive transcription factors. Since glutathione and thioredoxin can protect from drug-induced apoptosis, their upregulation in response to respiratory chain-generated reactive oxygen might represent a cellular adaptation to DNA damage that promotes cell survival.

Association of involvement in psychological self-regulation with longer survival in patients with metastatic cancer: an exploratory study

There have been numerous anecdotal claims that when patients are dedicated users of a variety of psychological self-regulation strategies, including relaxation, mental imaging, cognitive restructuring and meditation, such dedication may have a life-prolonging effect. Our aim was to test this possibility more rigorously, in patients with metastatic cancer.A prospective, longitudinal, correlative study was carried out on 22 patients with varying kinds of medically incurable metastatic cancer. The intervention was one year of weekly group psychological therapy. Extensive verbal data (patients' written homework and therapists' notes) were collected over the year. The extent of each patient's involvement with psychological work was estimated following a qualitative analysis of these data. Patients were classed as showing high, moderate, or low involvement on the basis of a quantitative rating of categories defined by the analysis. These three subgroups did not differ significantly in their expected median survival duration as estimated from independent quantitative predictions by a large panel of oncologists who analyzed the patients' medical charts at time of study entry.A significant relationship was found between degree of involvement in psychological work and survival duration. Results are presented as Kaplan-Meier survival curves (Fig. 2;P = 0. 006, Log Rank test) and as a graphic display of the median survival of each of the three groups (Fig. 3). The main likely confounders (medical status, age, quality of life, and attendance at therapy) were similar across subgroups and did not change the relation between psychological work and survival duration. Limitations in the design are discussed. However, the strong effects observed support clinical observations that dedicated involvement in psychological self-regulation may prolong the life of some patients with metastatic cancer.

Lovastatin induces a pronounced differentiation response in acute myeloid leukemias

We recently identified HMG-CoA reductase, the rate-limiting enzyme of the mevalonate pathway, as a potential therapeutic target of various retinoic acid responsive cancers. Lovastatin, a competitive inhibitor of HMG-CoA reductase, induced a retinoic acid-like differentiation response followed by extensive apoptosis in neuroblastoma cell lines at relatively low concentrations (<20 microM) of this agent. More recently, we demonstrated that acute myeloid leukemias but not acute lymphocytic leukemias also displayed increased sensitivity to lovastatin-induced apoptosis. In this study, we examined the ability of lovastatin to induce differentiation of acute myeloid leukemic cells and to evaluate the role differentiation may hold in the anti-leukemic properties of this agent. Increased expression of the leukocyte integrins CD11b and CD18 as well as down-regulation of the anti-apoptotic gene bcl-2 are associated with late stage differentiation of the myeloid lineage and retinoic acid induced maturation of acute myeloid leukemic cells. Lovastatin exposure induced increased expression of CD11b and CD18 markers similar to retinoic acid treatment. Following 24 hrs exposure to 20 microM lovastatin, all 7 acute myeloid leukemia cell lines tested showed a decrease in bcl-2 mRNA expression while only 1/5 acute lymphocytic leukemia cell lines showed a similar response. A role for bcl-2 in the apoptotic response of acute myeloid leukemia cells to lovastatin was demonstrated as exogenous constitutive expression of bcl-2 in the AML-5 cell line inhibited apoptosis in a time and dose dependent manner. Thus, lovastatin exposure of acute myeloid leukemia cells induced a differentiation response that may contribute to the therapeutic potential of this agent in the treatment of this disease.

Simultaneous detection of mitochondrial respiratory chain activity and reactive oxygen in digitonin-permeabilized cells using flow cytometry

BACKGROUND

Increased mitochondrial generation of reactive oxygen intermediates (ROI) due to defective respiratory chain activity has been implicated in physiological processes such as apoptosis, in the pathogenesis of mitochondrial diseases, and as part of the normal aging process. Established methods addressing activity of the respiratory chain complexes have been limited to bulk assays for single parameters. This study describes a flow cytometry-based method and its validation for the detection of respiratory chain function in single cells permeabilized by digitonin.

METHODS

Flow cytometry was used to measure mitochondrial membrane potential (DeltaPsi(m)) and reactive oxygen generation under differing conditions of respiration. This was brought about by the addition of substrates and inhibitors to digitonin-permeabilized cells. This method was validated by measurement of oxygen consumption and ATP production and by confocal microscopy.

RESULTS

Activity of the respiratory chain complexes assessed by DeltaPsi(m) responded to substrates and inhibitors as predicted from assessment by oxygen consumption and ATP synthesis. In addition, the flow cytometry method allows the simultaneous assessment of mitochondrial ROI generation. This was confirmed by the localization of the ROI probe, carboxy-DCF, to the same site as the mitochondrial probe observed by confocal microscopy.

CONCLUSIONS

This method allows the functional integrity of the respiratory chain complexes to be studied at the single-cell level, thus addressing the relationship between disordered function of respiratory chain complexes and mitochondrial ROI generation.

Equilibrative-sensitive nucleoside transporter and its role in gemcitabine sensitivity

Salvage of preformed nucleosides requires transport across the plasma membrane by sodium-dependent (concentrative) and sodium-independent (equilibrative) mechanisms. These transport systems are also the route of cellular uptake for nucleoside analogues, including gemcitabine (2',2'-difluorodeoxycytidine), a deoxycytidine analogue used in the treatment of pancreatic cancer. To determine whether gemcitabine cytotoxicity is influenced by the equilibrative-sensitive nucleoside transporter (es-NT), basal levels of the es-NT were quantified in three human pancreatic cancer cell lines (PANC-1, HS-766T, and PK-8) and one human bladder cancer cell line (MGH-U1) by flow cytometric analysis, and the results were compared with gemcitabine cytotoxicity assessed by clonogenic assay. To determine whether the salvage pathway of DNA synthesis can be up-regulated by inhibiting de novo DNA synthesis, combination experiments were carried out using the thymidylate synthase (TS) inhibitors 5-fluorouracil or raltitrexed with gemcitabine in a concurrent and sequential fashion. No relationship between basal es-NT and gemcitabine cytotoxicity was demonstrated. For two pancreatic cell lines, sequence-dependent effects of the combination of TS inhibitors and gemcitabine were seen with maximum effect when the TS inhibitors preceded gemcitabine. This was also associated with a significant increase in es-NT levels caused by the TS inhibitors. Thus, modulation of the es-NT by pretreatment with TS inhibitors may have the potential to improve the therapeutic benefit of gemcitabine.

Inhibition of phosphatidylinositide 3-kinase enhances gemcitabine-induced apoptosis in human pancreatic cancer cells

Human pancreatic adenocarcinoma cell lines PK1 and PK8 are resistant to the clinically relevant chemotherapy agent gemcitabine. Cell cycle analysis demonstrated an accumulation of cells in the early S phase during treatment with 20 microM gemcitabine, consistent with its mode of action as a DNA chain terminator. However, apoptosis was evident in only a small percentage of cells. Similar to pancreatic cancers in the clinic, PK1 and PK8 cells carry constitutively active Ki-Ras and overexpress multiple receptor tyrosine kinases. Both genetic abnormalities may potentially up-regulate the activity of the phosphatidylinositide 3-kinase (P13K)-protein kinase B (PKB)/Akt cell survival pathway. The current study examined the relevance of this pathway in the modulation of drug resistance in PK1 and PK8 cells. After exposure to 20 microM gemcitabine for 48 h and in the continuous presence of the drug, treatment with the P13K inhibitors wortmannin (50-200 nM) and LY294002 (15-120 microM) for 4 h substantially enhanced apoptosis in a concentration-dependent manner as compared with treatment with gemcitabine alone, as determined by the loss of mitochondrial membrane potential and the increase in propidium iodide uptake using flow cytometry. Furthermore, Western blotting showed that the reduction of phosphorylated PKB/Akt levels correlated with the enhancement of gemcitabine-induced apoptosis, suggesting that the PI3K-PKB/Akt pathway plays a significant role in mediating drug resistance in human pancreatic cancer cells. PI3K inhibitors may have therapeutic potential when combined with gemcitabine in the treatment of pancreatic cancers.

Protection from radiation-induced DNA single-strand breaks by induction of nuclear metallothionein

PURPOSE

To examine the extent to which nuclear metallothionein protects from radiation-induced DNA damage under aerobic and hypoxic conditions.

MATERIALS AND METHODS

A semiquantitative fluorescence image analysis method measured the nuclear content of metallothionein (MT) in ME180 and SiHa human squamous cervical carcinoma cell lines under normal growth conditions, and following MT induction by zinc. The extent of initial DNA damage following 60Co irradiation under aerobic and hypoxic conditions was assessed using the alkaline comet assay.

RESULTS

Provided that cells were maintained at 37 degrees C, most of the cellular content of MT was in the nucleus. Incubation at 4 degrees C caused the rapid translocation of MT from the nucleus into the cytoplasm in both cell lines, with no net loss of cellular MT. Baseline nuclear MT levels were about four times greater in ME180 cells, and were much more readily induced by treatment with 100 microM zinc acetate, compared with SiHa cells. Under aerobic conditions, MT induction by zinc resulted in no protection in either of the cell lines. Under hypoxic conditions, however, the number of DNA single-strand breaks in zinc-treated cells was reduced by approximately 40% in ME180, but not in SiHa cells, when compared with non-induced controls.

CONCLUSIONS

Nuclear MT can exert a significant level of protection from radiation by a mechanism that involves competition with oxygen for DNA radical sites and/or scavenging of free radicals. Because increased MT levels have been reported in hypoxic micro-regions of some solid tumours, this protective mechanism might have clinical relevance.

Microregional heterogeneity of non-protein thiols in cervical carcinomas assessed by combined use of HPLC and fluorescence image analysis

Under low oxygen conditions, non-protein thiols (NPSHs, non-protein sulfhydryls) can effectively compete for DNA radicals sites and hence represent a potentially important cause of radiation resistance in the clinic. Intra- and intertumoral heterogeneity of glutathione (GSH) and cysteine were assessed in cryostat sections of multiple biopsies obtained from 10 cervical carcinomas by the combined use of a sensitive high-performance liquid chromatography (HPLC) method and a fluorescence image analysis technique to examine the spatial distribution of NPSHs in tumor tissue. Glutathione concentrations ranged from 1.98 to 4.42 mM; significant (> or =1 mM) concentrations of cysteine, a more effective radioprotector than GSH, were found in some tumors. By HPLC, the intratumoral heterogeneity of NPSHs was relatively small compared with the intertumoral heterogeneity. The histochemical stain 1-(4-chloromercuryphenoylazo)-2-napthol (mercury orange), which binds to GSH and cysteine, was used to determine the spatial distribution of NPSHs in tumor tissue. A comparison of NPSH levels in serial cryostat sections showed a close correlation between NPSH values determined by HPLC and mercury orange fluorescence quantification. Using fluorescence image analysis, an approximately 2-fold increase of NPSHs in tumor versus nonmalignant tissue was observed in the same section. Because some cervical carcinomas contain radiobiologically important levels of cysteine, agents that target the biochemical pathways maintaining tumor cysteine have therapeutic potential as adjuncts to radiotherapy in cervix cancer patients.

Association between tissue hypoxia and elevated non-protein sulphydryl concentrations in human cervical carcinoma xenografts

A double staining technique was developed for the simultaneous measurement of tissue hypoxia and the concentration of non-protein sulphydryls (NPSH), based on the fluorinated nitroimidazole EF5 and the fluorescent histochemical NPSH stain 1-(4-chloromercuriphenoylazo)-naphthol-2 (mercury orange). Cryostat sections of tumour tissue were examined by fluorescence image analysis, using a computer-controlled microscope stage to generate large tiled field images of the cut tumour surface. This method was applied to the human cervical squamous cell carcinoma lines ME180 and SiHa, grown as xenografts in severe combined immunodeficient (SCID) mice, in order to determine if there is a systematic relationship between tissue hypoxia and NPSH levels. Hypoxic regions of the tumours, defined by EF5 labelling, were found to show greater NPSH concentrations relative to better oxygenated regions. This is probably due to increases in glutathione, since the ME180 and SiHa xenografts contained low levels of cysteine and metallothionein; the other major cellular thiols that can bind to mercury orange. Because the effects of glutathione on radiation and chemotherapy resistance are likely to be greater under hypoxic conditions, these results have potentially important implications for the study of resistance mechanisms in solid tumours.

p53 gene status and chemosensitivity of childhood acute lymphoblastic leukemia cells to adriamycin

The role of p53 as a determinant of sensitivity of ten childhood acute lymphoblastic leukemia (ALL) cell lines to Adriamycin (ADR) was investigated. ADR-sensitive cell lines were found to have wild-type (wt) p53, whereas resistant cell lines contained point mutations in the gene. The basal level of wt p53 protein in sensitive cells was lower than that of mutant p53 in resistant cells, however, after ADR treatment a 6- to 20-fold dose-dependent increase in wt p53 was observed, whereas mutant p53 increased only twofold. The percentage of apoptotic cells in ADR-sensitive lines with wt p53 ranged from 43 to 93% following ADR treatment, whereas that in resistant lines with mutant p53 was only 8-13%. The ratio of constitutive levels of Bax/Bcl-2 was significantly higher in cells containing wt p53 than in cells with mutant p53. These results suggest that p53 gene status and the ability of p53 to induce apoptosis may be determinants of sensitivity to ADR in childhood ALL cells.

Bcl-2 targeted to the endoplasmic reticulum can inhibit apoptosis induced by Myc but not etoposide in Rat-1 fibroblasts

Bcl-2 is a key inhibitor of a broad range of apoptotic pathways, yet neither the mechanism of action nor the role of Bcl-2 subcellular localization are well understood. The subcellular localization of Bcl-2 includes the mitochondrial membrane as well as the contiguous membrane of the endoplasmic reticulum and nuclear envelope. Most studies suggest that the ability of Bcl-2 to confer cell survival is dependent upon its localization to the mitochondria. In this manuscript, we show that Bcl-2 targeted to the endoplasmic reticulum can inhibit Myc-, but not etoposide-induced apoptosis in the Rat-1 fibroblast cell line. By contrast, wild type Bcl-2 can inhibit apoptosis triggered by either death agonist. We further show both Myc and etoposide trigger disruption of mitochondrial membrane potential (MMP) and induce poly-ADP ribose polymerase (PARP) cleavage, but release of calcium was not evident. Bcl-2 abrogates apoptosis at or upstream of MMP depletion showing that Bcl-2 does not have to reside at the mitochondria to prevent apoptosis. These results further elucidate the biochemical events associated with Myc- and etoposide-induced apoptosis and significantly advance our understanding of Bcl-2 function.

Increased sensitivity of acute myeloid leukemias to lovastatin-induced apoptosis: A potential therapeutic approach

We recently demonstrated that 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the rate-limiting enzyme of de novo cholesterol synthesis, was a potential mediator of the biological effects of retinoic acid on human neuroblastoma cells. The HMG-CoA reductase inhibitor, lovastatin, which is used extensively in the treatment of hypercholesterolemia, induced a potent apoptotic response in human neuroblastoma cells. This apoptotic response was triggered at lower concentrations and occurred more rapidly than had been previously reported in other tumor-derived cell lines, including breast and prostate carcinomas. Because of the increased sensitivity of neuroblastoma cells to lovastatin-induced apoptosis, we examined the effect of this agent on a variety of tumor cells, including leukemic cell lines and primary patient samples. Based on a variety of cytotoxicity and apoptosis assays, the 6 acute lymphocytic leukemia cell lines tested displayed a weak apoptotic response to lovastatin. In contrast, the majority of the acute myeloid leukemic cell lines (6/7) and primary cell cultures (13/22) showed significant sensitivity to lovastatin-induced apoptosis, similar to the neuroblastoma cell response. Of significance, in the acute myeloid leukemia, but not the acute lymphocytic leukemia cell lines, lovastatin-induced cytotoxicity was pronounced even at the physiological relevant concentrations of this agent. Therefore, our study suggests the evaluation of HMG-CoA reductase inhibitors as a therapeutic approach in the treatment of acute myeloid leukemia.

An oxidative stress-mediated death pathway in irradiated human leukemia cells mapped using multilaser flow cytometry

OCI/AML-2 acute myeloid leukemia cells were found to undergo apoptosis after treatment with y rays from a 137Cs source. Multilaser flow cytometry techniques using probes for live cell function were used to monitor the biochemical changes that occurred prior to the loss of surface membrane integrity. These showed increases in the generation of reactive oxygen species (ROS) and in the glutathione (GSH) content of irradiated cells. An additional population of cells that showed a further increase in ROS and depletion of GSH was seen in irradiated cells but not in controls. This population showed loss of mitochondrial membrane potential (deltapsim), indicative of the mitochondrial permeability transition, and exposure of phosphatidylserine on the cell surface. Increases in intracellular calcium were observed in a proportion of these low-deltapsi(m)/high-ROS cells. Similar findings were seen using the antileukemia drug cytosine arabinoside (ara-C), although cell cycle analysis showed that the loss of deltapsi(m) occurred mainly in G1 phase with ara-C treatment, and mainly in G2 phase with irradiation. Furthermore, the protective effect of overexpression of BCL2 was more pronounced after ara-C treatment than with radiation. Cells of the TP53 (formerly known as p53)-null human AML line OCI M2 showed growth arrest in G2 phase after radiation treatment, with no loss of deltapsi(m) or morphological changes indicative of apoptosis. The flavine-dependent oxidoreductase inhibitor diphenylene iodonium failed to inhibit generation of ROS in irradiated OCI/AML-2 cells, indicating that the mechanism is unlikely to involve the TP53-induced gene PIG3. These results show that oxidative stress can occur in irradiated human leukemia "blasts", and may play a direct role in radiation-induced apoptosis.

Antileukemic action of buthionine sulfoximine: evidence for an intrinsic death mechanism based on oxidative stress

The glutathione-depleting agent buthionine sulfoximine (BSO) was found to be toxic to some AML blast populations. This toxicity was manifested as the appearance of high levels of reactive oxygen generation in GSH-depleted cells, and later by the loss of mitochondrial membrane potential and an increase in intracellular calcium. Striking heterogeneity in BSO sensitivity was observed in a series of four human AML cell lines, and in fresh leukemic blasts obtained from eight AML patients. In some cases, toxicity was seen at BSO concentrations as low as 1 microM; approximately 100-fold less than the plasma levels achieved in patients treated with BSO as a drug resistance reversing agent. Based on these results we propose that some AML blast populations are unusually dependent on GSH-based antioxidant mechanisms, due to high intrinsic rates of reactive oxygen generation. The mitochondrial respiratory chain is the most likely source of this reactive oxygen. Because toxicity is seen at clinically achievable concentrations of BSO, this agent might have antileukemic activity in patients.

Relationships between the mitochondrial permeability transition and oxidative stress during ara-C toxicity

The mitochondrial permeability transition and oxidative stress seem to be critical alterations in cellular physiology that take place during programmed cell death. Failure to undergo apoptosis is associated with drug resistance in acute myeloid leukemia and other cancers. Therefore, it is important to establish causal relationships between the physiological changes that take place in apoptosis, because these are potential targets for novel treatment strategies to overcome this form of drug resistance. We describe the use of multilaser flow cytometry methods to make correlated measurements of mitochondrial membrane potential (MMP), the generation of reactive oxygen intermediates, the cellular content of reduced glutathione (GSH), intracellular calcium, and exposure of phosphatidylserine on the cell surface. Using these combined methods, we have mapped a "death sequence" that occurs after treatment of leukemic blasts with clinically relevant concentrations of 1-beta-D-arabinofuranosylcytosine (ara-C). Dual labeling of MMP and cellular glutathione content showed that loss of MMP, indicative of the permeability transition, took place in cells that were depleted of glutathione. The loss of MMP coincided with phosphatidylserine exposure and preceded a state of high reactive oxygen generation. Finally, there was an increase in intracellular calcium. These results demonstrate that the mitochondrial permeability transition takes place during ara-C toxicity but suggest that this occurs downstream of the loss of GSH. Thus, oxidative stress after ara-C-induced toxicity seems to be a biphasic phenomenon, with the permeability transition occurring after a depletion of GSH and preceding a state of high reactive oxygen generation.

Feasibility of obtaining breast epithelial cells from healthy women for studies of cellular proliferation

Increased dietary fat intake and rate of breast epithelial cell proliferation have each been associated with the development of breast cancer. The goal of this study was to measure the effect of a low fat, high carbohydrate diet on the rate of breast epithelial cell proliferation in women at high risk for breast cancer. Women were recruited from the intervention and control groups of a randomized low fat dietary intervention trial, breast epithelial cells were obtained by fine needle aspiration, and cell proliferation was assessed in these samples using immunofluorescent detection of Ki-67 and PCNA. The effects of needle size and study group on cell yield and cytologic features of the cells were also examined. Fifty three women (20 in the intervention group and 33 in the control group) underwent the biopsy procedure. Slides from 38 subjects were stained for Ki-67 and from 14 subjects for PCNA. No cell proliferation (fluorescence) was detected for either Ki-67 or PCNA in any of the slides. Epithelial cell yield and number of stromal fragments were greater with a larger needle size. Numbers of stromal fragments and bipolar naked nuclei were greater in the low fat as compared to the control group but no differences in epithelial cell yield were observed between the two groups. This study confirms that fine needle aspiration biopsy is a feasible method of obtaining epithelial cells from women without discrete breast masses, but suggests that cell proliferation cannot be assessed using Ki-67 and PCNA in such samples.

A novel energy dependent mechanism reducing daunorubicin accumulation in acute myeloid leukemia

Using cyclosporin A (CsA) to inhibit P-glycoprotein (P-gp) function we showed previously that there was a discordance between the ability of acute myeloid leukemic (AML) blast cells to accumulate daunorubicin and P-gp antigen expression (Xie et al, Leukemia 1995; 9:1882-1887). This discordance suggests that a CsA-sensitive drug efflux mechanism distinct from P-gp is expressed in many clinical samples. In the present study using the ATP depleting agents cyanide, azide, or dinitrophenol to inhibit energy dependent transport processes, we observed even larger increases in daunorubicin accumulation than were seen with CsA. Similar patterns were seen in a wide range of P-gp negative human cancer cell lines. Also the observed cyanide effect did not correlate with the expression of mRNA for multidrug resistance-associated protein (MRP), the only other member of the ABC family of membrane transporters that is known to be capable of effluxing daunorubicin. Thse results suggest that daunorubicin accumulation in many cases of AML is modulated by one or more novel energy-dependent processes that are distinct from P-gp or MRP. We speculate that this novel drug transport mechanism(s) may influence the response of AML patients to daunorubicin and other therapeutic agents.

Effects of topoisomerase I inhibition on the expression of topoisomerase II alpha measured with fluorescence image cytometry

A fluorescence image cytometry technique was developed to measure the effects of topotecan, a topoisomerase I inhibitor, on the nuclear expression of topoisomerase II alpha in a series of patients with refractory or relapsed acute myeloid leukemia (AML). We used a commercially available affinity-purified rabbit polyclonal antibody and a fluorescein-conjugated secondary antibody. By using DAPI as a DNA counterstain and dual wavelength excitation, it was possible to measure enzyme expression in the cell nucleus, and to examine its cell cycle phase distribution. In human acute leukemia cell lines, topoisomerase II alpha expression was greatest in late S and G2 phases, but in leukemia patient samples the enzyme expression appeared to be much less cell cycle dependent. There was considerable interpatient variation in the effects of topotecan on topoisomerase II alpha expression in the leukemia patients, with a threefold increase in the median value after 48 h followed by a decline to pretreatment levels after 5 days of treatment with the topoisomerase I inhibitor. Although these findings should be treated with caution because of the small number of cases studied, they support the prediction that topoisomerase I inhibitors might be capable of increasing sensitivity to topoisomerase II active drugs such as anthracyclines and epipodophyllotoxins by upregulating topoisomerase II expression. They also illustrate the potential value of fluorescence image cytometry for making sequential measurements of the effects of drug resistance modulating agents in cancer patients.

Generation of reactive oxygen intermediates after treatment of blasts of acute myeloblastic leukemia with cytosine arabinoside: role of bcl-2

Cytosine arabinoside is usually considered to be lethal by incorporation into DNA followed by chain termination. Recently, we have reported that the radical scavenger N-acetyl-cysteine (NAC) protects cultured clonogenic AML blast cells from the lethal affects of Ara-C if given before the drug. This observation provides indirect evidence that toxic reactive oxygen intermediates (ROI) are generated in AML blast cells following Ara-C-induced damage to DNA. In the present paper we present evidence in support of this hypothesis. Using flow cytometry and multiple fluorescent probes for live cell function, we have mapped a sequence of discrete stages that occur during Ara-C cytotoxicity. An early event was the increased generation of ROI. Initially this oxidative stress was countered by an increase in the cellular content of reduced glutathione (GSH), but cells then underwent an abrupt transition to a state characterized by low GSH and very high ROI generation indicative of collapse of cellular redox balance. Next, the capacity to maintain low intracellular ionized calcium was lost, probably due to lipid peroxidation at membrane sites of calcium regulation. Finally, surface membrane integrity was lost. Concurrent measurements of clonogenic cell survival insured the relevance of these flow cytometry measurements to the stem cell population. We used OCI/AML-2 cells transfected with bcl-2 to look for the place in this sequence where bcl-2 protein protects cells against apoptosis; bcl-2 transfectants showed an increase in ROI generation similar to controls, but were able to maintain GSH levels in the face of this oxidative stress. We conclude that oxidative stress plays a major role in Ara-C toxicity, and that bcl-2 protein protects cells by maintaining cellular redox balance in a reducing state. These studies complement previous work showing how regulators of AML growth affect the sensitivity of blast cells to Ara-C by changing the concentration or stability of bcl-2 protein.

Discordant P-glycoprotein antigen expression and transport function in acute myeloid leukemia

Expression of the multidrug resistance efflux pump P-glycoprotein (Pgp) was measured in a series of AML patients using two flow cytometry methods. Transport function was assessed by measuring the modulating effect of the Pgp inhibitor cyclosporin A (CsA) on the cellular accumulation of daunorubicin, and Pgp antigen expression by surface immunofluorescence using the MRK-16 antibody. Both methods showed a wide range of values for Pgp expression between individual patients, but in contrast to a series of cell lines expressing Pgp there was no correlation between antigen expression and transport function in the clinical samples. As previously reported for chronic lymphocytic leukemia (CLL), pretreatment with neuraminidase markedly improved MRK-16 staining in some cases, indicating that abnormal glycosylation can cause epitope masking in AML blasts. Because experience with cell lines shows that Pgp expression is a continuous variable which correlates with the level of drug resistance, rather than the 'positive' or 'negative' which are frequently reported by clinical flow cytometry laboratories, we used a calibration procedure to estimate the actual number of Pgp molecules expressed in the AML samples. Despite the additional refinements of neuraminidase treatment and antigen quantification, the correlation between Pgp antigen expression and daunorubicin accumulation remained extremely weak (r = 0.11; P = 0.63). It is suggested that the assay for transport function can detect molecules that affect daunorubicin accumulation but are antigenically distinct from classical P-glycoprotein. Heterogeneity of multidrug resistance efflux pumps might in part explain the relatively weak prognostic significance of immunofluorescence detection of Pgp in AML patients.

Modulation of the equilibrative nucleoside transporter by inhibitors of DNA synthesis

Expression of the equilibrative, S-(p-nitrobenzyl)-6-thioinosine (NBMPR)-sensitive nucleoside transporter (es), a component of the nucleoside salvage pathway, was measured during unperturbed growth and following exposure to various antimetabolites at growth-inhibitory concentrations. The probe 5-(SAENTA-x8)-fluorescein is a highly modified form of adenosine incorporating a fluorescein molecule. It binds. with high affinity and specificity to the (es) nucleoside transporter at a 1:1 stoichiometry, allowing reliable estimates of es expression by flow cytometry. Using a dual labelling technique which combined the vital DNA dye Hoechst-33342 and 5-(SAENTA-x8)-fluorescein, we found that surface expression of es approximately doubled between G1 and G2 + M phases of the cell cycle. To address the question of whether es expression could be modulated in cells exposed to drugs which inhibit de novo synthesis of nucleotides, cells were exposed to antimetabolite drugs having different modes of action. Hydroxyurea and 5-fluorouracil (5-FU), which inhibit the de novo synthesis of DNA precursors, produced increases in the expression of es. In contrast, cytosine arabinoside (ara-C) and aphidicolin, which directly inhibit DNA synthesis, produced no significant increase in es expression. Thymidine (TdR), which is an allosteric inhibitor of ribonucleotide reductase that depletes dATP, dCTP and dGTP pools while repleting the dTTP pool, had no significant effect on es expression. These data suggest that surface expression of the es nucleoside transporter is regulated by a mechanism which is sensitive to the supply of deoxynucleotides. Because 5-FU (which specifically depletes dTTP pools) causes a large increase in expression whereas TdR (which depletes all precursors except dTTP) does not, this mechanism might be particularly sensitive to dTTP pools.

Differential effects of depleting agents on cytoplasmic and nuclear non-protein sulphydryls: a fluorescence image cytometry study

The intracellular distribution of glutathione (GSH) was measured by a quantitative image cytometry method, using the sulphydryl-reactive agent mercury orange. This readily forms fluorescent adducts with GSH and other non-protein sulphydryls (NPSH), but reacts much more slowly with protein sulphydryls. Under optimum staining conditions mean integrated mercury orange fluorescence per cell was closely correlated with a standard biochemical assay for GSH. Use of the DNA dye DAPI as a counterstain allowed measurement of nuclear NPSH. The mean nuclear-cytoplasmic ratio was 0.57 +/- 0.05. Isolation of nuclei under aqueous conditions resulted in the loss of approximately 90% of mercury orange fluorescence, compared with nuclear fluorescence from intact cells, suggesting that background labelling of protein sulphydryls or other macromolecules is low. Depletion of GSH with N-ethylmaleimide or diethylmaleate decreased mercury orange fluorescence in the nucleus and cytoplasm to a similar extent. In contrast, mercury orange fluorescence in the nucleus was much more resistant to DL-buthionine-S,R-sulphoximine (BSO) depletion than that in the cytoplasm. This finding is compatible with a distinct pool of GSH in the nucleus that is comparatively resistant to BSO depletion. Alternatively, the retention of fluorescence in the nucleus following GSH depletion by BSO treatment might be due to accumulation of cysteine. These findings have implications for cancer treatment since the level of NPSH in the nucleus might be a more important determinant of resistance to DNA-damaging agents than that in cytoplasm. The image cytometry method described here is quantitative, allows a measure of tumour cell heterogeneity and can be applied to small biopsy samples obtained by fine-needle aspiration. Thus it appears suitable for prospective clinical studies in cancer patients, and for monitoring the effects of GSH-depleting agents used as adjuncts to cancer chemotherapy or radiotherapy.

Biochemical modulation of iododeoxyuridine by N6-[4-(morpholinosulfonyl)benzyl]-N6-methyl-2,6-diaminobenz[cd]indole glucuronate (AG-331) leading to enhanced cytotoxicity

Inhibition of thymidylate synthase (TS) may increase incorporation of thymidine analogues into DNA, leading to increased inhibition of colony formation in tumor cells. We have reported previously that TS inhibition by N-(5-[N-(3,4-dihydro-2-methyl-4-oxoquinazolin-6,-ylmethyl)-N -methylamino]-2 - thenoyl)-L-glutamic acid (ICI D1694 or Tomudex), a folate-based TS inhibitor, increases the cytotoxicity of iododeoxyuridine (IdUrd), a thymidine analogue, in MGH-U1 human bladder and HCT-8 human colon cancer cells. N6-[4-(Morpholinosulfonyl)benzyl]-N6-methyl-2,6-diaminobenz[ cd]-indole glucuronate (AG-331) differs from ICI D1694 in that it is a de novo designed lipophilic TS inhibitor, it does not require a specific carrier for cellular uptake, and it does not undergo intracellular polyglutamation. Exposure of MGH-U1 cells to 5 microM AG-331 for 24 hr decreased clonogenic survival by 30%, but almost completely inhibited TS activity. IdUrd is a cytotoxic thymidine analogue, with IC50 and IC90 values after 24-hr exposures in MGH-U1 cells of 13 and 81 microM, respectively. The combination of IdUrd and AG-331 resulted in an enhanced antitumor effect, as compared with the effect of either agent alone. The cytotoxic IC50 of IdUrd decreased from 13 to 1.5 microM, and the IC90 decreased from 81 to 5 microM with the addition of 5 microM AG-331. Biochemical studies of the combination revealed that pretreating MGH-U1 cells with 5 microM AG-331 increased IdUrd incorporation into cellular DNA by 3.8-fold. This increased incorporation was associated with a greater proportion of DNA single-strand breaks than observed with either agent alone, and the combination of 5 microM AG-331 plus IdUrd produced up to a 2.5-fold increase in DNA single-strand breaks as compared with IdUrd alone. The effects of AG-331, IdUrd, and the combination of IdUrd and AG-331 on the colony-forming ability of normal human bone marrow CFU-GM cells was determined as a measure of myelosuppression. The combination of IdUrd and AG-331, at the same concentrations as those used in the MGH-U1 cells, produced a wider therapeutic index relative to that of IdUrd alone, and the therapeutic index for the combination was 6.5, as compared with 4.0 for IdUrd plus ICI D1694 in previous studies from this laboratory. These observations suggest that the combination of IdUrd and AG-331 may enhance antitumor effects with minimal myelosuppression in vivo.

Effects of thymidylate synthase inhibition on thymidine kinase activity and nucleoside transporter expression

The effects of de novo dTMP inhibition by N-(5-[N-(3,4-dihydro-2-methyl-4-oxoquinazolin-6-ylmethyl)-N- methylamino]-2- thenoyl)-L-glutamic acid (D1694) or N6-[4-(morpholinosulfonyl)benz]-N6-diaminobenz[cd]indole glucuronate (AG-331) on clonogenic survival, thymidylate synthase (TS) and thymidine kinase (TK) activity, and expression of S-(p-nitrobenzyl)-6-thioinosine-sensitive nucleoside transporter (NT) sites were addressed in the human bladder cancer cell line, MGH-U1. These two TS inhibitors are structurally diverse. D1694 is a folate-based TS inhibitor, whereas AG-331 is a novel agent that inhibits the cofactor binding site of the enzyme. They also differ with respect to their cytotoxic effects in this cell line; D1694 cytotoxic 50% inhibitory concentration (IC50) and IC90 were 6.0 and 9.0 nM, respectively and IC50 and IC90 for TS inhibition were 2.5 and 4.8 nM, respectively. In contrast, AG-331 cytotoxic IC50 could not be achieved even at concentrations of up to 20 microM for 24-h exposures, and IC50 and IC90 for TS inhibition were 0.7 and 3.0 microM, respectively. Similar effects for D1694 and AG-331 were observed in their modulation of TK activity and NT expression. 5-(SAENTA-x8)-Fluorescein, a highly modified form of adenosine incorporating a fluorescein molecule which binds with a 1:1 stoichiometry to S-(p-nitrobenzyl)-6-thioinosine-sensitive NT sites, was used to investigate the expression of NT following exposure of cells to D1694 and AG-331. TK activity was addressed by the metabolism of [3H]thymidine to [3H]TMP by cellular extracted protein and by an alternative flow cytometric method using a modified form of thymidine incorporating a fluorescent molecule, dansyl-5-amino-2-deoxyuridine. Results obtained by both methods were comparable. At concentrations of 5 and 10 nM, D1694 increased TK activity 2.3-4.5-fold and NT expression 34-39-fold. AG-331, at concentrations of 5 and 10 microM, increased TK activity 1.8-2.5-fold and NT expression 22-31-fold, respectively. These data suggest that TK activity and NT expression have a common regulatory mechanism which is sensitive to endogenous dTTP pools and that the salvage pathway is a complex system of kinases coordinated with transport of nucleosides.

Relationship of DNA topoisomerase II alpha and beta expression to cytotoxicity of antineoplastic agents in human acute lymphoblastic leukemia cell lines

The levels of expression of topoisomerase II alpha and topoisomerase II beta were investigated in six established cell lines of human childhood acute lymphoblastic leukemia (ALL) as a function of doubling time, cell cycle distribution, and of sensitivity to the antineoplastic agents Adriamycin and etoposide. The slowest growing cell line, ALL-G, was most sensitive to both drugs, whereas the fastest growing cell line, ALL-C, was 15.3- and 6.4-fold more resistant than ALL-G to Adriamycin and etoposide, respectively. Furthermore, ALL-W, the second most rapidly dividing cell line, was most resistant to both Adriamycin (22.8-fold) and etoposide (14.1-fold). Expression of topoisomerase II alpha varied inversely with doubling time, whereas no correlation was found between topoisomerase II beta levels and doubling time. Expression of topoisomerase II beta varied inversely with that of topoisomerase II alpha. The level of topoisomerase II alpha correlated directly with the percentage of cells in S and G2-M phases, whereas topoisomerase II beta expression varied directly with the number of cells in G1. An inverse correlation was found between the level of expression of topoisomerase II beta and resistance to Adriamycin, whereas a direct correlation was observed between the level of expression of topoisomerase II alpha and resistance to Adriamycin. Studies with etoposide, although not statistically significant, were consistent with the pattern observed with Adriamycin. These findings suggest that in ALL cells, cytocidal activity of Adriamycin and etoposide may be mediated, at least in part, by topoisomerase II beta.

Constitutive expression of P-glycoprotein as a determinant of loading with fluorescent calcium probes

Determination of intracellular calcium levels in Chinese hamster ovary (CHO) cells using the fluorescent calcium probe indo-1AM was hindered by the low level of accumulation of indo-1 in these cells. CHO cells are known to express basal levels of the multidrug resistance efflux pump P-glycoprotein (P-gp). Rhodamine-123, which is a known substrate of P-gp, was used to confirm the presence of P-gp in CHO cells. Verapamil and cyclosporin (CsA), both inhibitors of P-gp, enhanced accumulation of indo-1 in these cells and therefore allowed for improved intracellular calcium measurements. P-gp overexpressing colchicine-resistant CHO cells (CHRC5) also displayed enhanced indo-1AM loading with P-gp inhibitors. Nondetectable levels of P-gp activity were found in wild-type CEM-CCRF cells (human T lymphoblasts), and these cells did not show any difference in indo-1AM loading in the presence or absence of P-gp inhibitors. Loading of a second calcium fluorescent probe fluo-3AM was improved in CHO cells by P-gp inhibition, whereas the structurally related pH probe BCECF-AM was minimally affected. Because low levels of P-gp may be expressed by a range of cell lines and normal tissues, it is suggested that this be considered if difficulties are encountered in loading fluorescent calcium probes.

ICI D1694 and idoxuridine: a synergistic antitumor combination

The cytotoxicity of idoxuridine (IdUrd), a thymidine analogue, and ICI D1694 (D1694), a folate-based thymidylate synthase (TS) inhibitor, were examined individually and in combination in two human tumor cell lines. MGH-U1 bladder cancer and HCT-8 colon cancer cells were grown as monolayer cultures with and without thymidine. The cytotoxicity of these agents alone and in combination were determined using normal human bone marrow colony-forming unit, granulocyte-macrophage (CFU-GM) as a surrogate for myelosuppression in vivo. Thymidylate synthase inhibition, IdUrd incorporation into DNA, and DNA single-strand breaks were measured in each cell line and related to cytotoxicity. The cytotoxicity of a 24-h exposure to IdUrd or D1694 increased with drug concentration in each cell line. The drug concentrations producing 50% and 10% clonogenic survival in MGH-U1 cells, respectively, were 0.006 and 0.009 microM for D1694 and 13.0 and 81.0 microM for IdUrd. Those for HCT-8 cells, respectively, were 0.009 and 0.018 microM for D1694 and 7.5 and 20.5 microM for IdUrd. The cytotoxicity of IdUrd combined with D1694 was synergistic in both MGH-U1 and HCT-8 cells as determined by median-effect analysis. The addition of thymidine at concentrations of 0.1, 0.3, and 1.0 microM to the culture medium did not decrease the cytotoxicity of D1694 in either tumor cell line. TS inhibition using the whole cell assay was observed with only D1694, producing 50% inhibition of TS activity at 0.002 microM for MGH-U1 and 0.007 microM for HCT-8 cells. IdUrd did not inhibit TS activity, nor did it enhance the TS inhibitory effects of D1694. The incorporation of IdUrd into DNA increased with increasing concentrations of D1694. This increased DNA incorporation correlated with the increase in DNA single-strand breaks. DNA single-strand breaks paralleled cytotoxicity. CFU-GM survival, exposed to the same drug concentrations as those used in the tumor cell lines, revealed that the therapeutic index was greater for the combination than for either agent alone. These findings suggest that IdUrd plus D1694 is a promising new drug combination, which may have a favorable therapeutic index in vivo.

Evaluation of methods for measuring cellular glutathione content using flow cytometry

The currently available flow cytometric stains for cellular glutathione were evaluated, examining the labelling of both human and rodent cell lines under various conditions of concentration, time, and temperature. Procedures were used that depleted glutathione (GSH) while having a minimal effect on other cellular sulphydryls in order to estimate linearity and the extent of background staining. As previously reported, monochlorobimane was highly specific for GSH in rodent cells but failed to label human cells adequately because of its low affinity for human glutathione S-transferases. Higher concentrations of monochlorobimane achieved more complete labelling of the human cellular GSH pool but gave increased background fluorescence due to non-GSH binding. The analogue monobromobimane, which binds nonenzymatically to sulphydryls, reacted more readily with GSH than with protein sulphydryls and, provided that stain concentration and incubation time were controlled, gave reproducible staining of human cells with approximately 20% of total fluorescence due to background staining. Of the currently available stains for measuring GSH in human cells, monobromobimane is the agent of choice. Mercury orange also binds more readily to GSH than to protein, giving a degree of specificity, and it has the additional advantage of being excited at 488 nm. However, the reproducibility of staining with mercury orange was less consistent than that using monobromobimane, and a higher background fluorescence was seen. Two additional stains, o-phthaldialdehyde and chloromethyl fluorescein, could also be used to label cellular GSH, but both gave an unacceptably high level of background staining. It is recommended that flow cytometric GSH assays should routinely include a sample of cells that have been depleted of GSH in order to determine the extent of background labeling.

Regulation of intracellular pH in subpopulations of cells derived from spheroids and solid tumours

Solid tumours are known to develop regions of extracellular acidity and survival of tumour cells in such regions depends on membrane-based mechanisms which regulate intracellular pH (pHi). We have therefore developed a method, based on dual staining of cells and flow cytometry, to study the regulation of pHi in subpopulations of tumours and spheroids. The activity of membrane-based pHi regulating transporters was studied in EMT-6 and MGH U1 cells grown in monolayer culture, spheroids, and tumours. pHi was measured with the fluorescent pH probe 2'7'-bis-(2-carboxyethyl)-5-(and-6)carboxyfluorescein, and Hoechst 33342 was used to identify cells from different regions of tumours and spheroids. In monolayer culture, incubation of cells for 18 h at pHe 6.6 led to a 1.3-1.5-fold enhancement in the activity of both the Na+/H+ exchanger and the Na(+)-dependent Cl-@HCO3- exchanger. This effect was prevented by the protein synthesis inhibitor cycloheximide. Cells from the centre of EMT-6 spheroids had increased activity of the Na+/H+ exchanger compared to cells from the periphery, when spheroids were grown in medium at pH 6.6, but not at 7.4. By contrast, in MGH U1 spheroids, cells from the centre had increased activity of the Na+/H+ antiport under both sets of conditions. There was no significant difference in the activity of the Na+/H+ exchanger in cells derived from different subpopulations of EMT-6 tumours or MGH U1 xenografts in nude mice. Although upregulation of Na+/H+ exchange occurs after exposure to acidic conditions in vitro, the microenvironmental conditions found within solid tumours do not appear to cause this effect. Our results suggest the feasibility of pharmacological inhibition of Na+/H+ exchange activity as an approach to therapy directed against nutrient-deprived tumour cells.

DNA Cytometry Consensus Conference. DNA flow cytometry and breast cancer

Measurement of cellular DNA content by flow cytometry is capable of detecting aneuploid stemlines, and also of giving an indication of tumor proliferation kinetics by approximating the percentage of cells in S-phase of the replicative cycle. Because it can be applied both to fresh frozen material submitted for steroid hormone receptor analysis and to fixed paraffin-embedded blocks, it is particularly well suited to the study of breast cancer. Despite being a relatively straightforward test which is now widely used in the risk assessment of patients with early breast cancer, in common with many other prognostic markers its precise clinical role remains uncertain. An extensive body of published data has appeared in the last few years, but the results often appear to be inconclusive or contradictory. In order to define the prognostic significance of DNA cytometry in malignant diseases of the breast, large bowel, bladder, prostate, and hematopoietic system, and to clarify some of the technical issues related to clinical laboratory standards and quality controls, a DNA Cytometry Consensus Conference was held in Prout's Neck, Maine, on October 1-4, 1992. This meeting was sponsored by the NCI, the International Society for Analytical Cytology, and industry. The significance of the meeting's conclusions for clinical breast cancer are discussed here. The consensus statement regarding the clinical utility of DNA cytometry in breast cancer, and the Guidelines for the Implementation of Clinical DNA Cytometry which were generated at this meeting, also appear in this issue of Breast Cancer Research and Treatment.

DNA Cytometry Consensus Conference. Consensus review of the clinical utility of DNA cytometry in carcinoma of the breast

This is the consensus statement regarding the clinical utility of DNA cytometry in breast cancer from the DNA Cytometry Consensus Conference held in Prout's Neck, Maine, USA, on October 1-4, 1992. Guidelines for clinical DNA cytometry generated at that meeting also appear in this issue of Breast Cancer Research and Treatment.