Radiosensitivity Of Prostate Cancer Cells Research Articles

Enhanced Radiosensitivity of Androgen-Resistant Prostate Cancer: AZD1152-Mediated Aurora Kinase B Inhibition

Aurora kinase B (AURKB) is critical to the process of mitosis, aiding in chromosome condensation by phosphorylating histone H3. We investigated the effects of AZD1152, an AURKB inhibitor, on radiosensitivity of androgen-insensitive prostate cancer cells. The goal of this study was to test whether AZD1152 increases the susceptibility of hormone-refractory prostate cancer cells to radiation-induced DNA damage and to determine the conditions of AZD1152 treatment that maximize radiosensitization. PC3 and DU145 cells were treated with various AZD1152 doses for various durations to elucidate the conditions that yielded maximal increases in G(2)/M-phase and polyploid cells. To assess DNA damage, γ-H2AX phosphorylation was quantified for cells grown under radiosensitizing conditions and subjected to either no radiation or 5 Gy radiation. Radiosensitivity was determined by clonogenic assays. Cell cycle effects in both cell lines were maximized by treatment with 60 nM AZD1152 for 48 h. AZD1152-treated cells exhibited significantly increased DNA damage 30 min postirradiation (PC3: 100% compared to 68%, P = 0.035; DU145: 100% compared to 69%, P = 0.034), with additional DNA damage 6 h postirradiation (PC3: 85% compared to 15%, P = 0.002; DU145: 67% compared to 21%, P = 0.012). Radiosensitivity was increased in both cell lines, with dose enhancement ratios of 1.53 for PC3 cells (P = 0.017) and 1.71 for DU145 cells (P = 0.02). This study identifies the optimal AZD1152 treatment conditions to maximize the radiosensitization of PC3 and DU145 cells. These results suggest a major role for DNA damage and impairment of DNA repair mechanisms in AZD1152-induced radiosensitization of prostate cancer cells.

Expression Profile of Apoptosis Related Genes and Radio-sensitivity of Prostate Cancer Cells

Radio-resistant or recurrent prostate cancer represents a serious health risk for approximately 20%-30% of patients treated with primary radiation therapy for clinically localized prostate cancer. In the present study, we investigated the expression profiles of 84 genes involved in various apoptosis pathways in two prostate cancer cell lines LNCaP (P53+ and AR+) and PC3 (P53- and AR-). We also studied the effect of monensin, an apoptosis inducing reagent, in X-ray-induced cell killing. Comparison of gene expressions between unirradiated LNCaP and PC3 cells revealed distinguished gene expression patterns. The data showed a significantly higher expression level of genes involved in the caspase/card family and the TNF ligand/receptor family in PC3 cells, whereas, LNCaP cells exhibited higher expressions in the p53 related genes. At 2 and 4 hrs post a 10 Gy X-ray exposure, changes of gene expressions were detected in a significant fraction of the genes in LNCaP cells, but no significant changes were found in PC3 cells. There was no significant apoptosis-inducing effect of X-rays (up to 10 Gy) in both cell lines; however, monensin was shown to be effective in inducing apoptosis in LNCaP, but not in PC3 cells. In addition, the effect of combined treatment of monensin and X-rays in LNCaP cells appeared to be synergistic. Our results suggest that monensin may be effective for both cancer cell killing and radiosensitizing, and the different expression profiles in apoptosis related genes in cancer cells may be correlated with their sensitivity to apoptosis inducing reagents.

Adenovirus E2F1 Overexpression Sensitizes LNCaP and PC3 Prostate Tumor Cells to Radiation In Vivo

We previously showed that E2F1 overexpression radiosensitizes prostate cancer cells in vitro. Here, we demonstrate the radiosensitization efficacy of adenovirus (Ad)-E2F1 infection in growing (orthotopic) LNCaP and (subcutaneous) PC3 nude mice xenograft tumors. Ad-E2F1 was injected intratumorally in LNCaP (3 × 10(8) plaque-forming units [PFU]) and PC3 (5 × 10(8) PFU) tumors treated with or without radiation. LNCaP tumor volumes (TV) were measured by magnetic resonance imaging, caliper were used to measure PC3 tumors, and serum prostate-specific antigen (PSA) levels were determined by enzyme-linked immunosorbent assay. Apoptosis was measured by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling, and key proteins involved in cell death signaling were analyzed by Western blotting. Intracellular overexpression of Ad-E2F1 had a significant effect on the regression of TV and reduction of PSA levels relative to that of adenoviral luciferase (Ad-Luc)-infected control. The in vivo regressing effect of Ad-E2F1 on LNCaP tumor growth was significant (PSA, 34 ng/ml; TV, 142 mm(3)) compared to that of Ad-Luc control (PSA, 59 ng/ml; TV, 218 mm(3); p <0.05). This effect was significantly enhanced by radiation therapy (compare: Ad-E2F1+RT/PSA, 16 ng/ml, and TV, 55 mm(3) to Ad-Luc+RT/PSA, 42 ng/ml, and TV, 174 mm(3), respectively; p <0.05). For PC3 tumors, the greatest effect was observed with Ad-E2F1 infection alone; there was little or no effect when radiotherapy (RT) was combined. However, addition of RT enhanced the level of in situ apoptosis in PC3 tumors. Molecularly, addition of Ad-E2F1 in a combination treatment abrogated radiation-induced BCL-2 protein expression and was associated with an increase in activated BAX, and together they caused a potent radiosensitizing effect, irrespective of p53 and androgen receptor functional status. We show here for the first time that ectopic overexpression of E2F1 in vivo, using an adenoviral vector, significantly inhibits orthotopic p53 wild-type LNCaP tumors and subcutaneous p53-null PC3 tumors in nude mice. Furthermore, we demonstrate that E2F1 strongly sensitizes LNCaP tumors to RT. These findings suggest that E2F1 overexpression can sensitize prostate tumor cells in vivo, independent of p53 or androgen receptor status.

Truncated E2F1 Radiosensitizes Prostate Cancer Cells by Inhibiting The E3 Ubiquitin Ligase SKP2

Ectopic overexpression of E2F1 via an adenoviral vector has been shown to induce apoptosis in cancer cells. Our earlier studies show that E2F1 overexpression sensitizes prostate cancer cells to radiation in vitro and in vivo. However, E2F1 overexpression induces proliferation and may potentiate transformation. To address this concern, a truncated version, E2F1[108], without the DP1, DNA, RB binding domains was constructed. E2F1[108] retains the binding domain for S-phase kinase associated protein 2 (SKP2), an F-box protein that degrades cell-cycle regulators and proapoptotic factors (FOXO) by ubiquitin dependent mechanism.

Abstract 1503: PSMA Aptamer-targeted siRNAs selectively Eehance prostate cancer radiation sensitivity

Abstract The application of dose escalated radiation therapy for localized prostate cancer has a clear therapeutic benefit; however, its clinical use is hampered by the increased risk of damage to surrounding normal tissues. One attractive alternative to elevated doses is radiation sensitizing agents. Prostate cancer cells can be made radiosensitive by targeted reductions of DNA repair proteins via RNA interference. However, without tissue-selective targeting, these agents will also sensitize surrounding tissue, leading to increased adverse effects. Here we describe a strategy where prostate-targeted RNA interference is used to selectively radio-sensitize prostate cancer cells. A high throughput siRNA screen was completed to identify DNA-repair pathway targets for prostate cancer radio-sensitization. Candidate siRNAs were then conjugated to PSMA-targeted RNA aptamers for selective delivery of siRNAs to Prostate cancer cells. In vitro and in vivo data support PSMA and aptamer-specific targeting and gene knock-down. These studies support continued development of this technology as a path to enhance radiotherapeutic outcomes for men with Prostate cancer and it is the first report of targeted, prostate-specific siRNAs for radio-sensitization. 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 1503.

The radiation response of androgen-refractory prostate cancer cell line C4-2 derived from androgen-sensitive cell line LNCaP

Radiation therapy is a relatively effective therapeutic method for localized prostate cancer (PCa) patients. However, radioresistance occurs in nearly 30% of patients treated with potentially curative doses. Therapeutic synergy between radiotherapy and androgen ablation treatment provides a promising strategy for improving the clinical outcome. Accordingly, the androgen deprivation-induced signaling pathway may also mediate radiosensitivity in PCa cells. The C4-2 cell line was derived from the androgen-sensitive LNCaP parent line under androgen-depleted condition and had acquired androgen-refractory characteristics. In our study, the response to radiation was evaluated in both LNCaP and C4-2. Results showed that C4-2 cells were more likely to survive from irradiation and appeared more aggressive in their resistance to radiation treatment compared with LNCaP, as measured by clonogenic assays and cell viability and cell cycle analyses. Gene expression analyses revealed that a set of genes involved in cell cycle arrest and DNA repair were differentially regulated in LNCaP and C4-2 in response to radiation, which was also consistent with the radiation-resistant property observed in C4-2 cells. These results strongly suggested that the radiation-resistant property may develop with progression of PCa to androgen-independent status. Not only can the LNCaP and C4-2 PCa progression model be applied for investigating androgen-refractory progression, but it can also be used to explore the development of radiation resistance in PCa.

Inhibition of p21‐activated kinase 6 (PAK6) increases radiosensitivity of prostate cancer cells

p21-activated kinase 6 (PAK6) is a serine/threonine kinase belonging to the p21-activated kinase (PAK) family. We investigated the role of PAK6 in radiation-induced cell death in human prostate cancer cells. We used a short hairpin RNA (shRNA) strategy to stably knock down PAK6 in PC3 and DU145 cells. Radiation sensitivities were compared in PAK6 stably knockdown cells versus the scrambled shRNA-expressing control cells. PAK6 mRNA and protein levels in PC3 and DU145 cells were upregulated upon exposure to 6 Gy of radiation. After irradiation, an increased percentage of apoptotic cells and cleaved caspase-3 levels were demonstrated in combination with a decrease in cell viability and a reduction in clonogenic survival in PAK6-knockdown cells. In addition, transfection with PAK6 shRNA blocked cells in a more radiosensitive G2-M phase and increased levels of DNA double-strand breaks. We further explored the potential mechanisms by which PAK6 mediates resistance to radiation-induced apoptosis. Inhibition of PAK6 caused a decrease in Ser(112) phosphorylation of BAD, a proapoptotic member of the Bcl-2 family, which led to enhanced binding of BAD to Bcl-2 and Bcl-X(L) and release of cytochrome c culminating into caspase activation and cell apoptosis. The combination of PAK6 inhibition and irradiation resulted in significantly decreased survival of prostate cancer cells. The underlying mechanisms by which targeting PAK6 may improve radiation response seem to be multifaceted, and involve alterations in cell cycle distribution and impaired DNA double-strand break repair as well as relieved BAD phosphorylation.

Synergistic Effect Between Curcumin (diferuloylmethane) and Radiation on Clonogenic Cell Death Independent of p53 in Prostate Cancer Cells

Radiotherapy is a common treatment for prostate cancer, but failure is observed 30 to 40% of the time. It is more common in patients with abnormal p53. The phytochemical diferuloylmethane (curcumin) a naturally occurring fl avinoid derived from the rhizome of Curcuma longa, shows potential radiosensitizing effects. In the present study, the effect of curcumin and radiation on cell viability, apoptosis and clonogenic cell death was examined in LNCaP (wild type p53) and PC3 (mutant p53) prostate cancer cells. The expression of p53 and p53 target genes was examined using RNase protection assay and Western blot analysis. Cell cycle and apoptosis was evaluated by fl ow cytometry. Cell viability and colony formation was examined using MTT assay and clonogenic assay respectively. The expression of p53, p21 and gadd45 increased in LNCaP cells after radiation exposure. In PC3 cells, there was no change in expression in mutant p53 after radiation or curcumin treatment. However, the expression of p21 and gadd45 increased after curcumin treatment in PC3 cells independent of p53. Curcumin induced cell growth arrest and apoptosis in both the cell lines. The cell viability and cell proliferation decreased in presence of both curcumin and radiation as compared to curcumin or radiation alone in both the cell lines. Regardless of p53 status, combination treatment with curcumin (2.5 to 10 μM) and radiation (2 Gy) had a synergistic effect on clonogenic cell death in both LNCaP and PC3 cells. Conclusion: Curcumin appears to radiosensitize prostate cancer cells and may be a possible adjuvant to radiotherapy in the treatment of prostate cancers.

Hypoxia-driven CDUPRT/5-FC Strategy Enhances the Radiosensitivity of Prostate Cancer Cells

Hypoxia-driven CDUPRT/5-FC Strategy Enhances the Radiosensitivity of Prostate Cancer Cells

Hypoxia response element‐driven cytosine deaminase/5‐fluorocytosine gene therapy system: a highly effective approach to overcome the dynamics of tumour hypoxia and enhance the radiosensitivity of prostate cancer cells in vitro

We proposed to exploit hypoxia-inducible factor (HIF)-1alpha overexpression in prostate tumours and use this transcriptional machinery to control the expression of the suicide gene cytosine deaminase (CD) through binding of HIF-1alpha to arrangements of hypoxia response elements. CD is a prodrug activation enzyme, which converts inactive 5-fluorocytosine to active 5-fluorouracil (5-FU), allowing selective killing of vector containing cells. We developed a pair of vectors, containing either five or eight copies of the hypoxia response element (HRE) isolated from the vascular endothelial growth factor (pH5VCD) or glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (pH8GCD) gene, respectively. The kinetics of the hypoxic induction of the vectors and sensitization effects were evaluated in 22Rv1 and DU145 cells in vitro. The CD protein as selectively detected in lysates of transiently transfected 22Rv1 and DU145 cells following hypoxic exposure. This is the first evidence of GAPDH HREs being used to control a suicide gene therapy strategy. Detectable CD levels were sustained upon reoxygenation and prolonged hypoxic exposures. Hypoxia-induced chemoresistance to 5-FU was overcome in both cell lines treated with this suicide gene therapy approach. Hypoxic transfectants were sensitized to prodrug concentrations that were ten-fold lower than those that are clinically relevant. Moreover, the surviving fraction of reoxygenated transfectants could be further reduced with the concomitant delivery of clinically relevant single radiation doses. This strategy thus has the potential to sensitize the hypoxic compartment of prostate tumours and improve the outcome of current therapies.

Radiosensitivity of prostate cancer cells is enhanced by EGFR inhibitor C225

Purpose To determine the direct effects of the epidermal growth factor receptor (EGFR) inhibitor C225 on the radiosensitivity of human prostate cancer cells. Experimental design Human prostate cancer DU145 cells were irradiated with 60Co (1.953 Gy/min) at various doses in the presence or absence of C225. The cellular proliferation and cell-survival rate were evaluated by MTT and colony-forming assays after irradiation. The cell-cycle distribution, cell apoptosis, and MAPK expression were investigated using FCM. The expression of Cyclin D1, CDK2, CDK4, and Survivin were determined by RT-PCR. Results The RBE in the C225 group compared with that in the control group was 1.39. Cells treated with C225 and irradiated at 4 Gy predominantly exhibited G 0/G 1 phase arrest and significant decrease in the fraction of cells in the S phase in comparison with those in the control cells, respectively. An evidently higher apoptosis rate on irradiation at 4 Gy was observed in C225-treated cells compared with that in the control cells. Decreased cell proliferation and increased cell death were further supported by the down-regulation of cyclin D1, CDK2, CDK4, and survivin in C225-treated DU145 cells, as determined by RT-PCR. Furthermore, C225 significantly inhibited the phosphorylation of P38-MAPK in DU145 cells. Conclusions The EGFR inhibitor C225 increased the radiosensitivity of DU145 cells through antiproliferative effect, inhibition of clonal growth, G 0/G 1 phase arrest, apoptosis induction, and inhibition of EGFR-signaling pathways by the down-regulation of MAPK activation.

Inhibition of p21-activated Kinase 6 (PAK6) Increases Radiosensitivity of Prostate Cancer Cells

For patients with high-risk localized prostate cancer, biochemical and clinical relapse after radiotherapy remains a significant problem. New therapeutic target for radiation sensitization is needed. p21-activated kinase 6 (PAK6), a serine/threonine kinase belonging to the p21-activated kinase (PAK) family, was first identified as an androgen receptor (AR)-interacting protein able to inhibit AR-mediated transcriptional responses. In this study, we investigated the role of PAK6 in radiation-induced cell death in human PC-3 and DU-145 prostate cancer cells. We used a short hairpin RNA (shRNA) strategy to stably knock down PAK6 in PC-3 and DU-145 cells. Radiation sensitivities were compared in PAK6 stably knockdown cells and in scrambled shRNA-expressing cells. 6 Gy of irradiation upregulated PAK6 mRNA and protein levels in PC-3 and DU-145 cells. Transfection with PAK6 shRNA resulted in a significant decrease of PAK6 mRNA and protein expression. After irradiation, an increased percentage of apoptotic cells and an increased cleaved caspase 3 level in parallel with a decreased cell viability and a reduced clonogenic survival was shown. Furthermore, transfection with PAK6 shRNA blocked cells in a more radiosensitive G2/M phase and increased levels of DNA double-strand breaks as indicated by a higher amount of γ-H2AX in irradiated cells. We further explored the potential mechanisms by which PAK6 mediates resistance to radiation-induced apoptosis. Inhibition of PAK6 caused a decrease in Ser112 phosphorylation of BAD, a proapoptotic member of the Bcl-2 family, which led to enhanced binding of BAD to Bcl-2 and Bcl-XL and release of cytochrome c culminating into caspase activation and cell apoptosis. The combination of PAK6 inhibition and irradiation resulted in significantly decreased survival of prostate cancer cells. The underlying mechanisms by which targeting Pak6 may improve radiation response seem to be multifaceted, and involve alterations in cell cycle distribution and impaired DNA double-strand break repair as well as relieved BAD phosphorylation. Additional studies are ongoing to validate PAK6 as a target for radiation sensitization in animal models of prostate cancer.

The Enhanced Radiosensitivity of Prostate Cancer Cells by EGFR Inhibitor C225

The Enhanced Radiosensitivity of Prostate Cancer Cells by EGFR Inhibitor C225

Triple Suicide Gene Therapy Radiosensitizes Prostate Cancer Cells

The double suicide gene therapy strategy combining herpes simplex virus type-1 thymidine kinase (HSV-1 TK) and cytosine deaminase (CD) with ganciclovir (GCV) and 5-fluorocytosine (5-FC) had been carried out in a phase I clinical trail for prostate cancer. The objective of the present study was to ascertain whether co-expression of HSV-1 TK, CD, and uracil phosphoribosyltransferase (UPRT) in conjunction with GCV and 5-FC treatment, a triple suicide gene therapy strategy, would be more effective at killing and radiosensitizing prostate cancer cells than double suicide gene therapy.

Targeting Cdc37 Radiosensitizes Prostate Cancer Cells by Inhibiting Multiple Kinase Cascades

Targeting Cdc37 Radiosensitizes Prostate Cancer Cells by Inhibiting Multiple Kinase Cascades

SN52, a novel nuclear factor-κB inhibitor, blocks nuclear import of RelB:p52 dimer and sensitizes prostate cancer cells to ionizing radiation

The activation of nuclear factor-kappaB (NF-kappaB) is thought to protect cancer cells against therapy-induced cytotoxicity. RelB, a member of the NF-kappaB family in the alternative pathway, is uniquely expressed at a high level in prostate cancer with high Gleason scores. Here, we show that ionizing radiation (IR) enhances nuclear import of RelB, leading to up-regulation of its target gene, manganese superoxide dismutase (MnSOD), and renders prostate cancer cells resistant to IR. To selectively block RelB nuclear import, we designed a cell-permeable SN52 peptide, a variant of the SN50 peptide that has been shown to block nuclear import of NF-kappaB family members in the classic pathway. Inhibition of IR-induced NF-kappaB activation by SN50 and SN52 was achieved by selectively interrupting the association of p50 and p52 with nuclear import factors importin-alpha1 and importin-beta1. Importantly, SN52 seems to be more efficient for radiosensitization of prostate cancer cells at clinically relevant radiation doses and has less cytotoxicity to normal prostate epithelial cells compared with the toxicity observed with SN50. These results suggest that targeting the alternative pathway is a promising approach to selectively radiosensitize prostate cancers and that SN52 may serve as a prototype biological agent for sensitizing prostate cancers to clinically relevant doses of IR.

PRIMA-1 met radiosensitizes prostate cancer cells independent of their MTp53-status

The novel agent PRIMA-1 met can reactivate WTp53 function in MTp53-expressing cells. We investigated PRIMA-1 met as a radiosensitizer of WTp53, p53Null or MTp53 prostate cancer cells. Radiosensitization was observed in PC3 (p53Null) cells, even under hypoxia. In certain circumstances, PRIMA-1 met may therefore act independently of MTp53 status and WTp53 transactivation.

No supra-additive effects of goserelin and radiotherapy on clonogenic survival of prostate carcinoma cells in vitro

BackgroundOncological results of radiotherapy for locally advanced prostate cancer (PC) are significantly improved by simultaneous application of LHRH analoga (e.g. goserelin). As 85% of PC express LHRH receptors, we investigated the interaction of goserelin incubation with radiotherapy under androgen-deprived conditions in vitro.MethodsLNCaP and PC-3 cells were stained for LHRH receptors. Downstream the LHRH receptor, changes in protein expression of c-fos, phosphorylated p38 and phosphorylated ERK1/2 were analyzed by means of Western blotting after incubation with goserelin and irradiation with 4 Gy. Both cell lines were incubated with different concentrations of goserelin in hormone-free medium. 12 h later cells were irradiated (0 – 4 Gy) and after 12 h goserelin was withdrawn. Endpoints were clonogenic survival and cell viability (12 h, 36 h and 60 h after irradiation).ResultsBoth tested cell lines expressed LHRH-receptors. Changes in protein expression demonstrated the functional activity of goserelin in the tested cell lines. Neither in LNCaP nor in PC-3 any significant effects of additional goserelin incubation on clonogenic survival or cell viability for all tested concentrations in comparison to radiation alone were seen.ConclusionThe clinically observed increase in tumor control after combination of goserelin with radiotherapy in PC cannot be attributed to an increase in radiosensitivity of PC cells by goserelin in vitro.

Suppression of RelB-mediated manganese superoxide dismutase expression reveals a primary mechanism for radiosensitization effect of 1α,25-dihydroxyvitamin D3 in prostate cancer cells

Nuclear factor-kappaB provides an adaptive response to protect cancer cells against cytotoxicity induced by redox active therapeutics. RelB is uniquely expressed at a high level in prostate cancer with high Gleason scores. Recently, we showed that the level of RelB rapidly increases in androgen-independent prostate cancer cells after exposure to ionizing radiation (IR), leading to a reduction in intrinsic radiosensitivity. Here, we show that interaction of 1alpha,25-dihydroxyvitamin D(3) [1alpha,25-(OH)(2)D(3)] with the vitamin D receptor significantly enhances radiosensitivity of prostate cancer cells at clinically relevant radiation doses. The radiosensitization effect of 1alpha,25-(OH)(2)D(3) is mediated, at least in part, by selectively suppressing IR-mediated RelB activation, leading to a reduced expression of its target gene MnSOD, a primary antioxidant enzyme in mitochondria. These results suggest that suppression of manganese superoxide dismutase is a mechanism by which 1alpha,25-(OH)(2)D(3) exerts its radiosensitization effect and that 1alpha,25-(OH)(2)D(3) may serve as an effective pharmacologic agent for selectively sensitizing prostate cancer cells to IR via suppression of antioxidant responses in mitochondria.

A Dominant Role for p53-Dependent Cellular Senescence in Radiosensitization of Human Prostate Cancer Cells

Because p53 inactivation may limit the effectiveness of radiation therapy for localized prostate cancer, it is important to understand how this gene regulates clonogenic survival after an exposure to ionizing radiation. Here, we show that premature cellular senescence is the principal mode of cell death accounting for the radiosensitivity of human prostate cancer cell lines retaining p53 function. Alternative stress response pathways controlled by this tumor suppressor, including cell cycle arrest, DNA damage repair, mitotic catastrophe and apoptosis, contributed significantly less to radiation-induced clonogenic death. Using a dominant negative C-terminal fragment of p53, we present the first evidence that a complete loss of endogenous p53 function is sufficient to limit the irradiation-induced senescence and clonogenic death of prostate cancer cells. Conversely, inheritance of wild-type p53 by prostate cancer cells lacking a functional allele of this gene (i.e., DU145) significantly increases clonogenic death through p53-dependent cellular senescence and apoptotic pathways. Our data provide evidence that mutations of even one p53 allele may be sufficient to alter their clonogenic fate. In addition, they support the idea that the p53 pathway can be used as a specific target for enhancing the radiosensitivity of prostate cancer cells. Activation of p53 by the drug nutlin-3 is shown to be an effective radiosensitizer of prostate cancer cells retaining functional alleles of p53 and this effect was entirely attributable to an increased induction of p53-dependent cellular senescence.