The phosphatidylinositol 3-kinase inhibitor wortmannin sensitizes quiescent but not proliferating MG-63 human osteosarcoma cells to radiation

Multimodal Mass Spectrometry Imaging of an Osteosarcoma Multicellular Tumour Spheroid Model to Investigate Drug-Induced Response

A multimodal mass spectrometry imaging (MSI) approach was used to investigate the chemotherapy drug-induced response of a Multicellular Tumour Spheroid (MCTS) 3D cell culture model of osteosarcoma (OS). The work addresses the critical demand for enhanced translatable early drug discovery approaches by demonstrating a robust spatially resolved molecular distribution analysis in tumour models following chemotherapeutic intervention. Advanced high-resolution techniques were employed, including desorption electrospray ionisation (DESI) mass spectrometry imaging (MSI), to assess the interplay between metabolic and cellular pathways in response to chemotherapeutic intervention. Endogenous metabolite distributions of the human OS tumour models were complemented with subcellularly resolved protein localisation by the detection of metal-tagged antibodies using Imaging Mass Cytometry (IMC). The first application of matrix-assisted laser desorption ionization-immunohistochemistry (MALDI-IHC) of 3D cell culture models is reported here. Protein localisation and expression following an acute dosage of the chemotherapy drug doxorubicin demonstrated novel indications for mechanisms of region-specific tumour survival and cell-cycle-specific drug-induced responses. Previously unknown doxorubicin-induced metabolite upregulation was revealed by DESI-MSI of MCTSs, which may be used to inform mechanisms of chemotherapeutic resistance. The demonstration of specific tumour survival mechanisms that are characteristic of those reported for in vivo tumours has underscored the increasing value of this approach as a tool to investigate drug resistance.

Radiotherapy in bone sarcoma: the quest for better treatment option

Bone sarcomas are rare tumors representing 0.2% of all cancers. While osteosarcoma and Ewing sarcoma mainly affect children and young adults, chondrosarcoma and chordoma have a preferential incidence in people over the age of 40. Despite this range in populations affected, all bone sarcoma patients require complex transdisciplinary management and share some similarities. The cornerstone of all bone sarcoma treatment is monobloc resection of the tumor with adequate margins in healthy surrounding tissues. Adjuvant chemo- and/or radiotherapy are often included depending on the location of the tumor, quality of resection or presence of metastases. High dose radiotherapy is largely applied to allow better local control in case of incomplete primary tumor resection or for unresectable tumors. With the development of advanced techniques such as proton, carbon ion therapy, radiotherapy is gaining popularity for the treatment of bone sarcomas, enabling the delivery of higher doses of radiation, while sparing surrounding healthy tissues. Nevertheless, bone sarcomas are radioresistant tumors, and some mechanisms involved in this radioresistance have been reported. Hypoxia for instance, can potentially be targeted to improve tumor response to radiotherapy and decrease radiation-induced cellular toxicity. In this review, the benefits and drawbacks of radiotherapy in bone sarcoma will be addressed. Finally, new strategies combining a radiosensitizing agent and radiotherapy and their applicability in bone sarcoma will be presented.

DNA damage response signaling pathways as important targets for combination therapy and chemotherapy sensitization in osteosarcoma

Osteosarcoma (OS) is the most common bone malignancy that occurs most often in young adults, and adolescents with a survival rate of 20% in its advanced stages. Nowadays, increasing the effectiveness of common treatments used in OS has become one of the main problems for clinicians due to cancer cells becoming resistant to chemotherapy. One of the most important mechanisms of resistance to chemotherapy is through increasing the ability of DNA repair because most chemotherapy drugs damage the DNA of cancer cells. DNA damage response (DDR) is a signal transduction pathway involved in preserving the genome stability upon exposure to endogenous and exogenous DNA-damaging factors such as chemotherapy agents. There is evidence that the suppression of DDR may reduce chemoresistance and increase the effectiveness of chemotherapy in OS. In this review, we aim to summarize these studies to better understand the role of DDR in OS chemoresistance in pursuit of overcoming the obstacles to the success of chemotherapy.

Effects of p53 Status of Tumor Cells and Combined Treatment With Mild Hyperthermia, Wortmannin or Caffeine on Recovery From Radiation-Induced Damage

Background

The aim of the study was to examine the dependency of p53 status and the usefulness of mild hyperthermia (MHT) as an inhibitor of recovery from radiation-induced damage, referring to the response of quiescent (Q) tumor cell population.

Methods

Human head and neck squamous cell carcinoma cells transfected with mutant TP53 (SAS/mp53) or with neo vector (SAS/neo) were injected subcutaneously into left hind legs of nude mice. Tumor-bearing mice received 5-bromo-2’-deoxyuridine (BrdU) continuously to label all intratumor proliferating (P) cells. They received high dose-rate γ-ray irradiation (HDR) immediately followed by localized MHT (40 °C for 2 h), or caffeine or wortmannin administration, or low dose-rate γ-ray irradiation simultaneously with localized MHT or caffeine or wortmannin administration. Nine hours after the start of irradiation, the tumor cells were isolated and incubated with a cytokinesis blocker, and the micronucleus (MN) frequency in cells without BrdU labeling (= Q cells) was determined using immunofluorescence staining for BrdU.

Results

SAS/neo tumor cells, especially intratumor Q cell populations, showed a marked reduction in sensitivity due to the recovery from radiation-induced damage, compared with the total or Q tumor cells within SAS/mp53 tumors that showed little recovery capacity. The recovery from radiation-induced damage was thought to be a p53-dependent event. In both total and Q tumor cells within SAS/neo tumors, especially the latter, MHT efficiently suppressed the reduction in sensitivity caused by leaving an interval between HDR irradiation and the assay and decreasing the irradiation dose-rate, as well as the combination with wortmannin administration.

Conclusions

From the viewpoint of solid tumor control as a whole, including intratumor Q-cell control, non-toxic MHT is useful for suppressing the recovery from radiation-induced damage, as well as wortmannin treatment combined with γ-ray irradiation.

Wortmannin efficiently suppresses the recovery from radiation-induced damage in pimonidazole-unlabeled quiescent tumor cell population

Labeling of proliferating (P) cells in mice bearing EL4 tumors was achieved by continuous administration of 5-bromo-2'-deoxyuridine (BrdU). Tumors were irradiated with γ-rays at 1 h after pimonidazole administration followed by caffeine or wortmannin treatment. Twenty-four hours later, assessment of the responses of quiescent (Q) and total (= P + Q) cell populations were based on the frequencies of micronucleation and apoptosis using immunofluorescence staining for BrdU. The response of the pimonidazole-unlabeled tumor cell fractions was assessed by means of apoptosis frequency using immunofluorescence staining for pimonidazole. The pimonidazole-unlabeled cell fraction showed significantly enhanced radio-sensitivity compared with the whole cell fraction more remarkably in Q cells than total cells. However, a significantly greater decrease in radio-sensitivity in the pimonidazole-unlabeled than the whole cell fraction, evaluated using an assay performed 24 hours after irradiation, was more clearly observed in Q cells than total cells. In both the pimonidazole-unlabeled and the whole cell fractions, wortmannin efficiently suppressed the reduction in sensitivity due to delayed assay. Wortmannin combined with γ-ray irradiation is useful for suppressing the recovery from radiation-induced damage especially in the pimonidazole-unlabeled cell fraction within the total and Q tumor cell populations.

Effectiveness of combined treatment using X-rays and a phosphoinositide 3-kinase inhibitor, ZSTK474, on proliferation of HeLa cells in vitro and in vivo

ZSTK474 is a novel orally applicable phosphoinositide 3-kinase-specific inhibitor that strongly inhibits cancer cell proliferation. To further explore the antitumor effect of ZSTK474 for future clinical usage, we studied its combined effects with radiation. The proliferation of HeLa cells was inhibited by treatment with X-rays alone or ZSTK474 alone. Combination treatment using X-rays then ZSTK474 given orally for 8 days, starting 24 h post-irradiation, significantly enhanced cell growth inhibition. The combined effect was also observed for clonogenic survival with continuous ZSTK474 treatment. Western blot analysis showed enhanced phosphorylation of Akt and GSK-3β by X-irradiation, whereas phosphorylation was inhibited by ZSTK474 treatment alone. Treatment with ZSTK474 after X-irradiation also inhibited phosphorylation, and remarkably inhibited xenograft tumor growth. Combined treatment with X-rays and ZSTK474 has greater therapeutic potential than radiation or drug therapy alone, both in vitro and in vivo.

Involvement of DNA-PK and ATM in radiation- and heat-induced DNA damage recognition and apoptotic cell death

Exposure to ionizing radiation and hyperthermia results in important biological consequences, e.g. cell death, chromosomal aberrations, mutations, and DNA strand breaks. There is good evidence that the nucleus, specifically cellular DNA, is the principal target for radiation-induced cell lethality. DNA double-strand breaks (DSBs) are considered to be the most serious type of DNA damage induced by ionizing radiation. On the other hand, verifiable mechanisms which can lead to heat-induced cell death are damage to the plasma membrane and/or inactivation of heat-labile proteins caused by protein denaturation and subsequent aggregation. Recently, several reports have suggested that DSBs can be induced after hyperthermia because heat-induced phosphorylated histone H2AX (γ-H2AX) foci formation can be observed in several mammalian cell lines. In mammalian cells, DSBs are repaired primarily through two distinct and complementary mechanisms: non-homologous end joining (NHEJ), and homologous recombination (HR) or homology-directed repair (HDR). DNA-dependent protein kinase (DNA-PK) and ataxia-telangiectasia mutated (ATM) are key players in the initiation of DSB repair and phosphorylate and/or activate many substrates, including themselves. These phosphorylated substrates have important roles in the functioning of cell cycle checkpoints and in cell death, as well as in DSB repair. Apoptotic cell death is a crucial cell suicide mechanism during development and in the defense of homeostasis. If DSBs are unrepaired or misrepaired, apoptosis is a very important system which can protect an organism against carcinogenesis. This paper reviews recently obtained results and current topics concerning the role of DNA-PK and ATM in heat- or radiation-induced apoptotic cell death.

Geldanamycin, an inhibitor of Hsp90, sensitizes human tumour cells to radiation

PURPOSE

The effects of the heat shock protein 90 (Hsp90) inhibitor geldanamycin (GA) were examined on the radiosensitivity and signal transduction pathways in human tumour cell lines.

MATERIALS AND METHODS

Two human cell lines, SQ-5 and DLD-1, derived from lung carcinoma and colon adenocarcinoma, respectively, were incubated for 16 h at 37 degrees C in medium containing 0.2 microM GA. The cells were then irradiated with X-rays and incubated with GA for a further 8 h. Radiation sensitivity was determined by clonogenic assays and protein levels were examined by Western blotting.

RESULTS

GA radiosensitized both cell lines, but potentiated X-ray sensitivity more in SQ-5 than in DLD-1 cells. It was found that GA depleted EGFR and ErbB-2 in DLD-1 cells and depleted only ErbB-2 in SQ-5 cells. GA also reduced the expression of Akt and phosphorylated Akt (pAkt) expression in SQ-5 cells. In addition, the ratio (%) of apoptotic cells and poly [ADP-ribose] polymerase cleavage increased in SQ-5 but not in DLD-1 cells after exposure to GA and X-ray irradiation. The findings suggest that GA enhances the radiation sensitivity of human tumour cells by inhibiting the EGFR signal transduction system and the Akt signalling pathway.

CONCLUSION

Targeting Hsp90 with GA provides a promising experimental strategy for radiosensitization of carcinoma.

Positive regulation of DNA double strand break repair activity during differentiation of long life span cells: the example of adipogenesis

Little information is available on the ability of terminally differentiated cells to efficiently repair DNA double strand breaks (DSBs), and one might reasonably speculate that efficient DNA repair of these threatening DNA lesions, is needed in cells of long life span with no or limited regeneration from precursor. Few tissues are available besides neurons that allow the study of DNA DSBs repair activity in very long-lived cells. Adipocytes represent a suitable model since it is generally admitted that there is a very slow turnover of adipocytes in adult. Using both Pulse Field Gel Electrophoresis (PFGE) and the disappearance of the phosphorylated form of the histone variant H2AX, we demonstrated that the ability to repair DSBs is increased during adipocyte differentiation using the murine pre-adipocyte cell line, 3T3F442A. In mammalian cells, DSBs are mainly repaired by the non-homologous end-joining pathway (NHEJ) that relies on the DNA dependent protein kinase (DNA-PK) activity. During the first 24 h following the commitment into adipogenesis, we show an increase in the expression and activity of the catalytic sub-unit of the DNA-PK complex, DNA-PKcs. The increased in DNA DSBs repair activity observed in adipocytes was due to the increase in DNA-PK activity as shown by the use of DNA-PK inhibitor or sub-clones of 3T3F442A deficient in DNA-PKcs using long term RNA interference. Interestingly, the up-regulation of DNA-PK does not regulate the differentiation program itself. Finally, similar positive regulation of DNA-PKcs expression and activity was observed during differentiation of primary culture of pre-adipocytes isolated from human sub-cutaneous adipose tissue.

Our results show that DNA DSBs repair activity is up regulated during the early commitment into adipogenesis due to an up-regulation of DNA-PK expression and activity. In opposition to the general view that DNA DSBs repair is decreased during differentiation, our results demonstrate that an up-regulation of this process might be observed in post-mitotic long-lived cells.

The chemopreventive flavonoid apigenin confers radiosensitizing effect in human tumor cells grown as monolayers and spheroids

Apigenin, a common dietary flavonoid present in many fruits and vegetables, is a nonmutagenic chemopreventive agent. In the present study, we investigated the effect of apigenin on the radiosensitivity of SQ-5 cells, which are derived from a human lung carcinoma. Actively growing cells were incubated for 16 h at 37 degrees C in medium containing 40 muM apigenin. The cells were then irradiated with X-rays and incubated with apigenin for a further 8 h. Radiosensitivity was assessed using a clonogenic assay. Apoptosis and necrosis were assessed using acridine orange/ethidium bromide double staining. Cells incubated with apigenin exhibited significantly greater radiosensitivity and apoptosis levels than cells not incubated with apigenin. Protein levels were measured by Western blotting. Incubation with apigenin increased protein expression of WAF1/p21 and decreased protein expression of Bcl-2. Furthermore, apigenin sensitized SQ-5 spheroids (cell aggregates growing in a three-dimensional structure that simulate the growth and microenvironmental conditions of in vivo tumors) to radiation. Thus, apigenin appears to be a promising radiosensitizing agent for use against human carcinomas.

Heat shock protein 90 inhibitor 17-allylamino-17-demethoxygeldanamycin potentiates the radiation response of tumor cells grown as monolayer cultures and spheroids by inducing apoptosis

Activation of the PI3K-Akt pathway is known to induce tumor radioresistance. In the current study, we examined the ability of 17AAG, which decreases the levels of Hsp90 client proteins including components of the PI3K-Akt pathway, to sensitize radioresistant human squamous cell carcinoma cells to X-irradiation. Human squamous cell carcinoma cell lines (SQ20B, SCC61 and SCC13) were incubated for 16 h at 37 degrees C in medium containing 17AAG. Radiation sensitivity was determined by clonogenic assays, and protein levels were examined by western blotting. Apoptosis was determined in monolayer cells by AO/EB double staining and in spheroids using the TdT-mediated dUTP nick end labeling assay. 17AAG (0.2 microM) enhanced the radiosensitivity more effectively in radioresistant SQ20B and SCC13 cells than in radiosensitive SCC61 cells. However, in all three cell lines, 17AAG increased radiation-induced apoptosis by reducing the expression of EGFR and ErbB-2 and inhibiting the phosphorylation of Akt. Furthermore, 17AAG (1 microM) sensitized SQ20B spheroids to radiation, and inhibition of Akt activation by 17AAG increased radiation-induced apoptosis in spheroids. The findings suggest that 17AAG effectively sensitizes radioresistant cells to radiation by inhibiting the PI3K-Akt pathway. Targeting the PI3K-Akt pathway with 17AAG could be a useful strategy for radiosensitization of carcinomas.

Preferential sensitization of tumor cells to radiation by heat shock protein 90 inhibitor geldanamycin

The purpose of this study was to investigate the radiosensitizing effect of geldanamycin (GA), an inhibitor of heat shock protein 90, on tumour cells and normal cells. We tested the effect of a combination of GA and radiation on cell survival, PI3K/Akt-related proteins and apoptosis induction. GA sensitized tumour cells to radiation in preference to normal cells. In addition, a combination of radiation and GA abolished Akt activities and strongly enhanced the induction of apoptosis in tumour cells which depend on Akt protein activities for cell survival. The present data support the hypothesis that GA sensitizes tumour cells by modulating the balance among mitogenic, antiproliferative and apoptotic pathways. Targeting Hsp90 in tumour cells may lead to the development of new radiosensitizing strategies in radiotherapy.

Role of PTEN and Akt in the regulation of growth and apoptosis in human osteoblastic cells

Cancer cells are characterized by either an increased ability to proliferate or a diminished capacity to undergo programmed cell death. PTEN is instrumental in regulating the balance between growth and death in several cell types and has been described as a tumor suppressor. The chromosome arm on which PTEN is located is deleted in a subset of human osteosarcoma tumors. Therefore, we predicted that the loss of PTEN expression was contributing to increased Akt activation and the subsequent growth and survival of osteosarcoma tumor cells. Immunoblot analyses of several human osteosarcoma cell lines and normal osteoblasts revealed relatively abundant levels of PTEN. Furthermore, stimulation of cell growth or induction of apoptosis in osteosarcoma cells failed to affect PTEN expression or activity. Therefore, routine regulation of osteosarcoma cell growth and survival appears to be independent of changes in PTEN. Subsequently, the activation of a downstream target of PTEN activity, the survival factor Akt, was analyzed. Inappropriate activation of Akt could bypass the negative regulation by PTEN. Analyses of Akt expression in several osteosarcoma cell lines and normal osteoblasts revealed uniformly low basal levels of phosphorylated Akt. The levels of phosphorylated Akt did not increase following growth stimulation. In addition, osteosarcoma cell growth was unaffected by inhibitors of phosphatidylinositol-3 kinase, an upstream activator of the Akt signaling pathway. These data further suggest that the Akt pathway is not the predominant signaling cascade required for osteoblastic growth. However, inhibition of PTEN activity resulted in increased levels of Akt phosphorylation and enhanced cell proliferation. These data suggest that while abundant levels of PTEN normally maintain Akt in an inactive form in osteoblastic cells, the Akt signaling pathway is intact and functional.

Radiosensitization of normal human cells by LY294002: cell killing and the rejoining of DNA and interphase chromosome breaks

The radiosensitizing effect of a phosphoinositide-3 kinase (PI3K) inhibitor, wortmannin, has been studied rather extensively, but there have been few studies on the radiosensitizing effect of another PI3K inhibitor, LY294002. In this report, we present the radiosensitizing effect of LY294002 using normal human cells. Clonogenic cell survival indicated that LY294002 enhanced the killing effect of gamma-rays in a dose-dependent manner, although this drug by itself did not affect the cell killing. To obtain a 10% cell survival, about one half of the radiation dose was needed when cells were treated with 50 microM LY294002 as compared to cells without the drug. A mild inhibition of repair of DNA double strand breaks (DSBs) was observed in irradiated normal human cells pre-treated with LY294002 (50 microM). At the interphase chromosome level, we also observed an increase in the number of residual breaks when irradiated cells were pre-treated with this drug (about 2-fold at 5 Gy). These results suggest that the inhibition of DSB repair mediated the radiosensitizing effect of LY294002 at the dose level that we used.

DNA-PK: the major target for wortmannin-mediated radiosensitization by the inhibition of DSB repair via NHEJ pathway

The effect of wortmannin posttreatment was studied in cells derived from different species (hamster, mouse, chicken, and human) with normal and defective DNA-dependent protein kinase (DNA-PK) activity, cells with and without the ataxia telangiectasia (ATM) gene, and cells lacking other regulatory proteins involved in the DNA double-strand break (DSB) repair pathways. Clonogenic assays were used to obtain all results. Wortmannin radiosensitization was observed in Chinese hamster cells (V79-B310H , CHO-K1), mouse mammary carcinoma cells (SR-1), transformed human fibroblast (N2KYSV), chicken B lymphocyte wild-type cells (DT40), and chicken Rad54 knockout cells (Rad54-/-). However, mouse mammary carcinoma cells (SX9) with defects in the DNA-PK and chicken DNA-PK catalytic subunit (DNA-PKcs) knockout cells (DNA-PKcs-/-/-) failed to exhibit wortmannin radiosensitization. On the other hand, SCID mouse cells (SC3VA2) exposed to wortmannin exhibited significant increases in radiosensitivity, possibly because of some residual function of DNA-PKcs. Moreover, the transformed human cells derived from AT patients (AT2KYSV) and chicken ATM knockout cells (ATM-/-) showed pronounced wortmannin radiosensitization. These studies demonstrate confirm that the mechanism underlying wortmannin radiosensitization is the inhibition of DNA-PK, but not of ATM, thereby resulting in the inhibition of DSB repair via nonhomologous endjoining (NHEJ).

Cell cycle-dependent effects of wortmannin on radiation survival and mutation

Wortmannin, a known radiation sensitizer, has been used in experiments with synchronized cells to compare its effect on radiation survival and mutation induction within the cell cycle. PL61 cells (CHO cells with an inactivated HPRT gene containing a single active copy of a bacterial gpt gene) were synchronized by mitotic selection. Wortmannin administered before gamma irradiation caused a greater sensitization in G(1)-phase cells relative to late S/G(2)-phase cells. Preferential radiosensitization of G(1)-phase cells by wortmannin sets a limit to the proposed use of wortmannin in radiation therapy, since, in contrast to normal tissues, tumors usually have high proportions of S-phase cells. Wortmannin increased mutation frequencies in both G(1)- and S/G(2)-phase cells. Interestingly, relative increases in radiation-induced mutations in G(1) and S/G(2) phases were comparable. The results are discussed in terms of the contributions of different repair modes in the production of mutations.

A low-pH culture condition enhances the radiosensitizing effect of wortmannin

PURPOSE

The radiosensitizing effect of wortmannin on human tumor cells in a low-pH microenvironment was compared with that in a neutral-pH environment.

METHODS AND MATERIALS

A172 human glioblastoma cells, A549 human lung adenocarcinoma cells, and HMV-1 human melanoma cells were treated with 20 microM wortmannin 2 h before irradiation, and cell survival was examined. A low-pH microenvironment was simulated by exposing cells to low-pH culture medium for 24 h before wortmannin treatment. The effects of wortmannin on the repair of DNA double-strand breaks (dsbs) after 50-Gy irradiation in both low- and neutral-pH conditions were measured by pulsed-field gel electrophoresis. Expression of the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) in low-pH conditions was also compared with that in neutral-pH conditions by Western blot analysis.

RESULTS

The radiosensitizing effect of wortmannin was greater in low-pH cultures than in neutral-pH cultures for all cell lines. The fast-rejoining component of DNA dsb repair was inhibited more strongly in low-pH than in neutral-pH conditions, although there was little difference in DNA-PKcs expression between groups.

CONCLUSIONS

The low-pH culture condition, which was designed to mimic the microenvironment of the central tumor mass in actively proliferating solid tumors, enhanced the radiosensitizing effect of wortmannin by inhibiting the fast-rejoining component of DNA dsb repair and by prolonging the retention of nonrejoined DNA dsbs.

Wortmannin sensitizes human glioblastoma cell lines carrying mutant and wild type TP53 gene to radiation

The purpose of this paper is to investigate the effect of wortmannin, a specific inhibitor of phosphatidylinositol 3-kinase (PI3K), on TP53 (formerly known as p53) signal transduction initiated by ionizing radiation and radiosensitization in isogenic derivatives of human glioblastoma cells differing in TP53 status. Wortmannin inhibited the accumulation of TP53 and CDKN1A (formerly known as WAF1) after 6 Gy irradiation in A-172/neo cells bearing wild-type TP53. In A-172/Trp248 cells carrying mutant TP53, X-rays induced no significant accumulation of TP53 and slight increase of CDKN1A. There were, consequently, little differences in the expression of TP53 and CDKN1A between A-172/Trp248 cells exposed to 6 Gy alone and wortmannin plus 6 Gy. However, wortmannin sensitized both A-172/neo and A-172/Trp248 cells to radiation. These studies indicate that wortmannin inhibits TP53 upregulation, but this suppression does not account for the radiosensitization by this drug. These results indicate that inhibitors of PI3K-related kinases may present a new class of radiosensitizers, regardless of the TP53 status of tumor cells.

Sensitization by wortmannin of heat- or X-ray induced cell death in cultured Chinese hamster V79 cells

Here we found that wortmannin sensitized Chinese hamster V79 cells to hyperthermic treatment at 44.0 degrees C as determined either by colony formation assay or by dye exclusion assay. Wortmannin enhanced heat-induced cell death accompanying cleavage of poly (ADP-ribose) polymerases (PARP). Additionally, the induction of heat shock protein HSP70 was suppressed and delayed in wortmannin-treated cells. Heat sensitizing effect of wortmannin was obvious at more than 5 or 10 microM of final concentrations, while radiosensitization was apparent at 5 microM. Requirement for high concentration of wortmannin, i.e., order of microM, suggests a possible role of certain protein kinases, such as DNA-PK and/or ATM among PI3-kinase family. The sensitization was minimal when wortmannin was added at the end of heat treatment. This was similar to the case of X-ray. Since heat-induced cell death and PARP cleavage preceded HSP70 induction phenomenon, the sensitization to the hyperthermic treatment was considered mainly caused by enhanced apoptotic cell death rather than secondary to suppression or delay by wortmannin of HSP70 induction. Further, in the present system radiosensitization by wortmannin was also at least partly mediated through enhancement of apoptotic cell death.