Exploiting Radiation-Induced Signaling to Increase the Susceptibility of Resistant Cancer Cells to Targeted Drugs: AKT and mTOR Inhibitors as an Example

Implementing targeted drug therapy in radio-oncologic treatment regimens has greatly improved the outcome of cancer patients. However, the efficacy of molecular targeted drugs such as inhibitory antibodies or small molecule inhibitors essentially depends on target expression and activity, which both can change during the course of treatment. Radiotherapy has previously been shown to activate prosurvival pathways, which can help tumor cells to adapt and thereby survive treatment. Therefore, we aimed to identify changes in signaling induced by radiation and evaluate the potential of targeting these changes with small molecules to increase the therapeutic efficacy on cancer cell survival. Analysis of "The Cancer Genome Atlas" database disclosed a significant overexpression of AKT1, AKT2, and MTOR genes in human prostate cancer samples compared with normal prostate gland tissue. Multifractionated radiation of three-dimensional-cultured prostate cancer cell lines with a dose of 2 Gy/day as a clinically relevant schedule resulted in an increased protein phosphorylation and enhanced protein-protein interaction between AKT and mTOR, whereas gene expression of AKT, MTOR, and related kinases was not altered by radiation. Similar results were found in a xenograft model of prostate cancer. Pharmacologic inhibition of mTOR/AKT signaling after activation by multifractionated radiation was more effective than treatment prior to radiotherapy. Taken together, our findings provide a proof-of-concept that targeting signaling molecules after activation by radiotherapy may be a novel and promising treatment strategy for cancers treated with multifractionated radiation regimens such as prostate cancer to increase the sensitivity of tumor cells to molecular targeted drugs. Mol Cancer Ther; 17(2); 355-67. ©2017 AACRSee all articles in this MCT Focus section, "Developmental Therapeutics in Radiation Oncology."

Defining molecular signature of pro-immunogenic radiotherapy targets in human prostate cancer cells

To understand the impact of clinically relevant radiation therapy (RT) on tumor immune gene expression and to utilize the changes that occur during treatment to improve cancer treatment outcome, we examined how immune response genes are modulated in prostate cancer cells of varying p53 status. LNCaP (p53 wild-type), PC3 (p53 null) and DU145 (p53 mutant) cells received a 10 Gy single dose or 1 Gy × 10 multifractionated radiation dose to simulate hypofractionated and conventionally fractionated prostate radiotherapy. Total RNA was isolated 24 h after multifractionated radiation treatment and single-dose treatments and subjected to microarray analysis and later validated by RT-PCR. RT-PCR was utilized to identify total-dose inflection points for significantly upregulated genes in response to multifractionated radiation therapy. Radiation-induced damage-associated molecular pattern molecules (DAMPs) and cytokine analyses were performed using bioluminescence and ELISA. Multifractionated doses activated immune response genes more robustly than single-dose treatment, with a relatively larger number of immune genes upregulated in PC3 compared to DU145 and LNCaP cells. The inflection point of multifractionated radiation-induced immune genes in PC3 cells was observed in the range of 8-10 Gy total radiation dose. Although both multifractionated and single-dose radiation-induced proinflammatory DAMPs and positively modulated the cytokine environment, the changes were of higher magnitude with multifractionated therapy. The findings of this study together with the gene expression data suggest that cells subjected to multifractionated radiation treatment would promote productive immune cell-tumor cell interactions.

Differential expression of stress and immune response pathway transcripts and miRNAs in normal human endothelial cells subjected to fractionated or single-dose radiation

Although modern radiotherapy technologies can precisely deliver higher doses of radiation to tumors, thus, reducing overall radiation exposure to normal tissues, moderate dose, and normal tissue toxicity still remains a significant limitation. The present study profiled the global effects on transcript and miR expression in human coronary artery endothelial cells using single-dose irradiation (SD, 10 Gy) or multifractionated irradiation (MF, 2 Gy × 5) regimens. Longitudinal time points were collected after an SD or final dose of MF irradiation for analysis using Agilent Human Gene Expression and miRNA microarray platforms. Compared with SD, the exposure to MF resulted in robust transcript and miR expression changes in terms of the number and magnitude. For data analysis, statistically significant mRNAs (2-fold) and miRs (1.5-fold) were processed by Ingenuity Pathway Analysis to uncover miRs associated with target transcripts from several cellular pathways after irradiation. Interestingly, MF radiation induced a cohort of mRNAs and miRs that coordinate the induction of immune response pathway under tight regulation. In addition, mRNAs and miRs associated with DNA replication, recombination and repair, apoptosis, cardiovascular events, and angiogenesis were revealed.

IMPLICATIONS

Radiation-induced alterations in stress and immune response genes in endothelial cells contribute to changes in normal tissue and tumor microenvironment, and affect the outcome of radiotherapy.

Abstract 425: Fractionated radiation-induced tumor suppressor microRNAs in human prostate carcinoma cells

To understand the role of microRNAs (miRNA) in regulation of radiation-induced gene expression and to help define potential radiation-inducible targets, miRNA expression was studied in wild type p53 LNCaP and p53-mutated PC3 and DU145 cells. We have previously investigated the changes in the molecular profiles of tumor cells that were exposed to single dose (SD) versus fractionated radiation (MF) in vitro, and identified immune and stress response pathways that were induced by fractionated but not single dose radiation. Methods: Cells were exposed to 5Gy and 10Gy either as SD or MF radiation radiation. Microarray analyses were done using human Agilent miRNA Microarray Kit (V2). Data were analyzed using Gene Spring software. Validation of the miRNA expression and gene expression of miRNA targets was evaluated by real-time RT-PCR analysis. Target filter analysis of differentially expressed miRNAs (>1.5 fold change and p value<0.05) and their mRNA targets were done. Results: Microarray analyses revealed that radiation differentially expressed 84, 68 and 8 miRNAs with high confidence (>1.5fold change, p<0.05) in LNCaP, PC3 and DU145 cells, respectively. MF radiation affected more miRNAs than SD radiation in all cell lines. In LNCaP and PC3 cells, MF radiation upregulated tumor suppressor microRNAs miR-34a, miR-200, miR-135, miR-221 and let7. Baseline expression of miR-34a was markedly reduced in PC3 cells and DU145 cells compared to LNCaP cells. However, miR-34a was upregulated by fractionated irradiation in LNCaP and PC3 cells, but not in DU145 cells. RT_PCR analysis of 22 experimentally verified miR-34a targets, showed distinct expression patterns in PC3 and LNCaP cells 6 and 24 hours after radiation. An inverse correlation of NOTCH1, E2F5 and MDM4 expression was observed in PC3 cells 6 and 24 hours after MF but not in LNCaP cells. On the other hand, IFNB1 showed an inverse correlation only in LNCaP cells. Majority of the differences in the expression patterns of miR-34a targets were at 24 hours after MF. Conclusion: Differences in the miRNA expression exist between cell lines and after varying radiation regimens. Among the three prostate carcinoma cell lines, tumor suppressor miRNAs are upregulated after MF radiation in LNCaP and PC3 cells. Currently, we are in the process of modulating miRNAs and their targets to examine their effects on radio sensitivity.

This work was supported by the Intramural Research Program of the NIH, NCI, CCR.

Citation Format: C. Norman Coleman, Molykutty John-Aryankalayil, Adeola Y. Makinde, Sanjeewani T. Palayoor. Fractionated radiation-induced tumor suppressor microRNAs in human prostate carcinoma cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 425. doi:10.1158/1538-7445.AM2013-425

Radiation survivors: understanding and exploiting the phenotype following fractionated radiation therapy

Radiation oncology modalities such as intensity-modulated and image-guided radiation therapy can reduce the high dose to normal tissue and deliver a heterogeneous dose to tumors, focusing on areas deemed at highest risk for tumor persistence. Clinical radiation oncology produces daily doses ranging from 1 to 20 Gy, with tissues being exposed to 30 or more daily fractions. Hypothesizing the cells that survive fractionated radiation therapy have a substantially different phenotype than the untreated cells, which might be exploitable for targeting with molecular therapeutics or immunotherapy, three prostate cancer cell lines (PC3, DU145, and LNCaP) and normal endothelial cells were studied to understand the biology of differential effects of multifraction (MF) radiation of 0.5, 1, and/or 2 Gy fraction to 10 Gy total dose, and a single dose of 5 and 10 Gy. The resulting changes in mRNA, miRNA, and phosphoproteome were analyzed. Significant differences were observed in the MF radiation exposures including those from the 0.5 Gy MF that produces little cell killing. As expected, p53 function played a major role in response. Pathways modified by MF include immune response, DNA damage, cell-cycle arrest, TGF-β, survival, and apoptotic signal transduction. The radiation-induced stress response will set forth a unique platform for exploiting the effects of radiation therapy as "focused biology" for cancer treatment in conjunction with molecular targeted or immunologically directed therapy. Given that more normal tissue is treated, albeit to lower doses with these newer techniques, the response of the normal tissue may also influence long-term treatment outcome.

Fractionated radiation alters oncomir and tumor suppressor miRNAs in human prostate cancer cells

We have previously demonstrated that prostate carcinoma cells exposed to fractionated radiation differentially expressed more genes compared to single-dose radiation. To understand the role of miRNA in regulation of radiation-induced gene expression, we analyzed miRNA expression in LNCaP, PC3 and DU145 prostate cancer cells treated with single-dose radiation and fractionated radiation by microarray. Selected miRNAs were studied in RWPE-1 normal prostate epithelial cells by RT-PCR. Fractionated radiation significantly altered more miRNAs as compared to single-dose radiation. Downregulation of oncomiR-17-92 cluster was observed only in the p53 positive LNCaP and RWPE-1 cells treated with single-dose radiation and fractionated radiation. Comparison of miRNA and mRNA data by IPA target filter analysis revealed an inverse correlation between miR-17-92 cluster and several targets including TP53INP1 in p53 signaling pathway. The base level expressions of these miRNAs were significantly different among the cell lines and did not predict the radiation outcome. Tumor suppressor miR-34a and let-7 miRNAs were upregulated by fractionated radiation in radiosensitive LNCaP (p53 positive) and PC3 (p53-null) cells indicating that radiation-induced miRNA expression may not be regulated by p53 alone. Our data support the potential for using fractionated radiation to induce molecular targets and radiation-induced miRNAs may have a significant role in predicting radiosensitivity.

Fractionated radiation therapy can induce a molecular profile for therapeutic targeting

To examine the possibility of using fractionated radiation in a unique way with molecular targeted therapy, gene expression profiles of prostate carcinoma cells treated with 10 Gy radiation administered either as a single dose or as fractions of 2 Gy × 5 and 1 Gy × 10 were examined by microarray analysis. Compared to the single dose, the fractionated irradiation resulted in significant increases in differentially expressed genes in both cell lines, with more robust changes in PC3 cells than in DU145 cells. The differentially expressed genes (>twofold change; P < 0.05) were clustered and their ontological annotations evaluated. In PC3 cells genes regulating immune and stress response, cell cycle and apoptosis were significantly up-regulated by multifractionated radiation compared to single-dose radiation. Ingenuity Pathway Analysis (IPA) of the differentially expressed genes revealed that immune response and cardiovascular genes were in the top functional category in PC3 cells and cell-to-cell signaling in DU145 cells. RT-PCR analysis showed that a flexure point for gene expression occurred at the 6th-8th fraction and AKT inhibitor perifosine produced enhanced cell killing after 1 Gy × 8 fractionated radiation in PC3 and DU145 cells compared to single dose. This study suggests that fractionated radiation may be a uniquely exploitable, non-oncogene-addiction stress pathway for molecular therapeutic targeting.

Abstract 3046: Differential microRNA patterns in prostate carcinoma cells after single and fractionated radiation

Our previous mRNA microarray analysis showed that fractionated radiation induced more differentially expressed genes compared to single dose radiation in PC3 and DU145 cells. MicroRNAs (miRNAs) are an important class of non-coding small RNAs capable of regulating gene expression at the translational level. To understand the role of miRNAs in regulation of radiation-induced gene expression patterns, we studied the expression levels of miRNAs by microarray analysis in prostate cancer cells. Methods: PC3 and DU145 cells were exposed to 5 Gy and 10Gy either as a single dose radiation or multi-fractionated (0.5Gyx10 and 1Gyx10) radiation. RNA was extracted at 24h after the final dose of radiation and miRNA microarray analysis was done using Agilent human miRNA Microarray Kit (V2). The chip contains probes for 723 human and 76 human viral microRNAs from the Sanger database v.10.1. Data were analyzed using GeneSpring software (Agilent technologies). Results: Of the total 723 miRNAs represented in the array, 141 miRNAs were differentially expressed (&gt; 1.5 fold change) by the 4 radiation protocols. Significant difference in the miRNA expression pattern was noted between the two cell lines. The number of radiation - induced miRNAs was higher in PC3 cells than in DU145 cells. In PC3 cells fractionated radiation resulted in more down regulated miRNAs compared to single dose radiation. In our previous study immune response genes were significantly up regulated by fractionated radiation in PC3 cells but not in DU145 cells. In the present study Mir-146a was significantly up regulated in DU145 cells after fractionated radiation but not in PC3 cells suggesting an inverse correlation between the differential expression of mir-146a and immune response genes in both cell lines. This study also demonstrated significant differences in let-7 family of miRNAs in the two cell lines and also between single and fractionated radiation. In PC3 cells, 8 of the let-7 miRNAs were upregulated after fractionated radiation whereas in DU145 cells they were either downregulated or not changed. Conclusion: Fractionated radiation resulted in expression of more number of miRNAs compared to single dose radiation. We are currently in the process of evaluating radiation-induced differential gene expression changes by a combined approach of mRNA, miRNA and protein array analysis to identify radiation induced molecular targets for cancer therapy.

This work was supported by the Intramural Research Program of Center for Cancer Research, National Cancer Institute, National Institutes of Health.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3046.

NS-398, ibuprofen, and cyclooxygenase-2 RNA interference produce significantly different gene expression profiles in prostate cancer cells

Cyclooxygenase-2 (COX-2) plays a significant role in tumor development and progression. Nonsteroidal anti-inflammatory drugs (NSAID) exhibit potent anticancer effects in vitro and in vivo by COX-2-dependent and COX-2-independent mechanisms. In this study, we used microarray analysis to identify the change of expression profile regulated by a COX-2-specific NSAID NS-398 (0.01 and 0.1 mmol/L), a nonspecific NSAID ibuprofen (0.1 and 1.5 mmol/L) and RNA interference (RNAi)-mediated COX-2 inhibition in PC3 prostate cancer cells. A total of 3,362 differentially expressed genes with 2-fold change and P<0.05 were identified. Low concentrations of NSAIDs and COX-2 RNAi altered very few genes (1-3%) compared with the higher concentration of NS-398 (17%) and ibuprofen (80%). Ingenuity Pathway Analysis was used for distributing the differentially expressed genes into biological networks and for evaluation of functional significance. The top 3 networks for both NSAIDs included functional categories of DNA replication, recombination and repair, and gastrointestinal disease. Immunoresponse function was specific to NS-398, and cell cycle and cellular movement were among the top functions for ibuprofen. Ingenuity Pathway Analysis also identified renal and urologic disease as a function specific for ibuprofen. This comprehensive study identified several COX-2-independent targets of NSAIDs, which may help explain the antitumor and radiosensitizing effects of NSAIDs. However, none of these categories were reflected in the identified networks in PC3 cells treated with clinically relevant low concentrations of NS-398 and ibuprofen or with COX-2 RNAi, suggesting the benefit to fingerprinting preclinical drug concentrations to improve their relevance to the clinical setting.

PX-478, an inhibitor of hypoxia-inducible factor-1alpha, enhances radiosensitivity of prostate carcinoma cells

Overexpression of hypoxia-inducible factor-1alpha (HIF-1alpha) in human tumors is associated with poor prognosis and poor outcome to radiation therapy. Inhibition of HIF-1alpha is considered as a promising approach in cancer therapy. The purpose of this study was to test the efficacy of a novel HIF-1alpha inhibitor PX-478 as a radiosensitizer under normoxic and hypoxic conditions in vitro. PC3 and DU 145 prostate carcinoma cells were treated with PX-478 for 20 hr, and HIF-1alpha protein level and clonogenic cell survival were determined under normoxia and hypoxia. Effects of PX-478 on cell cycle distribution and phosphorylation of H2AX histone were evaluated. PX-478 decreased HIF-1alpha protein in PC3 and DU 145 cells. PX-478 produced cytotoxicity in both cell lines with enhanced toxicity under hypoxia for DU-145. PX-478 (20 mumol/L) enhanced the radiosensitivity of PC3 cells irradiated under normoxic and hypoxic condition with enhancement factor (EF) 1.4 and 1.56, respectively. The drug was less effective in inhibiting HIF-1alpha and enhancing radiosensitivity of DU 145 cells compared to PC3 cells with EF 1.13 (normoxia) and 1.25 (hypoxia) at 50 mumol/L concentration. PX-478 induced S/G2M arrest in PC3 but not in DU 145 cells. Treatment of PC3 and DU 145 cells with the drug resulted in phosphorylation of H2AX histone and prolongation of gammaH2AX expression in the irradiated cells. PX-478 is now undergoing Phase I clinical trials as an oral agent. Although the precise mechanism of enhancement of radiosensitivity remains to be identified, this study suggests a potential role for PX-478 as a clinical radiation enhancer.

Radiation sensitivity of human carcinoma cells transfected with small interfering RNA targeted against cyclooxygenase-2

PURPOSE

Cyclooxygenase-2 (COX-2) is considered a potential target for cancer therapy, because COX-2 levels are elevated in the majority of human tumors compared with the normal tissues. COX-2 inhibitors inhibit tumor growth and enhance radiation response in vitro as well as in vivo. However, the precise role of COX-2 in radiation response is not clear. The purpose of the present study was to investigate the in vitro radiosensitivity of tumor cells as a function of COX-2 expression.

EXPERIMENTAL DESIGN AND RESULTS

PC3 and HeLa cells express COX-2 protein constitutively. We silenced the COX-2 gene in these cells using small interfering RNA (siRNA). Transfection of PC3 cells with 100 nmol/L siRNA targeted against COX-2 resulted in reduction of COX-2 protein by 75% and inhibition of arachidonic acid-induced prostaglandin E2 synthesis by approximately 50% compared with the vehicle control. In HeLa cells, 100 nmol/L COX-2 siRNA inhibited COX-2 protein expression by 80%. Cell cycle analysis showed that transfection with COX-2 siRNA did not alter the cell cycle distribution. Radiosensitivity was determined by clonogenic cell survival assay. There was no significant difference in the radiosensitivity of cells in which COX-2 was silenced compared with the cells transfected vehicle or with negative control siRNAs (enhancement ratio = 1.1).

CONCLUSIONS

These data indicate that the in vitro radiosensitivity of tumor cells is minimally dependent on the cellular COX-2 status. Given that a number of potential mechanisms are attributed to COX-2 inhibitors for radiosensitization, specific intervention of COX-2 by RNA interference could help elucidate the precise role of COX-2 in cancer therapy and to optimize strategies for COX-2 inhibition.

Effect of Radiation and Ibuprofen on Normoxic Renal Carcinoma Cells Overexpressing Hypoxia-Inducible Factors by Loss of von Hippel–Lindau Tumor Suppressor Gene Function

PURPOSE

Tumor hypoxia is a major limiting factor for radiation therapy. Hypoxia-inducible factors (HIFs) are overexpressed in several human cancers and are considered prognostic markers and potential targets for cancer therapy. The purpose of the present study was to investigate the impact of HIFs on radiosensitivity.

EXPERIMENTAL DESIGN

Renal clear cell carcinoma (RCC) cell lines overexpressing HIFs under normoxic conditions because of inactivation of von Hippel-Lindau tumor suppressor gene function (VHL-ve) and their matched pairs in which overexpression of HIFs was abolished by expression of functional VHL (VHL+ve) were irradiated. Radiosensitivity was determined by clonogenic assay. HIF and VHL protein levels were evaluated by Western blot analysis. RCC cells were also treated with ibuprofen, a radiosensitizer and HIF inhibitor in prostate cancer cells. The effect of ibuprofen on radiosensitization and HIF and VHL proteins was compared in RCC matched-pair cell lines.

RESULTS

The data showed only small differences in the radiosensitivity between the cells overexpressing HIFs and cells with basal HIF levels. The dose-modifying factors for C2, 786-0, and A498 RCC cells were 1.14, 1.14 and 1.15, respectively. Radiation did not alter HIF or VHL protein levels. Ibuprofen inhibited HIFs in VHL+ve cells expressing basal levels of HIFs. In VHL-ve cells overexpressing HIFs, the inhibition was very modest. Ibuprofen radiosensitized C2 RCC cells to the same extent irrespective of their HIF status.

CONCLUSIONS

Overexpression of HIFs in RCC cells harboring VHL mutations has only a modest effect on the radiosensitivity. Radiosensitization by ibuprofen appears to be independent of HIF status.

Ibuprofen-mediated reduction of hypoxia-inducible factors HIF-1alpha and HIF-2alpha in prostate cancer cells

PURPOSE

Hypoxia-inducible factors HIF-1alpha and HIF-2alpha are considered to be potential targets for antineoplastic therapy because they regulate the expression of genes that contribute to tumor cell survival, aggressiveness, and angiogenesis. Nonsteroidal anti-inflammatory drugs (NSAIDs) have gained considerable interest as anticancer agents because of their cytotoxic and antiangiogenic properties. The aim of this study was to investigate whether NSAIDs inhibit HIFs and HIF-regulated gene expression in prostate cancer cells.

EXPERIMENTAL DESIGN

PC3 and DU-145 cells were treated with ibuprofen (Ibu) and other NSAIDs under normoxic and hypoxic (95% N(2), 5% CO(2); <10 ppm O(2)) conditions. The effect of NSAIDs on HIF proteins was analyzed by Western blot analysis. HIF-regulated proteins, vascular endothelial growth factor (VEGF) and glucose transporter-1 (Glut-1), were analyzed by ELISA and Western blot analysis, respectively.

RESULTS

Exposure of PC3 and DU-145 cells to hypoxic condition up-regulated HIF-1alpha and HIF-2alpha proteins. Treatment with Ibu under normoxic and hypoxic conditions reduced the level of HIF-1alpha and HIF-2alpha. Ibu-mediated down-regulation of HIFs was associated with down-regulation of HIF-regulated proteins VEGF and Glut-1 in cells exposed to hypoxia. Other nonspecific NSAIDs, diclofenac and ketorolac, also inhibited HIF-1alpha and HIF-2alpha. The reduction in HIFs was observed in PC3 cells that expressed cyclooxygenase-2 (COX-2) protein as well as in DU-145 cells, which did not express COX-2 protein. COX-2-specific inhibitor NS-398 did not inhibit HIF-1alpha or VEGF and GLUT-1.

CONCLUSIONS

These data indicate that one of the effects of NSAIDs is to reduce HIF protein levels. The inhibition of HIFs by NSAIDs was COX-2 independent.

Constitutive activation of IkappaB kinase alpha and NF-kappaB in prostate cancer cells is inhibited by ibuprofen

Apoptotic pathways controlled by the Rel/NF-kappaB family of transcription factors may regulate the response of cells to DNA damage. Here, we have examined the NF-kappaB status of several prostate tumor cell lines. In the androgen-independent prostate tumor cells PC-3 and DU-145, the DNA-binding activity of NF-kappaB was constitutively activated and IkappaB-alpha levels were decreased. In contrast, the androgen-sensitive prostate tumor cell line LNCaP had low levels of NF-kappaB which were upregulated following exposure to cytokines or DNA damage. The activity of the IkappaB-alpha kinase, IKKalpha, which mediates NF-kappaB activation, was also measured. In PC-3 cells, IKKalpha activity was constitutively active, whereas LNCaP cells had minimal IKKalpha activity that was activated by cytokines. The anti-inflammatory agent ibuprofen inhibited the constitutive activation of NF-kappaB and IKKalpha in PC-3 and DU-145 cells, and blocked stimulated activation of NF-kappaB in LNCaP cells. However, ibuprofen did not directly inhibit IkappaB-alpha kinase. The results demonstrate that NF-kappaB is constitutively activated in the hormone-insensitive prostate tumor cell lines PC-3 and DU-145, but not in the hormone responsive LNCaP cell line. The constitutive activation of NF-kappaB in prostate tumor cells may increase expression of anti-apoptotic proteins, thereby decreasing the effectiveness of anti-tumor therapy and contributing to the development of the malignant phenotype.

Combined antitumor effect of radiation and ibuprofen in human prostate carcinoma cells

Recent clinical observations indicate that ibuprofen may alleviate the radiation-induced dysuria that almost invariably occurs during radiation therapy for prostate cancer. Because the use of ibuprofen could consequently become common during radiation therapy for prostate cancer, we have been interested in the potential interactions between ibuprofen and ionizing radiation on prostate tumor cells. The effects of gamma-irradiation and/or ibuprofen on PC3 and DU-145 human prostate carcinoma cells were evaluated in vitro using three model systems. Clonogenic survival was determined by plating cells 24 h after treatment of nearly confluent monolayers. Analysis of cell growth, cell detachment, and apoptotic cell death was carried out over a period of up to 9 days after treatment of PC3 and DU-145 monolayers. The effect of ibuprofen and/or radiation was also probed by observing the inhibition of growth of established PC3 and DU-145 colonies that were treated on the 14th day of colony growth. Ibuprofen enhanced the radiation response of prostate cancer cells in all three in vitro models. Both the cytotoxic and radiosensitizing effects of ibuprofen seem to require concentrations that are higher than those reported to inhibit prostaglandin synthesis, suggesting that other molecular mechanisms may be responsible for ibuprofen cytotoxicity.

Apoptosis and clonogenic cell death in PC3 human prostate cancer cells after treatment with gamma radiation and suramin

Suramin is a novel cytostatic/cytotoxic agent that is currently undergoing clinical trials in the treatment of hormone- and chemo-refractory tumors. Its unusual mechanism of action and its activity against prostate cancer raise the possibility that it could be particularly suitable for combined-modality treatment of prostate cancer. PC3 human prostate cancer cells were used as an in vitro model to test the possible interaction between suramin and ionizing radiation. Treatment with gamma radiation resulted in detachment of PC3 cells from the monolayer, and the detached cells exhibited internucleosomal DNA fragmentation characteristic of apoptosis. Low concentration of suramin (50-100 micrograms/ml, 35-70 microM) increased spontaneous as well as radiation-enhanced apoptosis. However, suramin inhibited spontaneous and radiation-enhanced apoptosis at 300 micrograms/ml (210 microM), a concentration that is more commonly used in the clinic. At this concentration suramin inhibited DNA fragmentation induced by chemotherapeutic drugs as well. The effect of suramin on inhibition of DNA fragmentation was reversible if the suramin was removed 24 h after irradiation. Despite inhibition of radiation-induced apoptosis by 300 micrograms/ml suramin (from 5% to 2.9% at 48 h), clonogenic cell death was enhanced by the combination of suramin and radiation. The effects of radiation and suramin on clonogenic cell survival appeared to be additive by isobologram analysis at clinically relevant radiation doses. Continuous exposure to a lower concentration of suramin (100 micrograms/ml) during the clonogenic assay period was as effective in decreasing clonogenic survival as 48 h exposure to 300 micrograms/ml suramin in decreasing clonogenic survival. Our data indicate that, when used in combination with radiation, suramin may be effective at concentrations that are lower than those required for efficacy as a single agent.

Radiosensitization of human tumor cells by the phosphatidylinositol3-kinase inhibitors wortmannin and LY294002 correlates with inhibition of DNA-dependent protein kinase and prolonged G2-M delay

Members of the phosphatidylinositol (PI) 3-kinase gene family, including the ataxia telangiectasia gene and the DNA-dependent protein kinase (DNA-PK), are involved in regulating cellular radiosensitivity. We have investigated two structurally unrelated PI 3-kinase inhibitors, wortmannin and LY294002, to determine whether they inhibit DNA-PK and increase cellular radiosensitivity. The PI 3-kinase inhibitors wortmannin and LY294002 were effective radiosensitizers of human tumor cells, with sensitizer enhancement ratios (at 10% survival) of 2.8 and 1.9, respectively, in SW480 cells. Wortmannin and LY294002 inhibited the kinase activity of purified DNA-PK and inactivated cellular DNA-PK kinase activity. Inhibition of cellular DNA-PK activity occurred at the same concentrations of wortmannin that caused radiosensitization, and this correlation was found in a range of tumor cell lines. However, cells deficient in either DNA-PK (scid cells) or the ataxia telangiectasia protein were also partly sensitized to radiation by wortmannin, indicating the involvement of more than one protein kinase in the mechanism of action of wortmannin. Wortmannin also affected the G2-M checkpoint. SW480 cells had a reversible G2-M delay of 20 h following irradiation. However, wortmannin-treated SW480 cells had a prolonged G2-M delay; more than 75% of cells were arrested in G2 at 50 h postirradiation. This suggests the accumulation of significant unrepaired DNA damage following inhibition of PI 3-kinase family members. Therefore, PI 3-kinase inhibitors may represent a new class of radiosensitizers that inhibit the repair of DNA damage.

Effect of BSO and etanidazole on neurofilament degradation in neonatal rat spinal cord cultures

Peripheral neuropathy is the major dose-limiting toxicity of the hypoxic cell sensitiser, etanidazole. Previous work from this laboratory using culture neuronal cell lines suggested that nitroimidazole-induced degradation of neurofilament proteins might be the critical biological event mediating this neurotoxicity. The purpose of the present study was to develop the neurofilament degradation assay in an organotypic spinal cord culture system with the goal of developing strategies for optimising sensitiser efficacy as well as ameliorating nitroimidazole-induced neurotoxicity. Spinal cord cultures were treated with etanidazole and neurofilament protein degradation was analysed by immunoblot analysis. Spinal cord cultures exposed to etanidazole exhibited a dose-dependent loss of parent neurofilament proteins, with concomitant appearance of low molecular weight degradation products. The potential neurotoxic effect of L, S-buthionine sulphoximine (BSO), a compound that enhances the radiosensitising effectiveness of 2-nitroimidazoles, was also screened in this assay system. BSO alone, at concentrations up to 100 microM, did not promote neurofilament degradation. BSO (20 microM) enhanced the effect of etanidazole on neurofilament degradation by a dose-modifying factor of 1.6 +/- 0.5. Since 20 microM BSO is expected to enhance etanidazole radiosensitisation of hypoxic cells by a larger factor, this suggests that a therapeutic gain could be achieved using BSO in combination with etanidazole in radiation therapy.

Modulation of radiation-induced apoptosis and G2/M block in murine T-lymphoma cells

Radiation-induced apoptosis in lymphocyte-derived cell lines is characterized by endonucleolytic cleavage of cellular DNA within hours after radiation exposure. We have studied this phenomenon qualitatively (DNA gel electrophoresis) and quantitatively (diphenylamine reagent assay) in murine EL4 T-lymphoma cells exposed to 137Cs gamma irradiation. Fragmentation was discernible within 18-24 h after exposure. It increased with time and dose and reached a plateau after 8 Gy of gamma radiation. We studied the effect of several pharmacological agents on the radiation-induced G2/M block and DNA fragmentation. The agents which reduced the radiation-induced G2/M-phase arrest (caffeine, theobromine, theophylline and 2-aminopurine) enhanced the degree of DNA fragmentation at 24 h. In contrast, the agents which sustained the radiation-induced G2/M-phase arrest (TPA, DBcAMP, IBMX and 3-aminobenzamide) inhibited the DNA fragmentation at 24 h. These studies on EL4 lymphoma cells are consistent with the hypothesis that cells with radiation-induced genetic damage are eliminated by apoptosis subsequent to a G2/M block. Furthermore, it may be possible to modulate the process of radiation-induced apoptosis in lymphoma cells with pharmacological agents that modify the radiation-induced G2/M block, and to use this effect in the treatment of patients with malignant disease.

Novel concepts in modification of radiation sensitivity

PURPOSE

To determine whether biological effects of radiation, such as apoptosis, that differ from classical clonogenic cell killing, can be modified with agents that would not be expected to modify classical clonogenic cell killing. This would expand the range of potential modifiers of radiation therapy.

METHODS AND MATERIALS

EL4 murine lymphoma cell apoptosis was determined by electrophoretic analysis of deoxyribonucleic acid (DNA) fragmentation. DNA was extracted 24 h after irradiation or addition of inducing agents. Modifiers of radiation-induced apoptosis were added immediately after irradiation. The effects of radiation on wounded endothelial monolayers were studied by scraping a line across the monolayer 30 min after irradiation. Cell detachment was used as an endpoint to determine the protective effect of prolonged exposure to retinol prior to irradiation.

RESULTS

EL4 cell apoptosis can be induced by tert-butyl hydroperoxide or the glutathione oxidant SR-4077. Radiation-induced EL4 cell apoptosis can be inhibited with 3-aminobenzamide, an agent that sensitizes cells to classical clonogenic cell killing. Radiation-induced endothelial cell detachment from confluent monolayers can be modified by pretreatment with retinol.

CONCLUSION

These results raise the possibility that radiation could induce apoptosis by an oxidative stress mechanism that is different from that involved in classical clonogenic cell killing. These and other recent findings encourage the notion that differential modification of classical clonogenic cell killing and other important endpoints of radiation action may be possible.

Modification of the aerobic cytotoxicity of etanidazole

PURPOSE

To determine the feasibility of modifying the aerobic cytotoxicity of etanidazole without interfering with the tumoricidal action of radiation plus etanidazole.

METHODS AND MATERIALS

The aerobic cytotoxicity of etanidazole was studied using two different models: (1) Induction of apoptosis in EL4 cells: apoptotic DNA fragmentation was analyzed by agarose gel electrophoresis following 24 h treatment with etanidazole alone or in combination with various modifiers. (2) Spinal cord neuronal loss in organotypic roller tube cultures: Survival of acetylcholinesterase positive ventral horn neurons was analyzed morphometrically following 72 h treatment with etanidazole alone or in combination with vitamin E succinate.

RESULTS

Etanidazole (10 mM) induced apoptosis in EL4 cells. This effect was suppressed by 24 h treatment with TPA, IBMX, the free radical scavenger TEMPOL or vitamin E succinate. Vitamin E succinate also protected spinal cord cultures from etanidazole-induced neuronal loss.

CONCLUSION

These results suggest that it might be possible to modify the neurotoxicity of etanidazole with agents that would not be expected to interfere with the tumoricidal action of radiation plus etanidazole.

Radiation-induced apoptosis in F9 teratocarcinoma cells

We have found that F9 murine teratocarcinoma cells undergo morphological changes and internucleosomal DNA fragmentation characteristic of apoptosis after exposure to ionizing radiation. We studied the time course, radiation dose-response, and the effects of protein and RNA synthesis inhibitors on this process. The response is dose dependent in the range 2-12 Gy. Internucleosomal DNA fragmentation can be detected as early as 6 h postirradiation and is maximal by 48 h. Cycloheximide, a protein synthesis inhibitor, and 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole, an RNA synthesis inhibitor, both induced internucleosomal DNA fragmentation in the unirradiated cells and enhanced radiation-induced DNA fragmentation. F9 cells can be induced to differentiate into cells resembling endoderm with retinoic acid. After irradiation, differentiated F9 cells exhibit less DNA fragmentation than stem cells. This indicates that ionizing radiation can induce apoptosis in non-lymphoid tumours. We suggest that embryonic tumour cells may be particularly susceptible to agents that induce apoptosis.

Modifiers of radiation-induced apoptosis

EL4 murine lymphoma cells and F9 murine teratocarcinoma cells undergo apoptosis-like cell death after exposure to ionizing radiation. Apoptosis differs in several ways from classical clonogenic cell killing by radiation. We have tested several modifiers and radiomimetic agents in an effort to determine if the mechanism of induction of apoptosis by radiation differs from the mechanism of classical clonogenic cell killing by radiation, and consequently that these two end points of radiation action might be differentially modifiable. We found that internucleosomal DNA fragmentation, characteristics of apoptosis, can result from treatment of EL4 and F9 cells with agents that have diverse modes of action: tert-butyl hydroperoxide, diazenedicarboxylic acid bis(N,N-piperidide), and etoposide. Hydrogen peroxide did not induce internucleosomal DNA fragmentation at concentrations expected to be produced by the doses of ionizing radiation that we used. Radiation-induced DNA fragmentation could be inhibited by 3-aminobenzamide, dibutryl cyclic AMP, or 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl, although in this respect there appear to be marked differences between the cell lines.

G2M arrest and apoptosis in murine T lymphoma cells following exposure to 212Bi alpha particle irradiation

Asynchronous exponentially growing EL4 murine T lymphoma cells were exposed either to high LET alpha-radiation from 212Bi-DTPA or to gamma-radiation from a 137Cs source. Radiation-induced cell cycle perturbation was studied by flow cytometry. Alpha irradiation, like gamma, transiently arrested cells in the G2M phase in a dose-dependent manner. The maximum percentages of cells accumulated in G2M 18 h after alpha- and gamma-irradiation were comparable, though the dose-response relationships differed. The "RBE" value for G2M block for alpha- versus gamma-radiation was approx. 4. Electron microscopic studies of the cell samples where a large proportion of cells were arrested in G2M showed subcellular changes in nuclear membrane and the presence of morphologically apoptotic cells. Biochemical analysis of DNA from irradiated cells by agarose gel electrophoresis revealed more extensive DNA fragmentation for alpha- vs gamma-irradiation, even at relatively low total doses. We conclude that the high LET radiation is more efficient in inducing G2M block and apoptosis in EL4 lymphoma cells. The overall radiosensitivity of some high and low grade malignant lymphoma cells to radiation may correlate with these processes. The clinical implications of 212Bi-induced G2M delay may be particularly important for biologically targeted high LET radiopharmaceutical therapy.

Induction of mutations by bismuth-212 alpha particles at two genetic loci in human B-lymphoblasts

The human lymphoblast cell line TK6 was exposed to the alpha-particle-emitting radon daughter 212Bi by adding DTPA-chelated 212Bi directly to the cell suspension. Cytotoxicity and mutagenicity at two genetic loci were measured, and the molecular nature of mutant clones was studied by Southern blot analysis. Induced mutant fractions were 2.5 x 10(-5)/Gy at the hprt locus and 3.75 x 10(-5)/Gy at the tk locus. Molecular analysis of HPRT- mutant DNAs showed a high frequency (69%) of clones with partial or full deletions of the hprt gene among radiation-induced mutants compared with spontaneous mutants (31%). Chi-squared analyses of mutational spectra show a significant difference (P < or = 0.005) between spontaneous mutants and alpha-particle-induced mutants. Comparison with published studies of accelerator-produced heavy-ion exposures of TK6 cells indicates that the induction of mutations at the hprt locus, and perhaps a subset of mutations at the tk locus, is a simple linear function of particle fluence regardless of the ion species or its LET.

Brain cationic ATPase activities in epileptic (El) mice

Total, Mg2+-, Na+,K+-, and Ca2+-ATPase activities were studied in fresh brain membrane preparations from adult epileptic (El) mice and nonepileptic C57BL/6J (B6) mice. The El mice have an inherited type of temporal lobe epilepsy. No significant differences were observed between the El and B6 mice for any of the ATPase activities in the hippocampus, brain stem, or cerebellum. These findings indicate that seizure susceptibility in El mice is not associated with differences in the activities of these cationic ATPases and that seizure susceptibility in El mice and audiogenic DBA/2 mice may involve different biochemical mechanisms.

Inhibition of protein kinase C by antineoplastic agents: implications for drug resistance

One mechanism by which drugs alter the function of enzymes is through chronic inhibition. To determine whether commonly used cancer chemotherapeutic agents could alter protein kinase C (PKC) and thereby modify the calcium-messenger system, we studied the effect of anthracyclines and vinca alkaloids on the activity of PKC. Doxorubicin, daunomycin, vincristine and vinblastine inhibited the activity of PKC by 50% at concentrations of 150, 120, 350 and 140 microM respectively. Furthermore, we demonstrated the potential for this interaction to occur in intact cells, since doxorubicin blocked the binding of the phorbol ester, PDBu, to its receptor, PKC. The mode of inhibition of PKC was due, at least in part, to interference with the activation of the enzyme by phosphatidylserine. The activity of PKC was increased 15 fold in a highly resistant human breast cancer line, but this increase in enzymic activity was not seen in all lines tested. These studies demonstrate that anthracyclines and vinca alkaloids inhibit PKC, and suggest that chronic antagonism could lead to changes in its activity and function.

Calcium ATPase activities in synaptic plasma membranes of seizure-prone mice

Audiogenic seizure (AGS)-susceptible DBA/2 (D2) mice have a significant reduction in brain Ca2+-ATPase activity compared to AGS-resistant C57BL/6 (B6) mice. This reduction is inherited together with AGS susceptibility in B6 X D2 recombinant inbred strains. The Ca2+-ATPase reduction occurs in microsomes and synaptosomes, but not in mitochondria. This enzyme activity is measured at a high Ca2+ concentration (2 mM) with no added Mg2+ or EGTA. We further studied this Ca2+-ATPase activity and a Mg2+-dependent (Ca2+ + Mg2+)-ATPase activity in synaptic plasma membranes (SPM) from the B6 and D2 strains. Using EGTA or CDTA to adjust free Ca2+ concentrations, we measured Ca2+-ATPase activities at Ca2+ concentrations from 0.8 microM to 436 microM. The Ca2+-ATPase activity is consistently lower in the D2 than in the B6 SPM over all Ca2+ concentrations. The basal Mg2+-ATPase activity measured at 2 mM MgCl2, is also lower in SPM of D2 than B6 mice. Calcium stimulates the basal Mg2+-ATPase activity to the same extent in the SPM of the B6 and the D2 mice. Maximum stimulation in both strains occurs at 150 microM added CaCl2 (buffered with 100 microM EGTA). Higher Ca2+ concentrations inhibit this ATPase activity similarly in both strains. The EGTA-EDTA washing of SPM significantly reduces by 50% of the (Ca2+ + Mg2+)-ATPase activities of both strains, whereas calmodulin treatment restored these activities. Neither of these treatments, however, has any noticeable effects on the Ca2+-ATPase activities of the strains.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective antimitochondrial agents inhibit calmodulin

Certain cationic-lipophilic compounds are known to selectively accumulate in tumor mitochondria and inhibit energy production. Since these substances bear a structural resemblance to known inhibitors of calmodulin, we studied whether rhodamine-123 or a bis-4-aminoquinaldinium could antagonize the action of calmodulin. Rhodamine-123 (IC50 = 58 microM) and dequalinium (IC50 = 1 microM) inhibited the activity of a calmodulin-stimulated cyclic nucleotide phosphodiesterase. Propylinium, a compound similar to dequalinium except for having a 3 rather than 10 carbon alkyl bridge connecting two non-substituted quinoline rings, had no inhibitory effect. Kinetic analysis showed that dequalinium competitively inhibited calmodulin's activation of phosphodiesterase. We also studied the antiproliferative effects of the compounds on the C6 astrocytoma cell line. Rhodamine-123 and dequalinium inhibited the proliferation of this cell line while propylinium had no effect. These studies demonstrate that rhodamine-123 and dequalinium are calmodulin-antagonists and inhibit cellular proliferation.

Genetic association between Ca2+-ATPase activity and audiogenic seizures in mice

Ca2+-ATPase activity was studied in fresh brain stem homogenates of the audiogenic seizure (AGS)-resistant C57BL/6 and AGS-susceptible DBA/2 inbred strains and in 21 B6 X D2 recombinant inbred strains. A highly significant negative correlation was found between Ca2+-ATPase activity and AGS susceptibility among these strains. In general, strains with low Ca2+-ATPase activities were more AGS-susceptible than strains with high activities. Further, Ca2+-ATPase activity appears to be influenced by a major gene associated with the Ah locus. This gene is designated Caa for Ca2+-ATPase activity and is different from Ias, which is closely linked to the Ah locus. Ias influences AGS spread by a yet unknown biochemical mechanism, whereas Caa may influence AGS susceptibility by regulating Ca2+-ATPase activity in brain tissue.

Genetic study of cationic ATPase activities and audiogenic seizure susceptibility in recombinant inbred and congenic strains of mice

Total, Mg2+, and Na+, K+ ATPase activities were studied in fresh brain homogenates of the audiogenic seizure (AGS)-resistant C57BL/6J (B6) and AGS-susceptible DBA/2J (D2) inbred strains and in 13 B6 X D2 (BXD) recombinant inbred (RI) strains. These activities were also studied in the D2.B6-Iasb congenic mice, that are similar genetically to D2 mice, except for the Iasb gene which inhibits the spread of AGS activity. The total and Mg2+ ATPase activities of the brainstem were significantly lower in the D2 than in the B6 mice at 21 days of age. No differences were found between these strains for Na+,K+ ATPase activity. The total, Mg2+, and Na+,K+ ATPase activities in the B6 brainstem did not change noticeably from 21 to 80 days of age. In the D2 brainstem, however, the Mg2+ activity increased with age, and the Na+,K+ ATPase activity decreased from 30 to 80 days of age. No genetic associations could be found between AGS susceptibility and total or Mg2+ ATPase activities in the D2.B6-Iasb mice or among the 13 BXD RI strains. Hence, differences in genetic background, rather than differences in AGS susceptibility, can account for the lower ATPase activities in 21-day-old D2 mice. Further, the Mg2+ and Na+,K+ ATPase activities appear to be regulated by more than one gene. This study emphasizes the utility of RI and congenic strains for testing the biochemical basis of AGS susceptibility in mice.