Effects of the protein kinase inhibitors wortmannin and KN62 on cellular radiosensitivity and radiation-activated S phase and G1/S checkpoints in normal human fibroblasts

Intratumor Heterogeneity and Therapy Resistance: Contributions of Dormancy, Apoptosis Reversal (Anastasis) and Cell Fusion to Disease Recurrence

A major challenge in treating cancer is posed by intratumor heterogeneity, with different sub-populations of cancer cells within the same tumor exhibiting therapy resistance through different biological processes. These include therapy-induced dormancy (durable proliferation arrest through, e.g., polyploidy, multinucleation, or senescence), apoptosis reversal (anastasis), and cell fusion. Unfortunately, such responses are often overlooked or misinterpreted as “death” in commonly used preclinical assays, including the in vitro colony-forming assay and multiwell plate “viability” or “cytotoxicity” assays. Although these assays predominantly determine the ability of a test agent to convert dangerous (proliferating) cancer cells to potentially even more dangerous (dormant) cancer cells, the results are often assumed to reflect loss of cancer cell viability (death). In this article we briefly discuss the dark sides of dormancy, apoptosis, and cell fusion in cancer therapy, and underscore the danger of relying on short-term preclinical assays that generate population-based data averaged over a large number of cells. Unveiling the molecular events that underlie intratumor heterogeneity together with more appropriate experimental design and data interpretation will hopefully lead to clinically relevant strategies for treating recurrent/metastatic disease, which remains a major global health issue despite extensive research over the past half century.

Calmodulin-dependent protein kinase II (CaMKII) mediates radiation-induced mitochondrial fission by regulating the phosphorylation of dynamin-related protein 1 (Drp1) at serine 616

Mitochondrial dynamics are suggested to be indispensable for the maintenance of cellular quality and function in response to various stresses. While ionizing radiation (IR) stimulates mitochondrial fission, which is mediated by the mitochondrial fission protein, dynamin-related protein 1 (Drp1), it remains unclear how IR promotes Drp1 activation and subsequent mitochondrial fission. Therefore, we conducted this study to investigate these concerns. First, we found that X-irradiation triggered Drp1 phosphorylation at serine 616 (S616) but not at serine 637 (S637). Reconstitution analysis revealed that introduction of wild-type (WT) Drp1 recovered radiation-induced mitochondrial fission, which was absent in Drp1-deficient cells. Compared with cells transfected with WT or S637A Drp1, the change in mitochondrial shape following irradiation was mitigated in S616A Drp1-transfected cells. Furthermore, inhibition of CaMKII significantly suppressed Drp1 S616 phosphorylation and mitochondrial fission induced by IR. These results suggest that Drp1 phosphorylation at S616, but not at S637, is prerequisite for radiation-induced mitochondrial fission and that CaMKII regulates Drp1 phosphorylation at S616 following irradiation.

Pharmacological inhibition of DNA repair enzymes differentially modulates telomerase activity and apoptosis in two human leukaemia cell lines

PURPOSE

To investigate the effect of wortmannin and 3-aminobenzamide (3-AB) on telomerase activity and apoptosis in two human leukaemia cells.

MATERIALS AND METHODS

MOLT-4 (p53-wild type) and KG1a (p53-null) cells were irradiated with gamma-rays (3 Gy at 1.57 Gy min(-1)) and the effects of wortmannin and 3-AB were evaluated. Telomerase activity was measured by polymerase chain reaction and the expression of human telomerase reverse transcriptase, human telomerase RNA and telomerase-associated protein 1 was assessed by reverse transcriptase-polymerase chain reaction. Apoptosis was evaluated by fluorescence microscopy and flow cytometry.

RESULTS

A radiation-induced up-regulation of telomerase activity was observed from 4 h post-irradiation in both cell lines. This up-regulation was abrogated by wortmannin and 3-AB. Telomerase activity was maximal 24 h post-irradiation, coinciding with an accumulation of human telomerase reverse transcriptase mRNA. Apoptosis and G2/M arrest were evident from 4 h post-irradiation in MOLT-4 cells. KG1a cells exhibited a G2/M block at 24 h post-irradiation and apoptosis increased between 24 and 48 h post-irradiation. 3-AB abolished G2/M blockage and enhanced radiation-induced apoptosis in both cell lines, while wortmannin increased apoptosis only in MOLT-4 cells.

CONCLUSIONS

3-AB inhibits the radiation-associated telomerase activity increase and enhances apoptosis in MOLT-4 and KG1a cells. Wortmannin, which also inhibits the radiation-associated telomerase activity increase in both cell lines, does not modify radiation-induced apoptosis in KG1a cells. DNA repair enzymes might be selective targets for enhancing radiosensitivity in certain tumour cells.

Enhanced induction of apoptosis in a radio-resistant bladder tumor cell line by combined treatments with X-rays and wortmannin

The radiosensitizing effect of wortmannin (WM) treatment during and after irradiation was studied in radioresistant bladder tumor cell lines with normal (MGH-U1 cells) or defective p53 activity (RT112 cells). WM modulated G(2)/M cell cycle arrest induced by higher X-ray doses (10 Gy) in both cell lines, although the alteration was significant only in RT112 cells. The observation suggests that WM activity is independent of p53. Constitutive expression of DNA-PKcs was found to be higher in RT112 cells than in MGH-U1. Treatment with WM enhanced radiation-induced apoptosis significantly in RT112 cells while it had no effect on MGH-U1 cells. Although a variety of PI3-kinases and PI3-K like kinases (including ATM) could be inhibited by WM, our observation of increased early lethality by WM treatment in RT112 is in agreement with previous results. They suggest that the WM-dependent radiosensitization of RT112 is a direct consequence of the inhibition of DNA-PK, resulting in the inhibition of DSB repair in the fast component. This early effect in the p53 deficient cell line could also indicate that processes other than apoptosis may contribute to the increased radiosensitization. In our opinion, the expression level of DNA-PKcs in human tumor cells may be a good predictor for the success of DNA-PKcs inhibitors when used as radiosensitizers.

Linking kinetochore-microtubule binding to the spindle checkpoint

The spindle checkpoint blocks cell-cycle progression until chromosomes are properly attached to the mitotic spindle. Popular models propose that checkpoint proteins associate with kinetochores to produce a "wait anaphase" signal that inhibits anaphase. Recent data suggest that a two-state switch results from using the same kinetochore proteins to bind microtubules and checkpoint proteins. At least eight protein kinases are implicated in spindle checkpoint signaling, arguing that a traditional signal transduction cascade is integral to spindle checkpoint signaling.

Activation of telomerase by ionizing radiation: differential response to the inhibition of DNA double-strand break repair by abrogation of poly (ADP-ribosyl)ation, by LY294002, or by Wortmannin

PURPOSE

Telomerase activity represents a radiation-inducible function, which may be targeted by a double-strand break (DSB)-activated signal transduction pathway. Therefore, the effects of DNA-PK inhibitors (Wortmannin and LY294002) on telomerase upregulation after irradiation were studied. In addition, the role of trans-dominant inhibition of poly(ADP-ribosyl)ation, which strongly reduces DSB rejoining, was assessed in comparison with 3-aminobenzamide.

METHODS AND MATERIALS

COM3 rodent cells carry a construct for the dexamethasone-inducible overexpression of the DNA-binding domain of PARP1 and exhibit greatly impaired DSB rejoining after irradiation. Telomerase activity was measured using polymerase chain reaction ELISA 1 h after irradiation with doses up to 10 Gy. Phosphorylation status of PKB/Akt and of PKCalpha/beta(II) was assessed by western blotting.

RESULTS

No telomerase upregulation was detectable for irradiated cells with undisturbed DSB rejoining. In contrast, incubation with LY294002 or dexamethasone yielded pronounced radiation induction of telomerase activity that could be suppressed by Wortmannin. 3-Aminobenzamide not only was unable to induce telomerase activity but also suppressed telomerase upregulation upon incubation with LY294002 or dexamethasone. Phospho-PKB was detectable independent of irradiation or dexamethasone pretreatment, but was undetectable upon incubations with LY294002 or Wortmannin, whereas phospho-PKC rested detectable.

CONCLUSIONS

Telomerase activation postirradiation was triggered by different treatments that interfere with DNA DSB processing. This telomerase upregulation, however, was not reflected by the phosporylation status of the putative mediators of TERT activation, PKB and PKC. Although an involvement of PKB in TERT activation is not supported by the present findings, a respective role of PKC isoforms other than alpha/beta(II) cannot be ruled out.

Proteomic Analysis of the Effects of Tetramethylpyrazine on Irradiated QXMSC1 Cells

Tetramethylpyrazine is the active ingredient of a Chinese herbal medicine. In this study, tetramethylpyrazine was tested for its activities in irradiated bone marrow stromal QXMSC1 cells. The proliferation of QXMSC1 cells was measured by MTS assay kit and flow cytometry. To identify proteins involved in the processes of cellular and molecular response of tetramethylpyrazine to irradiation damage, we comparatively analyzed the proteome of nonirradiated, irradiated and tetramethylpyrazine treated QXMSC1 cells. Reverse transcriptase polymerase chain reaction (RT-PCR) were used to validate the differentially expressed proteins. 20 Gy 60Co gamma irradition inhibited QMSC1 cells growth and tetramethylpyrazine could reverse of this action due to stimulating QXMSC1 cells from G1 to S progression. Proteomic analytical results showed that 18 spots were changed in irradiated QXMSC1 cells, and 15 spots matched with known proteins after database searching. The expression level of proteins such as translationally controlled tumor protein (TCTP), and galectin-3, were increased in irradiated QXMSC1 cells, while calmodulin, pyruvate kinase were decreased. Tetramethylpyrazine could prevent this change or reverse to some degree. The function of these proteins involves in hematopoiesis, cell cycle and signal transduction. The changes of these proteins were confirmed by RT-PCR at mRNA levels. This study suggested that stimulating proliferation via tetramethylpyrazine played an important role in the cure effect on irradiated QXMSC1 cells and was helpful to deeply understand the mechanism of tetramethylpyrazine at the molecular level.

Radiation-induced gene expression profile of human cells deficient in 8-hydroxy-2′-deoxyguanine glycosylase

The human OGG1 gene encodes a DNA glycosylase that is involved in the base excision repair of 8-hydroxy-2'-deoxyguanine (8-OH-dG) from oxidatively damaged DNA. Cellular 8-OH-dG levels accumulate in the absence of this activity and could be deleterious for the cell. To assess the role of 8-oxoguanine glycosylase (OGG1) in the cellular defense mechanism in a specific DNA repair defect background, we set out to determine the expression pattern of base excision repair genes and other cellular genes not involved in the base excision pathway in OGG1-deficient human KG-1 cells after ionizing radiation exposure. KG-1 cells have lost OGG1 activity due to a homozygous mutation of Arg229Gln. Gene expression alterations were monitored at 4, 8, 12 and 24 hr in 2 Gy irradiated cells. Large-scale gene expression profiling was assessed with DNA microarray technology. Gene expression analysis identified a number of ionizing radiation-responsive genes, including several novel genes. There were 2 peaks of radiation-induced gene induction or repression: one at 8 hr and the other at 24 hr. Overall the number of downregulated genes was higher than the number of upregulated genes. The highest number of downregulated genes was at 8 hr postirradiation. Genes corresponding to cellular, physiologic, developmental and extracellular processes were identified. The highest number of radiation-induced genes belonged to the signal transduction category, followed by genes involved in transcription and response to stress. Microarray gene expression data were independently validated by relative quantitative RT-PCR. Surprisingly, none of the genes involved in the base excision repair of radiation-induced DNA damage showed altered expression.

Polyethyleneimine grafted with pluronic P85 enhances Ku86 antisense delivery and the ionizing radiation treatment efficacy in vivo

In an effort to improve the efficacy of antisense delivery, we evaluated polyethyleneimine (PEI, 2 kDa) alone or grafted with nonionic amphiphilic block copolymer Pluronic (P85) as a carrier for Ku86 antisense oligonucleotide (ASO) delivery. Ku86 is an abundant nuclear protein that plays an important role in nonhomologous DNA end joining and has implications in tumorigenesis and acquired drug resistance. Transfection of adherent and suspension cell lines with Ku86 ASOs complexed with P85-g-PEI (2 kDa) conjugates was associated with a specific decrease in Ku86 mRNA levels (EC50<75 nM and EC50<250 nM, respectively, n=3). More importantly, no requirement for reduced serum conditions was necessary during transfection. In contrast, whereas Ku86 ASOs complexed with PEI (2 kDa) alone were effective in decreasing Ku86 mRNA levels in adherent cell lines (EC50<75 nM, n=3), the formulation did not produce any detectable decrease in Ku86 mRNA levels in suspension cell lines. Transfection of adherent cell lines with 500 nM Ku86 ASOs formulated with P85-g-PEI (2 kDa) was associated with a specific decrease (<10% remaining of control) in Ku86 protein expression and a two-fold increased cell death after treatment with ionizing radiation (IR). In athymic nude mice bearing subcutaneous human HT29 colon adenocarcinoma xenografts, Ku86 ASO-P85-g-PEI (2 kDa) administration (15 mg/kg, subcutaneously) with a Q1D x 7 treatment schedule, when combined with a single dose of IR (6 Gy), caused a significant inhibition of HT29 tumor growth compared with mismatch- and naked antisense-pretreated control groups (time from 200 to 1000 mm3, 126.9 versus 84.18 and 87.76 days, P<0.005). A potentiation of the antitumor activity was observed in all mice treated with Ku86 ASO-P85-g-PEI (2 kDa) formulation; however, tumor growth inhibition was reversible upon treatment cessation. No morbidity/mortality or changes in histopathology were observed under this treatment regiment. Our results indicate that P85-g-PEI (2 kDa) conjugates may increase the efficacy of Ku86 ASO delivery in management of resistant malignancies, thus providing a rationale for their evaluation in cancer patients in combination with conventional anticancer therapies.

Relationship between the radiosensitizing effect of wortmannin, DNA double-strand break rejoining, and p21WAF1 induction in human normal and tumor-derived cells

Wortmannin (WM) is a potent inhibitor of the catalytic sub-unit of DNA-PK, which is involved in one pathway of DNA double-strand break (DSB) rejoining, and of ATM, which functions upstream in the p53 signaling pathway. WM is known to be an efficient radiosensitizer in a variety of mammalian cell types, to inhibit DSB rejoining following exposure to supralethal doses (> or =30 Gy) of ionizing radiation, and to abrogate the induction of p53 at early times after radiation exposure. We report here that WM is a more effective radiosensitizer in A549 human lung carcinoma cells than in normal human fibroblasts (NHFs). In addition, WM strongly inhibits DSB rejoining in A549 cells exposed to relatively low doses (e.g., 10 Gy) of ionizing radiation, without having any detectable effect in NHFs. We further demonstrate that WM significantly potentiates the induction of p21WAF1, a p53-regulated gene that encodes for a key mediator of cell-cycle/growth arrest, when determined at late times (e.g., 24 h) after irradiation. This late WM-dependent potentiation of p21WAF1 induction following radiation exposure is observed in NHFs and in the p53 wild-type tumor cell lines A549, A172, and SKNSH, but not in the p53-deficient tumor cell lines DLD-1, HeLa, and SKNSH-E6. We conclude that: (i) inhibition of DSB rejoining by WM may be an important contributor to its radiosensitizing effect in A549 cells but not in NHFs; and (ii) radiosensitization of p53-proficient human cells by WM may in part be associated with the delayed induction of p21WAF1, which can lead to a sustained growth-arrested phenotype resembling senescence.

Regulation of the Multifunctional Ca2+/Calmodulin-dependent Protein Kinase II by the PP2C Phosphatase PPM1F in Fibroblasts

The regulation of the multifunctional calcium/calmodulin dependent protein kinase II (CaMKII) by serine/threonine protein phosphatases has been extensively studied in neuronal cells; however, this regulation has not been investigated previously in fibroblasts. We cloned a cDNA from SV40-transformed human fibroblasts that shares 80% homology to a rat calcium/calmodulin-dependent protein kinase phosphatase that encodes a PPM1F protein. By using extracts from transfected cells, PPM1F, but not a mutant (R326A) in the conserved catalytic domain, was found to dephosphorylate in vitro a peptide corresponding to the auto-inhibitory region of CaMKII. Further analyses demonstrated that PPM1F specifically dephosphorylates the phospho-Thr-286 in autophosphorylated CaMKII substrate and thus deactivates the CaMKII in vitro. Coimmunoprecipitation of CaMKII with PPM1F indicates that the two proteins can interact intracellularly. Binding of PPM1F to CaMKII involves multiple regions and is not dependent on intact phosphatase activity. Furthermore, overexpression of PPM1F in fibroblasts caused a reduction in the CaMKII-specific phosphorylation of the known substrate vimentin(Ser-82) following induction of the endogenous CaM kinase. These results identify PPM1F as a CaM kinase phosphatase within fibroblasts, although it may have additional functions intracellularly since it has been presented elsewhere as POPX2 and hFEM-2. We conclude that PPM1F, possibly together with the other previously described protein phosphatases PP1 and PP2A, can regulate the activity of CaMKII. Moreover, because PPM1F dephosphorylates the critical autophosphorylation site of CaMKII, we propose that this phosphatase plays a key role in the regulation of the kinase intracellularly.

Influence of Oxygen on the Radiosensitivity of Human Glioma Cell Lines

We have investigated the influence of hypoxia on the radiosensitivity of 4 early-passage tumor cell lines that were established from malignant glioma patients at our Institute. These cell lines were M006, M059J (a highly radiosensitive line), M059K (a radioresistant line derived from the same biopsy as M059J), and M010b. The GM637 human fibroblast cell line was used as a normal control. The oxygen enhancement ratios (OERs) for these cell lines, determined using a clonogenic survival assay, were approximately 3.6 (GM637), approximately 3.7 (M006), approximately 2.5 (M010b), approximately 2.1 (M059K), and approximately 3.5 (M059J). The broad range of OERs for these glioma lines was not related to cellular glutathione levels or to differences in intrinsic cellular radiosensitivity. Because studies with rodent cell lines indicate that defects in certain DNA repair genes, including ERCC1, can greatly influence cellular OERs, and because several such repair genes, including ERCC1, localize to a region of chromosome 19q that is close to a common deletion in human glioma, we reasoned that such deletions might contribute to the diverse OERs of these tumor cell lines. However, measurements of ERCC1 protein levels using immunofluorescence staining or Western blotting, of ERCC1 mRNA levels using Northern blotting, and of functional nucleotide excision repair capability using the UV/adenovirus reactivation assay, failed to indicate any deficit in these activities. Thus, although the effect of hypoxia on the radiosensitivity of different human glioma cell lines can vary widely, the mechanism of this effect remains unknown. The potential implications of this finding for radiation therapy, and especially for hypoxia imaging-guided intensity-modulated radiation therapy (IMRT) treatment planning, are discussed.

Metabolic labeling of human cells with tritiated nucleosides results in activation of the ATM-dependent p53 signaling pathway and acceleration of DNA repair

We investigated the effects of metabolic labeling with [(3)H]thymidine, [(3)H]uridine, and [(14)C]thymidine on human cells in terms of cell growth, p53 signaling, and nucleotide excision repair. Labeling with [(3)H] nucleosides resulted in growth inhibition by both p53-dependent and -independent mechanisms. Tritium labeling also led to nuclear accumulation of p53 and induction of the p53-regulated gene p21(WAF1) and its encoded protein (p21). ATM-deficient cells, however, did not increase their p53 and p21 protein levels in response to radiolabeling. Thus, labeling of human cells with tritiated nucleosides activates the radiation-responsive, ATM-dependent, DNA-damage surveillance network. Labeling of normal cells with [(3)H]thymidine significantly accelerated the repair of ultraviolet (UV) light-induced cyclobutane pyrimidine dimers, as monitored by a sensitive immunofluorescence assay. Unlike [(3)H] labeling, [(14)C] labeling did not produce any impact on proliferation, p53 signaling, or DNA repair. In the light of these findings, the validity of results obtained with nucleic acid synthesis and DNA repair assays that involve [(3)H] and [(14)C] labeling is discussed. Our immunofluorescence approach detected pyrimidine dimers after exposure to UV fluences as low as 1 J/m(2) (the lowest fluence examined). This approach may prove particularly useful for monitoring DNA damage and its repair following exposure to extremely low levels of genotoxic agents.

Cell adhesion to the extracellular matrix protein fibronectin modulates radiation-dependent G2 phase arrest involving integrin-linked kinase (ILK) and glycogen synthase kinase-3beta (GSK-3beta) in vitro

Cell adhesion to extracellular matrix (ECM) is thought to confer resistance against cell-damaging agents, that is, drugs and radiation, in tumour and normal cells in vitro. The dependence of cell survival on beta1-integrin-linked kinase (ILK), protein kinase Balpha/Akt (PKBalpha/Akt) and glycogen synthase kinase-3beta (GSK-3beta) activity, which participate in beta1-integrin signalling and cell cycle progression was investigated as a function of radiation exposure. Colony-formation assays on polystyrene, fibronectin (FN), laminin (LA), bovine serum albumin (BSA) or poly-L-lysine (poly-L) (0-8 Gy), kinase assays, flow cytometric DNA and annexin-V analysis and immunoblotting were performed in nonirradiated and irradiated (2 or 6 Gy) A549 human lung cancer cells and CCD32 normal human lung fibroblasts. Cell contact to FN in contrast to polystyrene elevated basal ILK, PKBalpha/Akt and GSK-3beta kinase activities in A549 and CCD32 cells, as well as the basal amount of A549 G2 phase cells. Irradiation on FN or LA as compared to polystyrene, BSA or poly-L significantly improved cell survival. Following irradiation, kinase activities were stimulated strongly on polystyrene but showed to be less prominent on FN, which was because of the FN-related basal induction. Following irradiation, FN compared to polystyrene enlarged and prolonged G2 arrest in both the cell lines. For the analysis of phosphatidylinositol-3 kinase (PI3-K) dependence of protein kinases and cell cycle transition, the PI3-K inhibitors LY294002 and wortmannin were used showing decreased kinase activities, antiproliferative and radiation-dependent G2 accumulation-abrogating effects accompanied by downregulation of cyclin D1 and phospho-pRb in cells attached to polystyrene. Fibronectin partly abrogated these effects PI3-K-independently. These findings suggest a novel pathway that makes direct phosphorylation of GSK-3beta by ILK feasible after irradiation. Conclusively, the data indicate that ILK, PKBalpha/Akt and GSK-3beta are involved in modulations of the cell cycle after irradiation. These interactions are strictly dependent on ECM components in a cell line-specific manner. Our findings provide molecular insights into mechanisms likely to be important for ECM-dependent cell survival and cellular radioresistance as well as tumour growth.

Aberrant sensing of extracellular Ca2+ by cultured ataxia telangiectasia fibroblasts

Ataxia telangiectasia (AT) is a human hereditary syndrome whose underlying gene product, ataxia telangiectasia mutated (ATM) protein kinase, is involved in multiple intracellular signaling pathways. We demonstrated previously that AT fibroblasts are defective in intracellular Ca(2+) mobilization in response to both stress-inducing and mitogenic stimuli. To extend these findings, normal and AT cells were exposed to serum in the presence of different concentrations of extracellular Ca(2+) ([Ca(2+)](o)), and release of intracellular Ca(2+), activation of calmodulin-dependent protein kinase II and phosphorylation of kinases ERK1 and 2 were monitored. When maintained in high [Ca(2+)](o) (0.42 mM), normal fibroblasts responded to serum introduction more rapidly and efficiently than did AT cells. Unexpectedly, decreasing the [Ca(2+)](o) in the medium had a diametrically opposite effect. Under low [Ca(2+)](o) (0.0022 mM) conditions, normal cells were slow and inefficient in their responses, whereas AT cells showed a substantial improvement in all three end points. These findings demonstrate that loss of ATM kinase function deregulates the extracellular calcium-sensing receptor (CaR). This malfunction presumably arises from a post-transcriptional event, since CaR mRNA proved to be normal in AT cells. Together, our data suggest that ATM may mediate cell response to mitogenic factors by tightly regulating the set point of the CaR and thereby modulating the crosstalk between this metabotropic receptor and growth factor receptors. Alternatively, the faulty sensing of extracellular calcium in AT cells may be secondary to a state of chronic oxidative stress attributable to ATM deficiency.

Correction of radioresistant DNA synthesis in ataxia telangiectasia fibroblasts by prostaglandin E2 treatment

Cultured cells from patients inheriting the rare cancer-prone and radiotherapy-sensitive disorder ataxia telangiectasia (AT) exhibit defects in the activation of cell-cycle checkpoints after exposure to ionizing radiation. In particular, the failure of AT cells to arrest transiently the DNA de novo replication machinery immediately after irradiation--so-called radioresistant DNA synthesis (RDS)--is often taken as a molecular hallmark of the disease. Recently we reported that: (i) the radiation-responsive S-phase checkpoint operating in normal human cells is mediated by a signal transduction pathway involving Ca2+/calmodulin-dependent protein kinase II (CaMKII); and (ii) the RDS phenotype of AT cells is associated with failure to mobilize Ca2+ from intracellular stores, which is required for activation of the CaMKII-dependent S-phase arrest. In the present study, we demonstrate that the RDS phenotype of AT dermal fibroblasts can be rectified in the absence of ectopic expression of functional ATM, the 350-kDa protein kinase encoded by the gene mutated in AT. Correction of RDS was observed when AT fibroblasts were coincubated with normal fibroblasts under conditions in which the 2 different cell cultures shared the same medium but were completely separated physically. The RDS trait was also rectified when AT fibroblasts were briefly incubated with prostaglandin E2 in the absence of normal feeder cells, signifying that this ubiquitous eicosanoid can serve as the diffusible "RDS-correction factor" for AT cells in the aforementioned cocultivation studies. It would therefore appear that prostaglandin E2 can assume the role of an extracellular signaling modulator of the S-phase checkpoint in AT cells exposed to ionizing radiation, inducing DNA synthesis shutdown via an alternative, ATM-independent signal transduction pathway.