Cell death in irradiated prostate epithelial cells: role of apoptotic and clonogenic cell kill

Therapy-induced senescence as a component of tumor biology: Evidence from clinical cancer

Therapy-Induced Senescence (TIS) is an established response to anticancer therapy in a variety of cancer models. Ample evidence has characterized the triggers, hallmarks, and functional outcomes of TIS in preclinical studies; however, limited evidence delineates TIS in clinical cancer (human tumor samples). We examined the literature that investigated the induction of TIS in samples derived from human cancers and highlighted the major findings that suggested that TIS represents a main constituent of tumor biology. The most frequently utilized approach to identify TIS in human cancers was to investigate the protein expression of senescence-associated markers (such as cyclins, cyclin-dependent kinase inhibitors, Ki67, DNA damage repair response markers, DEC1, and DcR1) via immunohistochemical techniques using formalin-fixed paraffin-embedded (FFPE) tissue samples and/or testing the upregulation of Senescence-Associated β-galactosidase (SA-β-gal) in frozen sections of unfixed tumor samples. Collectively, and in studies where the extent of TIS was determined, TIS was detected in 31-66% of tumors exposed to various forms of chemotherapy. Moreover, TIS was not only limited to both malignant and non-malignant components of tumoral tissue but was also identified in samples of normal (non-transformed) tissue upon chemo- or radiotherapy exposure. Nevertheless, the available evidence continues to be limited and requires a more rigorous assessment of in vivo senescence based on novel approaches and more reliable molecular signatures. The accurate assessment of TIS will be beneficial for determining its relevant contribution to the overall outcome of cancer therapy and the potential translatability of senotherapeutics.

Distinct mechanisms mediating therapy-induced cellular senescence in prostate cancer

BACKGROUND

Prostate cancer (PCa) is an age-related malignancy in men with a high incidence rate. PCa treatments face many obstacles due to cancer cell resistance and many bypassing mechanisms to escape therapy. According to the intricacy of PCa, many standard therapies are being used depending on PCa stages including radical prostatectomy, radiation therapy, androgen receptor (AR) targeted therapy (androgen deprivation therapy, supraphysiological androgen, and AR antagonists) and chemotherapy. Most of the aforementioned therapies have been implicated to induce cellular senescence. Cellular senescence is defined as a stable cell cycle arrest in the G1 phase and is one of the mechanisms that prevent cancer proliferation.

RESULTS

In this review, we provide and analyze different mechanisms of therapy-induced senescence (TIS) in PCa and their effects on the tumor. Interestingly, it seems that different molecular pathways are used by cancer cells for TIS. Understanding the complexity and underlying mechanisms of cellular senescence is very critical due to its role in tumorigenesis. The most prevalent analyzed pathways in PCa as TIS are the p53/p21^WAF1/CIP1, the p15^INK4B/p16^INK4A/pRb/E2F/Cyclin D, the ROS/ERK, p27^Kip1/CDK/pRb, and the p27^Kip1/Skp2/C/EBP β signaling. Despite growth inhibition, senescent cells are highly metabolically active. In addition, their secretome, which is termed senescence-associated secretory phenotype (SASP), affects within the tumor microenvironment neighboring non-tumor and tumor cells and thereby may regulate the growth of tumors. Induction of cancer cell senescence is therefore a double-edged sword that can lead to reduced or enhanced tumor growth.

CONCLUSION

Thus, dependent on the type of senescence inducer and the specific senescence-induced cellular pathway, it is useful to develop pathway-specific senolytic compounds to specifically targeting senescent cells in order to evict senescent cells and thereby to reduce SASP side effects.

The MTT Assay: Utility, Limitations, Pitfalls, and Interpretation in Bulk and Single-Cell Analysis

The MTT assay for cellular metabolic activity is almost ubiquitous to studies of cell toxicity; however, it is commonly applied and interpreted erroneously. We investigated the applicability and limitations of the MTT assay in representing treatment toxicity, cell viability, and metabolic activity. We evaluated the effect of potential confounding variables on the MTT assay measurements on a prostate cancer cell line (PC-3) including cell seeding number, MTT concentration, MTT incubation time, serum starvation, cell culture media composition, released intracellular contents (cell lysate and secretome), and extrusion of formazan to the extracellular space. We also assessed the confounding effect of polyethylene glycol (PEG)-coated gold nanoparticles (Au-NPs) as a tested treatment in PC-3 cells on the assay measurements. We additionally evaluated the applicability of microscopic image cytometry as a tool for measuring intracellular MTT reduction at the single-cell level. Our findings show that the assay measurements are a result of a complicated process dependant on many of the above-mentioned factors, and therefore, optimization of the assay and rational interpretation of the data is necessary to prevent misleading conclusions on variables such as cell viability, treatment toxicity, and/or cell metabolism. We conclude, with recommendations on how to apply the assay and a perspective on where the utility of the assay is a powerful tool, but likewise where it has limitations.

Functional and mutational analysis after radiation and cetuximab treatment on prostate carcinoma cell line DU145

Background

Epidermal Growth Factor Receptor is often overexpressed in advanced prostate carcinoma. In-vitro-studies in prostate carcinoma cell line DU145 have demonstrated increased sensibility to radiation after cetuximab treatment, but clinical data are not sufficient to date.

Methods

We analyzed effects of radiation and cetuximab in DU145 and A431 using proliferation, colony-forming-unit- and annexin-V-apoptosis-assays. Changes in protein expression of pEGFR and pERK1/2 after radiation and cetuximab treatment were analyzed. Using NGS we also investigated the impact of cetuximab long-term treatment.

Results

Cell counts in DU145 were reduced by 44% after 4 Gy (p = 0.006) and 55% after 4 Gy and cetuximab (p < 0.001). The surviving fraction (SF) was 0.69 after 2 Gy, 0.41 after 4 Gy and 0.15 after 6 Gy (each p < 0.001). Cetuximab treatment did not alter significantly growth reduction in 4 Gy radiated DU145 cells, p > 0.05 or SF, p > 0.05, but minor effects on apoptotic cell fraction in DU145 were detected. Using western blot, there were no detectable pEGFR and pERK1/2 protein signals after cetuximab treatment. No RAS mutation or HER2 amplification was detected, however a TP53 gen-mutation c.820G > T was found.

Conclusions

Radiation inhibits cell-proliferation and colony-growth and induces apoptosis in DU145. Despite blocking MAP-Kinase-pathway using cetuximab, no significant radiation-sensitizing-effect was detected. Cetuximab treatment did not induce resistance-mutations. Further research must clarify which combination of anti-EGFR treatment strategies can increase radiation-sensitizing-effects.

When dormancy fuels tumour relapse

Tumour recurrence is a serious impediment to cancer treatment, but the mechanisms involved are poorly understood. The most frequently used anti-tumour therapies-chemotherapy and radiotherapy-target highly proliferative cancer cells. However non- or slow-proliferative dormant cancer cells can persist after treatment, eventually causing tumour relapse. Whereas the reversible growth arrest mechanism allows quiescent cells to re-enter the cell cycle, senescent cells are largely thought to be irreversibly arrested, and may instead contribute to tumour growth and relapse through paracrine signalling mechanisms. Thus, due to the differences in their growth arrest mechanism, metabolic features, plasticity and adaptation to their respective tumour microenvironment, dormant-senescent and -quiescent cancer cells could have different but complementary roles in fuelling tumour growth. In this review article, we discuss the implication of dormant cancer cells in tumour relapse and the need to understand how quiescent and senescent cells, respectively, may play a part in this process.

MicroRNAs as Epigenetic Determinants of Treatment Response and Potential Therapeutic Targets in Prostate Cancer

Prostate cancer (PCa) is the second most common tumor in men worldwide, and the fifth leading cause of male cancer-related deaths in western countries. PC is a very heterogeneous disease, meaning that optimal clinical management of individual patients is challenging. Depending on disease grade and stage, patients can be followed in active surveillance protocols or undergo surgery, radiotherapy, hormonal therapy, and chemotherapy. Although therapeutic advancements exist in both radiatiotherapy and chemotherapy, in a considerable proportion of patients, the treatment remains unsuccessful, mainly due to tumor poor responsiveness and/or recurrence and metastasis. microRNAs (miRNAs), small noncoding RNAs that epigenetically regulate gene expression, are essential actors in multiple tumor-related processes, including apoptosis, cell growth and proliferation, autophagy, epithelial-to-mesenchymal transition, invasion, and metastasis. Given that these processes are deeply involved in cell response to anti-cancer treatments, miRNAs have been considered as key determinants of tumor treatment response. In this review, we provide an overview on main PCa-related miRNAs and describe the biological mechanisms by which specific miRNAs concur to determine PCa response to radiation and drug therapy. Additionally, we illustrate whether miRNAs can be considered novel therapeutic targets or tools on the basis of the consequences of their expression modulation in PCa experimental models.

DNA Damage- But Not Enzalutamide-Induced Senescence in Prostate Cancer Promotes Senolytic Bcl-xL Inhibitor Sensitivity

Cellular senescence is a natural tumor suppression mechanism defined by a stable proliferation arrest. In the context of cancer treatment, cancer cell therapy-induced senescence (TIS) is emerging as an omnipresent cell fate decision that can be pharmacologically targeted at the molecular level to enhance the beneficial aspects of senescence. In prostate cancer (PCa), TIS has been reported using multiple different model systems, and a more systematic analysis would be useful to identify relevant senescence manipulation molecular targets. Here we show that a spectrum of PCa senescence phenotypes can be induced by clinically relevant therapies. We found that DNA damage inducers like irradiation and poly (ADP-ribose) polymerase1 (PARP) inhibitors triggered a stable PCa-TIS independent of the p53 status. On the other hand, enzalutamide triggered a reversible senescence-like state that lacked evidence of cell death or DNA damage. Using a small senolytic drug panel, we found that senescence inducers dictated senolytic sensitivity. While Bcl-2 family anti-apoptotic inhibitor were lethal for PCa-TIS cells harboring evidence of DNA damage, they were ineffective against enzalutamide-TIS cells. Interestingly, piperlongumine, which was described as a senolytic, acted as a senomorphic to enhance enzalutamide-TIS proliferation arrest without promoting cell death. Overall, our results suggest that TIS phenotypic hallmarks need to be evaluated in a context-dependent manner because they can vary with senescence inducers, even within identical cancer cell populations. Defining this context-dependent spectrum of senescence phenotypes is key to determining subsequent molecular strategies that target senescent cancer cells.

microRNAs identified in prostate cancer: Correlative studies on response to ionizing radiation

As the most frequently diagnosed non-skin cancer in men and a leading cause of cancer-related death, understanding the molecular mechanisms that drive treatment resistance in prostate cancer poses a significant clinical need. Radiotherapy is one of the most widely used treatments for prostate cancer, along with surgery, hormone therapy, and chemotherapy. However, inherent radioresistance of tumor cells can reduce local control and ultimately lead to poor patient outcomes, such as recurrence, metastasis and death. The underlying mechanisms of radioresistance have not been fully elucidated, but it has been suggested that miRNAs play a critical role. miRNAs are small non-coding RNAs that regulate gene expression in every signaling pathway of the cell, with one miRNA often having multiple targets. By fine-tuning gene expression, miRNAs are important players in modulating DNA damage response, cell death, tumor aggression and the tumor microenvironment, and can ultimately affect a tumor's response to radiotherapy. Furthermore, much interest has focused on miRNAs found in biofluids and their potential utility in various clinical applications. In this review, we summarize the current knowledge on miRNA deregulation after irradiation and the associated functional outcomes, with a focus on prostate cancer. In addition, we discuss the utility of circulating miRNAs as non-invasive biomarkers to diagnose, predict response to treatment, and prognosticate patient outcomes.

αvβ3 Integrin Mediates Radioresistance of Prostate Cancer Cells through Regulation of Survivin

The α_vβ₃ integrin is involved in various physiologic and pathologic processes such as wound healing, angiogenesis, tumor growth, and metastasis. The impact of α_vβ₃ integrin on the radiosensitivity of prostate cancer cells and the molecular mechanism controlling cell survival in response to ionizing radiation (IR) was investigated. Both LNCaP cells stably transfected with α_vβ₃ integrin and PC-3 cells that contain endogenous β₃ integrin were used. This study demonstrated that α_vβ₃ integrin increases survival of α_vβ₃-LNCaP cells upon IR while small hairpin RNA (shRNA)-mediated knockdown of α_vβ₃ integrin in PC-3 cells sensitizes to radiation. Expression of α_vβ₃ integrin in LNCaP cells also enhances anchorage-independent cell growth while knockdown of α_vβ₃ integrin in PC-3 cells inhibits anchorage-independent cell growth. The α_vβ₃ antagonist, cRGD, significantly increases radiosensitivity in both α_vβ₃-LNCaP and PC-3 cells. Moreover, α_vβ₃ integrin prevents radiation-induced downregulation of survivin. Inhibition of survivin expression by siRNA or shRNA enhances IR-induced inhibition of anchorage-independent cell growth. Overexpression of wild-type survivin in PC-3 cells treated with α_vβ₃ integrin shRNA increases survival of cells upon IR. These findings reveal that α_vβ₃ integrin promotes radioresistance and regulates survivin levels in response to IR. IMPLICATIONS: Future translational research on targeting α_vβ₃ integrin and survivin may reveal novel approaches as an adjunct to radiotherapy for patients with prostate cancer.

Dipeptidyl Vinyl Sulfone as a Novel Chemical Tool to Inhibit HMGB1/NLRP3-Inflammasome and Inflamma-miRs in Aβ-Mediated Microglial Inflammation

Rapid microglial activation and associated inflammatory pathways contribute to immune-defense and tissue repair in the central nervous system (CNS). However, persistent activation of these cells will ultimately result in vast production of pro-inflammatory mediators and other neurotoxic factors, which may induce neuronal damage and contribute to chronic neurodegenerative diseases, as Alzheimer's disease (AD). Therefore, small molecules with immunomodulatory effects on microglia may be considered as potential tools to counteract their proinflammatory phenotype and neuroimmune dysregulation in such disorders. Indeed, reducing amyloid-β (Aβ)-induced microglia activation is believed to be effective in treating AD. In this study, we investigated whether dipeptidyl vinyl sulfone (VS) was able to attenuate Aβ-mediated inflammatory response using a mouse microglial (N9) cell line and a solution containing a mixture of Aβ aggregates. We show that low levels of VS are able to prevent cell death while reducing microglia phagocytosis upon Aβ treatment. VS also suppressed Aβ-induced expression of inflammatory mediators in microglia, such as matrix metalloproteinase (MMP)-2 and MMP-9, as well as high-mobility group box protein-1 (HMGB1), nod-like receptor protein 3 (NLRP3)-inflammasome, and interleukin (IL)-1β. Interestingly, increased expression of the two critical inflammation-related microRNAs (miR)-155 and miR-146a in microglia upon Aβ treatment was also prevented by VS coincubation. Taken together, VS emerges as a potential new therapeutic strategy worthy of further investigation in improved cellular and animal models of AD.

In vivo studies of the PARP inhibitor, AZD-2281, in combination with fractionated radiotherapy: An exploration of the therapeutic ratio

BACKGROUND AND PURPOSE

Pre-clinical data have shown that PARP inhibitors (PARPi) may increase the efficacy of radiotherapy in prostate cancer. However, it is uncertain as to whether PARPi lead to clonogenic kill when combined with radiotherapy (RT).

MATERIAL AND METHODS

We tested the PARP inhibitor AZD-2281 as a radiosensitizing agent under oxic and hypoxic conditions for clonogenic survival in vitro and in vivo using the human prostate cancer cell line, 22Rv1. In addition, the effects of PARPi+RT on normal tissue were investigated using a crypt clonogenic assay.

RESULTS

AZD-2281 inhibited cellular PARP activity under both oxic and hypoxic conditions. The addition of AZD-2281 radiosensitized 22Rv1 cells under oxia, acute hypoxia and chronic hypoxia in vitro. The combination of AZD-2281 with fractionated radiotherapy resulted in a significant growth delay and clonogenic kill in vivo. No increased gut toxicity was observed using this combined PARPi+radiotherapy regimen.

CONCLUSIONS

This is the first preclinical study to demonstrate direct clonogenic kill in vivo by the addition of AZD-2281 to radiotherapy. As we did not observe gut toxicity, the use of PARPi in the context of prostate cancer radiotherapy warrants further investigation in clinical trials.

Nrf2 sensitizes prostate cancer cells to radiation via decreasing basal ROS levels

Androgen deprivation therapy (ADT) was reported to lower basal ROS level in prostate cancer (PCa) and to sensitize PCa to radiation. We aimed to seek for the underlying molecular mechanism and to develop novel additive treatments to ADT in this regard. We simulated human androgen milieu in vitro and tested the ROS level in PCa cells undergoing ADT. We also tested the Nrf2 level in PCa cells with or without ADT. Genetic and pharmaceutical upregulation of Nrf2 was applied in vitro and in vivo in transgenic adenocarcinoma of the mouse prostate (TRAMP) mice with or without castration to investigate whether Nrf2 overexpression supplemented the effect of ADT in PCa. We first discovered that androgen deprivation increased basal ROS level in PCa cells with AR expression. We then found that genetic Nrf2 upregulation lowered basal ROS similar to ADT. Also, SFN sensitized PCa cell to radiation via upregulation of Nrf2. We then found that Nrf2 level in control TRAMP groups was lower than castration or SFN groups. The SFN treated TRAMP mice showed similar level of Nrf2 to castration. Genetic and pharmaceutical upregulation of Nrf2 lowered the ROS in PCa cells and sensitized PCa cells to radiation similar to ADT, implicating possible administration of SFN in place of ADT for PCa patients requiring radiotherapy.

Hypoxia provokes base excision repair changes and a repair-deficient, mutator phenotype in colorectal cancer cells

Regions of acute and chronic hypoxia exist within solid tumors and can lead to increased rates of mutagenesis and/or altered DNA damage and repair protein expression. Base excision repair (BER) is responsible for resolving small, non-helix-distorting lesions from the genome that potentially cause mutations by mispairing or promoting DNA breaks during replication. Germline and somatic mutations in BER genes, such as MutY Homolog (MUTYH/MYH) and DNA-directed polymerase (POLB), are associated with increased risk of colorectal cancer. However, very little is known about the expression and function of BER proteins under hypoxic stress. Using conditions of chronic hypoxia, decreased expression of BER proteins was observed because of a mechanism involving suppressed BER protein synthesis in multiple colorectal cancer cell lines. Functional BER was impaired as determined by MYH- and 8-oxoguanine (OGG1)-specific glycosylase assays. A formamidopyrimidine-DNA glycosylase (Fpg) Comet assay revealed elevated residual DNA base damage in hypoxic cells 24 hours after H2O2 treatment as compared with normoxic controls. Similarly, high-performance liquid chromatography analysis demonstrated that 8-oxo-2'-deoxyguanosine lesions were elevated in hypoxic cells 3 and 24 hours after potassium bromate (KBrO3) treatment when compared with aerobic cells. Correspondingly, decreased clonogenic survival was observed following exposure to the DNA base damaging agents H2O2 and MMS, but not to the microtubule interfering agent paclitaxel. Thus, a persistent downregulation of BER components by the microenvironment modifies and facilitates a mutator phenotype, driving genetic instability and cancer progression.

IMPLICATIONS

Aberrant BER is a contributing factor for the observed genetic instability in hypoxic tumor cells.

Radiosensitization of Pancreatic Cancer Cells In Vitro and In Vivo through Poly (ADP-ribose) Polymerase Inhibition with ABT-888

OBJECTIVES

To determine whether poly (ADP-ribose) polymerase-1/2 (PARP-1/2) inhibition enhances radiation-induced cytotoxicity of pancreatic adenocarcinoma in vitro and in vivo, and the mechanism by which this occurs.

METHODS

Pancreatic carcinoma cells were treated with ABT-888, radiation, or both. In vitro cell viability, apoptosis, and PARP activity were measured. Orthotopic xenografts were generated in athymic mice and treated with ABT-888 (25mg/kg), radiation (5Gy), both, or no treatment. Mice were monitored with bioluminescence imaging.

RESULTS

In vitro, treatment with ABT-888 and radiation led to higher rates of cell death after 8days (P < .01). Co-treatment with 5Gy and 1, 10 or 100μmol/l of ABT-888 led to dose enhancement factors of 1.29, 1.41 and 2.36, respectively. Caspase activity was not significantly increased when treated with ABT-888 (10 μmol/l) alone (1.28-fold, P = .08), but became significant when radiation was added (2.03-fold, P < .01). PARP activity increased post-radiation and was abrogated following co-treatment with ABT-888. In vivo, treatment with ABT-888, radiation or both led to tumor growth inhibition (TGI) of 8, 30 and 39days, and survival at 60days of 0%, 0% and 40%, respectively.

CONCLUSIONS

ABT-888 with radiation significantly enhanced tumor response in vitro and in vivo. ABT-888 inhibited PAR protein polymerization resulting in dose-dependent feedback up-regulation of PARP and p-ATM suggesting increased DNA damage. This translated into enhancement in TGI and survival with radiation in vivo. In vitro PAR levels correlated with levels of tumor apoptosis suggesting potential as a predictive biomarker. These data are being used to support a Phase I study in locally advanced pancreatic cancer.

Experimental investigation of the cell survival in dose cold spot

The aim of this work was to investigate the impact of intercellular contact during radiation exposure on cell survival in regions of reduced dose. Methods. The PC3 human prostate adenocarcinoma cell line was irradiated using a 6 MV x-ray beam to assess clonogenic cell deaths with the specific aim to investigate cell survival in a dose cold spot. Radiation-induced cell survival in a 20% lower dose region, compared to that of cells receiving 100% of the prescribed dose (2 Gy), was assessed for experimental set-ups when under-irradiated cells were either in direct contact with cells receiving 2 Gy or irradiated separately. In addition, the results were compared against non-irradiated controls. Results. A significant (p < 0.001) decrease in cell survival was found when cells, collocated in the same flask, received either 100% or 80% of the prescribed dose (the dose distribution contained a cold spot of 20% lower dose) compared to non-irradiated cells. However, in the experiment in which the entire flask was exposed to only 80% of the prescribed dose, the mean difference in cell survival compared to non-irradiated control was not significant (p > 0.05). This was contrary to a significant decrease (p < 0.001) in survival of cells receiving 100% of the prescribed dose versus the control. Additionally, significant reduction (p < 0.05) in cell survival was observed for cells which were under-irradiated by 20% but collocated in the same flask with cells receiving 100% dose compared to cells where the entire flask was irradiated with 80% of the prescribed dose. Conclusion. For the given cell line, under existing growing and treatment conditions, the cell survival in the dose cold spot region was significantly lower when under-irradiated cells were in contact with the cells receiving 100% of the prescribed dose compared to survival of cells under-irradiated by the same amount of radiation but treated separately to cells receiving 100% dose.

Improved functionality of the vasculature during conventionally fractionated radiation therapy of prostate cancer

Although endothelial cell apoptosis participates in the tumor shrinkage after single high-dose radiotherapy, little is known regarding the vascular response after conventionally fractionated radiation therapy. Therefore, we evaluated hypoxia, perfusion and vascular microenvironment changes in an orthotopic prostate cancer model of conventionally fractionated radiation therapy at clinically relevant doses (2 Gy fractions, 5 fractions/week). First, conventionally fractionated radiation therapy decreased tumor cell proliferation and increased cell death with kinetics comparable to human prostate cancer radiotherapy. Secondly, the injection of Hoechst 33342 or fluorescent-dextrans showed an increased tumor perfusion within 14 days in irradiated tumors, which was correlated with a clear reduction of hypoxia. Improved perfusion and decreased hypoxia were not explained by increased blood vessel density, size or network morphology. However, a tumor vascular maturation defined by perivascular desmin+/SMA+ cells coverage was clearly observed along with an increase in endothelial, zonula occludens (ZO)-1 positive, intercellular junctions. Our results show that, in addition to tumor cell killing, vascular maturation plays an uncovered role in tumor reoxygenation during fractionated radiation therapy.

HDAC inhibitor confers radiosensitivity to prostate stem-like cells

Background:

Radiotherapy can be an effective treatment for prostate cancer, but radiorecurrent tumours do develop. Considering prostate cancer heterogeneity, we hypothesised that primitive stem-like cells may constitute the radiation-resistant fraction.

Methods:

Primary cultures were derived from patients undergoing resection for prostate cancer or benign prostatic hyperplasia. After short-term culture, three populations of cells were sorted, reflecting the prostate epithelial hierarchy, namely stem-like cells (SCs, α₂β₁integrin^hi/CD133⁺), transit-amplifying (TA, α₂β₁integrin^hi/CD133⁻) and committed basal (CB, α₂β₁integrin^lo) cells. Radiosensitivity was measured by colony-forming efficiency (CFE) and DNA damage by comet assay and DNA damage foci quantification. Immunofluorescence and flow cytometry were used to measure heterochromatin. The HDAC (histone deacetylase) inhibitor Trichostatin A was used as a radiosensitiser.

Results:

Stem-like cells had increased CFE post irradiation compared with the more differentiated cells (TA and CB). The SC population sustained fewer lethal double-strand breaks than either TA or CB cells, which correlated with SCs being less proliferative and having increased levels of heterochromatin. Finally, treatment with an HDAC inhibitor sensitised the SCs to radiation.

Interpretation:

Prostate SCs are more radioresistant than more differentiated cell populations. We suggest that the primitive cells survive radiation therapy and that pre-treatment with HDAC inhibitors may sensitise this resistant fraction.

Contemporary issues in radiotherapy for clinically localized prostate cancer

Radiotherapy is a valid curative alternative to surgery for prostate cancer. However, patient selection is critical to ensure patients obtain benefits from therapy delivered with curative intent. Dose-escalated radiation has been shown to improve patient outcomes, facilitated by development of robust image guidance and better target delineation imaging technologies. These concepts have also rekindled interest in hypofractionated radiotherapy in the forms of stereotactic body radiotherapy and brachytherapy. Postprostatectomy radiotherapy also improves long-term biochemical outcome in men at high risk of local recurrence.

Radiosensitization of prostate cancer cells by the dual PI3K/mTOR inhibitor BEZ235 under normoxic and hypoxic conditions

BACKGROUND AND PURPOSE

Despite appropriate radiotherapy, high-risk prostate cancer patients often experience local relapse and progression to metastatic disease. Radioresistance may be due to tumor-hypoxia but also due to the PTEN mutation/deletion present in 70% prostate cancers. We investigated whether the novel PI3K/mTOR inhibitor BEZ235 might sensitize prostate cancer cells to radiation and reduce hypoxia-induced radioresistance.

MATERIALS AND METHODS

The potential radiosensitizing properties of BEZ235 were investigated in vitro and in vivo using two prostate cancer cell lines, PC3 (PTEN(-/-)) and DU145 (PTEN(+/+)), under normoxic (21% O(2)) and hypoxic (0.5% O(2)) conditions.

RESULTS

BEZ235 rapidly inhibited PI3K and mTOR signaling in a dose dependent manner and limited tumor cell proliferation and clonogenic survival in both cell lines independently of PTEN status. In vivo, BEZ235 pretreatment enhanced the efficacy of radiation therapy on PC3 xenograft tumors in mice without inducing intestinal radiotoxicity. In culture, BEZ235 radiosensitized both cell lines in a comparable manner. Moreover, BEZ235 inhibited PI3K/mTOR activation and radiosensitized both cell lines under normoxia and hypoxia. BEZ235 radiosensitizing effects correlated with a decrease in γH2AX foci repair and increased G2/M cell cycle arrest.

CONCLUSIONS

BEZ235 is a potent radiosensitizer of normoxic and hypoxic prostate cancer cells.

Quantitative ultrasound for the monitoring of novel microbubble and ultrasound radiosensitization

There is a need for cancer imaging to provide "real-time" information about the metabolic and cellular responses of tumours. Quantitative ultrasound techniques have recently been demonstrated to be a potential method of assessing tumour response at the cellular level. Anti-cancer treatments administered to xenograft-bearing mice consisted of radiotherapy and a novel antivascular therapy utilizing encapsulated microbubble agents in the presence of ultrasound. Radiation dose and microbubble concentrations were varied and the treatment modalities were given in combination to assess the possible enhancement of tumour cell death. Quantitative methods were used to non-invasively assess responses. Results demonstrated statistically significant changes in backscatter parameters (midband fit, spectral intercept) in tumours treated with high doses of radiotherapy or a high concentration of microbubbles. Combined treatments demonstrated further increases in ultrasound parameters. Histopathologic assessment was used and tumour cell death was found to correlate with increases in ultrasound parameters.

Chronic hypoxia compromises repair of DNA double-strand breaks to drive genetic instability

Hypoxic cells have been linked to genetic instability and tumor progression. However, little is known about the exact relationship between DNA repair and genetic instability in hypoxic cells. We therefore tested whether the sensing and repair of DNA double-strand breaks (DNA-dsbs) is altered in irradiated cells kept under continual oxic, hypoxic or anoxic conditions. Synchronized G0–G1 human fibroblasts were irradiated (0–10 Gy) after initial gassing with 0% O2 (anoxia), 0.2% O2 (hypoxia) or 21% O2 (oxia) for 16 hours. The response of phosphorylated histone H2AX (γ-H2AX), phosphorylated ataxia telangiectasia mutated [ATM(Ser1981)], and the p53 binding protein 1 (53BP1) was quantified by intranuclear DNA repair foci and western blotting. At 24 hours following DNA damage, residual γ-H2AX, ATM(Ser1981) and 53BP1 foci were observed in hypoxic cells. This increase in residual DNA-dsbs under hypoxic conditions was confirmed using neutral comet assays. Clonogenic survival was also reduced in chronically hypoxic cells, which is consistent with the observation of elevated G1-associated residual DNA-dsbs. We also observed an increase in the frequency of chromosomal aberrations in chronically hypoxic cells. We conclude that DNA repair under continued hypoxia leads to decreased repair of G1-associated DNA-dsbs, resulting in increased chromosomal instability. Our findings suggest that aberrant DNA-dsb repair under hypoxia is a potential factor in hypoxia-mediated genetic instability.

NKX3.1 haploinsufficiency is prognostic for prostate cancer relapse following surgery or image-guided radiotherapy

BACKGROUND

Despite the use of prostate specific antigen (PSA), Gleason-score, and T-category as prognostic factors, up to 40% of patients with intermediate-risk prostate cancer will fail radical prostatectomy or precision image-guided radiotherapy (IGRT). Additional genetic prognosticators are needed to triage these patients toward intensified combination therapy with novel targeted therapeutics. We tested the role of the NKX3.1 gene as a determinant of treatment outcome given its reported roles in tumor initiating cell (TIC) renewal, the DNA damage response, and cooperation with c-MYC during prostate cancer progression.

METHODS

Using high-resolution array comparative genomic hybridization (aCGH), we profiled the copy number alterations in TIC genes using tumor DNA from frozen needle biopsies derived from 126 intermediate-risk patients who underwent IGRT. These data were correlated to biochemical relapse-free rate (bRFR) by the Kaplan-Meier method and Cox proportional hazards models.

RESULTS

A screen of the aCGH-IGRT data for TIC genes showed frequent copy number alterations for NKX3.1, PSCA, and c-MYC. NKX3.1 haploinsufficiency was associated with increased genomic instability independent of PSA, T-category, and Gleason-score. After adjusting for clinical factors in a multivariate model, NKX3.1 haploinsufficiency was associated with bRFR when tested alone (HR = 3.05, 95% CI: 1.46-6.39, P = 0.0030) or when combined with c-MYC gain (HR = 3.88, 95% CI: 1.78-8.49, P = 0.00067). A similar association was observed for patients following radical prostatectomy with a public aCGH database. NKX3.1 status was associated with positive biopsies post-IGRT and increased clonogen radioresistance in vitro.

CONCLUSIONS

Our results support the use of genomic predictors, such as NKX3.1 status, in needle biopsies for personalized approaches to prostate cancer management.

Resveratrol enhances prostate cancer cell response to ionizing radiation. Modulation of the AMPK, Akt and mTOR pathways

BACKGROUND

Prostate cancer (PrCa) displays resistance to radiotherapy (RT) and requires radiotherapy dose escalation which is associated with greater toxicity. This highlights a need to develop radiation sensitizers to improve the efficacy of RT in PrCa. Ionizing radiation (IR) stimulates pathways of IR-resistance and survival mediated by the protein kinase Akt but it also activates the metabolic energy sensor and tumor suppressor AMP-Activated Protein Kinase (AMPK). Here, we examined the effects of the polyphenol resveratrol (RSV) on the IR-induced inhibition of cell survival, modulation of cell cycle and molecular responses in PrCa cells.

METHODS

Androgen-insensitive (PC3), sensitive (22RV1) PrCa and PNT1A normal prostate epithelial cells were treated with RSV alone (2.5-10 μM) or in combination with IR (2-8 Gy). Clonogenic assays, cell cycle analysis, microscopy and immunoblotting were performed to assess survival, cell cycle progression and molecular responses.

RESULTS

RSV (2.5-5 μM) inhibited clonogenic survival of PC3 and 22RV1 cells but not of normal prostate PNT1A cells. RSV specifically sensitized PrCa cells to IR, induced cell cycle arrest at G1-S phase and enhanced IR-induced nuclear aberrations and apoptosis. RSV enhanced IR-induced expression of DNA damage (γH2Ax) and apoptosis (cleaved-caspase 3) markers as well as of the cell cycle regulators p53, p21(cip1) and p27(kip1). RSV enhanced IR-activation of ATM and AMPK but inhibited basal and IR-induced phosphorylation of Akt.

CONCLUSIONS

Our results suggest that RSV arrests cell cycle, promotes apoptosis and sensitizes PrCa cells to IR likely through a desirable dual action to activate the ATM-AMPK-p53-p21(cip1)/p27(kip1) and inhibit the Akt signalling pathways.

Molecular targets for radiation oncology in prostate cancer

Recent selected developments of the molecular science of prostate cancer (PrCa) biology and radiation oncology are reviewed. We present potential targets for molecular integration treatment strategies with radiation therapy (RT), and highlight potential strategies for molecular treatment in combination with RT for patient care. We provide a synopsis of the information to date regarding molecular biology of PrCa, and potential integrated research strategy for improved treatment of PrCa. Many patients with early-stage disease at presentation can be treated effectively with androgen ablation treatment, surgery, or RT. However, a significant portion of men are diagnosed with advanced stage/high-risk disease and these patients progress despite curative therapeutic intervention. Unfortunately, management options for these patients are limited and are not always successful including treatment for hormone refractory disease. In this review, we focus on molecules of extracellular matrix component, apoptosis, androgen receptor, RUNX, and DNA methylation. Expanding our knowledge of the molecular biology of PrCa will permit the development of novel treatment strategies integrated with RT to improve patient outcome.

Low-dose valproic acid enhances radiosensitivity of prostate cancer through acetylated p53-dependent modulation of mitochondrial membrane potential and apoptosis

Histone deacetylase inhibitors (HDI) have shown promise as candidate radiosensitizers for many types of cancers, including prostate cancer. However, the mechanisms of action are not well understood. In this study, we show in prostate cancer cells that valproic acid (VPA) at low concentrations has minimal cytotoxic effects yet can significantly increase radiation-induced apoptosis. VPA seems to stabilize a specific acetyl modification (lysine 120) of the p53 tumor suppressor protein, resulting in an increase in its proapoptotic function at the mitochondrial membrane. These effects of VPA are independent of any action of the p53 protein as a transcription factor in the nucleus, since these effects were also observed in native and engineered prostate cancer cells containing mutant forms of p53 protein having no transcription factor activity. Transcription levels of p53-related or Bcl-2 family member proapoptotic proteins were not affected by VPA exposure. The results of this study suggest that, in addition to nuclear-based pathways previously reported, HDIs may also result in radiosensitization at lower concentrations via a specific p53 acetylation and its mitochondrial-based pathway(s).

Tumor cell kill by c-MYC depletion: role of MYC-regulated genes that control DNA double-strand break repair

MYC regulates a myriad of genes controlling cell proliferation, metabolism, differentiation, and apoptosis. MYC also controls the expression of DNA double-strand break (DSB) repair genes and therefore may be a potential target for anticancer therapy to sensitize cancer cells to DNA damage or prevent genetic instability. In this report, we studied whether MYC binds to DSB repair gene promoters and modulates cell survival in response to DNA-damaging agents. Chromatin immunoprecipitation studies showed that MYC associates with several DSB repair gene promoters including Rad51, Rad51B, Rad51C, XRCC2, Rad50, BRCA1, BRCA2, DNA-PKcs, XRCC4, Ku70, and DNA ligase IV. Endogenous MYC protein expression was associated with increased RAD51 and KU70 protein expression of a panel of cancer cell lines of varying histopathology. Induction of MYC in G(0)-G(1) and S-G(2)-M cells resulted in upregulation of Rad51 gene expression. MYC knockdown using small interfering RNA (siRNA) led to decreased RAD51 expression but minimal effects on homologous recombination based on a flow cytometry direct repeat green fluorescent protein assay. siRNA to MYC resulted in tumor cell kill in DU145 and H1299 cell lines in a manner independent of apoptosis. However, MYC-dependent changes in DSB repair protein expression were not sufficient to sensitize cells to mitomycin C or ionizing radiation, two agents selectively toxic to DSB repair-deficient cells. Our results suggest that anti-MYC agents may target cells to prevent genetic instability but would not lead to differential radiosensitization or chemosensitization.

Antagonistic interaction between bicalutamide (Casodex) and radiation in androgen-positive prostate cancer LNCaP cells

BACKGROUND

Bicalutamide (Casodex) reportedly improves high-risk prostate cancer survival as an adjuvant treatment following radiotherapy. However, biological data for the interaction between bicalutamide and ionizing radiation in concomitant association are lacking.

METHODS

To explore this issue, androgen-dependent (LNCaP) and -independent (DU145) human prostate cancer cells were exposed for 48 hr to 20, 40, or 80 microM bicalutamide introduced before (neoadjuvant), during (concomitant), or following (adjuvant) radiation. Growth inhibition and cytotoxicity, cell cycle distribution and expression of the prostate serum antigen (PSA) and androgen receptor (AR), were measured as endpoints.

RESULTS

Bicalutamide-induced cytotoxic and cytostatic effects were found to be correlated with a marked G1 phase arrest and S phase depression. The drug down-regulated PSA and AR proteins and psa mRNA in LNCaP cells. However, transient up-regulation of the expression of ar mRNA was observed in the presence of 40 microM drug. DU145 cells did not express PSA and proved to be comparatively resistant to the drug from both cytostatic and cytotoxic effects. Bicalutamide dose-dependently induced a significant decrease of radiation susceptibility among drug survivors in LNCaP cells, whilst the interaction appeared to be additive in DU145 cells.

CONCLUSIONS

The antagonistic radiation-drug interaction observed in LNCaP cells is of significance in relation to combined radiotherapy-bicalutamide treatments directed against tumors expressing the AR. The results suggest that bicalutamide is amenable to combined schedule with radiotherapy provided the drug and radiation are not given in close temporal proximity.

Relative biological effectiveness and cell-killing efficacy of continuous low-dose-rate 125I seeds on prostate carcinoma cells in vitro

The aim of this study was to determine the effects of (125)I seeds on prostate carcinoma (PC3) cells. The relative biological effectiveness of (125)I seeds on PC3 cells with respect to (60)Co gamma rays was 1.4. Both 4 Gy of (60)Co gamma ray and (125)I seed irradiation increased the percentage of cells in G(2) phase, but there was no significant difference between these 2 types of radiation. Significantly, (125)I seeds induced higher apoptotic rates of PC3 cells compared with (60)Co gamma ray irradiation. Furthermore, Bcl-2 expression, but not caspase-3 activity, in PC3 cells was downregulated after irradiation with (125)I seed or (60)Co gamma rays.

Leucinostatin A inhibits prostate cancer growth through reduction of insulin-like growth factor-I expression in prostate stromal cells

Targeting stroma in tumor tissues is an attractive new strategy for cancer treatment. We developed in vitro coculture system, in which the growth of human prostate cancer DU-145 cells is stimulated by prostate stromal cells (PrSC) through insulin-like growth factor I (IGF-I). Using this system, we have been searching for small molecules that inhibit tumor growth through modulation of tumor-stromal cell interactions. As a result, we have found that leucinostatins and atpenins, natural antifungal antibiotics, inhibit the growth of DU-145 cells cocultured with PrSC more strongly than that of DU-145 cells alone. In this study we examined the antitumor effects of these small molecules in vitro and in vivo. When DU-145 cells were coinoculated with PrSC subcutaneously in nude mice, leucinostatin A was found to significantly suppress the tumor growth more than atpenin B. The antitumor effect of leucinostatin A in vivo was not obtained against the tumors of DU-145 cells alone. RT-PCR experiments revealed that leucinostatin A specifically inhibited IGF-I expression in PrSC without effect on expressions of other IGF axis molecules. Leucinostatins and atpenins are known to abrogate mitochondrial functions. However, when we used mitochondrial DNA-depleted, pseudo-rho(0) cells, we found that one of leucinostain A actions certainly depended on mitochondrial function, but it actually inhibited the growth of DU-145 cells more strongly in coculture with pseudo-rho(0) PrSC and reduced IGF-I expression in pseudo-rho(0) PrSC. Taken together, our results suggested that leucinostatin A inhibited prostate cancer cell growth through reduction of IGF-I expression in PrSC.

Hypofractionation: what does it mean for prostate cancer treatment?

PURPOSE

Using current radiobiologic models and biologic parameters, we performed an exploratory study of the clinical consequences of hypofractionation in prostate cancer radiotherapy.

METHODS AND MATERIALS

Four hypofractionated treatment regimens were compared with standard fractionation of 2 Gy x 39 for prostate carcinoma using a representative set of anatomical structures. The linear-quadratic model and generalized equivalent uniform dose formalism were used to calculate normalized equivalent uniform dose (gEUD(2)), from which tumor control probability and normal tissue complication probability were calculated, as well as "complication-free tumor control probability" (P+). The robustness of the results was tested for various tumor alpha/beta values and broad interval of biologic parameters such as surviving fraction after a dose of 2 Gy (SF2).

RESULTS

A 2.5% and 5.8% decrease in NTCP for rectum and bladder, respectively, was predicted for the 6.5 Gy/fraction regimen compared with the 2 Gy/fraction. Conversely, TCP for hypofractionated regimens decreased significantly with increasing SF2 and alpha/beta. For tumor cells with SF2 = 0.4-0.5, P+ was superior for nearly all hypofractionated regimens even for alpha/beta values up to 6.5 Gy. For less responsive tumor cells (SF2 = 0.6), hypofractionation regimens were inferior to standard fractionation at much lower alpha/beta.

CONCLUSION

For a sample set of anatomical structures, existing radiobiologic data and models predict improved clinical results from hypofractionation over standard fractionation not only for prostate carcinoma with low alpha/beta but also for high alpha/beta (up to 6.5 Gy) when SF2 < 0.5. Predicted results for specific patients may vary with individual anatomy, and large-scale clinical conclusions can be drawn only after performing similar analysis on an appropriate population of patients.

Molecular fingerprinting of radiation resistant tumors: can we apprehend and rehabilitate the suspects?

Radiation therapy continues to be one of the more popular treatment options for localized prostate cancer. One major obstacle to radiation therapy is that there is a limit to the amount of radiation that can be safely delivered to the target organ. Emerging evidence suggests that therapeutic agents targeting specific molecules might be combined with radiation therapy for more effective treatment of tumors. Recent studies suggest that modulation of these molecules by a variety of mechanisms (e.g., gene therapy, antisense oligonucleotides, small interfering RNA) may enhance the efficacy of radiation therapy by modifying the activity of key cell proliferation and survival pathways such as those controlled by Bcl-2, p53, Akt/PTEN and cyclooxygenase-2. In this article, we summarize the findings of recent investigations of radiosensitizing agents in the treatment of prostate cancer.

Influence of DNA double‐strand break rejoining on clonogenic survival and micronucleus yield in human cell lines

PURPOSE

To examine the role of DNA double-strand break (DSB) rejoining in cell survival and micronucleus yield after 60Co gamma-irradiation.

MATERIALS AND METHOD

Thirteen human cell lines (six glioblastoma, five prostate, one melanoma, one squamous cell carcinoma) were irradiated with 60Co gamma-rays to doses of 0-10Gy for cell survival and micronucleus measurements and 0-100Gy for DSB rejoining. Measurements were performed using standard clonogenic, micronucleus and constant-field gel electrophoresis assays.

RESULTS

Radioresistance and micronucleus yield were positively correlated (r=0.74, p=0.004). A significant cell type-dependent correlation was demonstrated between total (0-20 h) DSB rejoining and cell survival (r=0.86, p=0.03 for glioblastomas; r=0.79, p=0.04 for other cell lines), with more resistant cell lines showing higher levels of DSB rejoining. No relationship was apparent between fast (0-2 h) or slow (2-20 h) DSB rejoining and clonogenic survival. While there was no relationship between total or slow DSB rejoining and micronucleus yield, a significant and cell type-specific correlation emerged between fast rejoining and micronucleus yield for the glioblastomas (r=0.89, p=0.04) and other cell lines (r=0.76, p=0.04). Cell lines with higher levels of DSB rejoining within 2 h of irradiation showed higher yields of micronuclei.

CONCLUSION

Fast DSB rejoining, possibly through interaction with slow DSB rejoining, appears to play an important role in the formation of micronuclei. However, total DSB rejoining reflects intrinsic radiosensitivity. Consideration of differences in DSB rejoining kinetics might contribute to a better understanding of the significance of cell survival and micronucleus data in the clinical and radiation protection setting.

Androgens induce oxidative stress and radiation resistance in prostate cancer cells though NADPH oxidase

Androgen deprivation therapy (ADT) facilitates the response of prostate cancer (PC) to radiation. Androgens have been shown to induce elevated basal levels of reactive oxygen species (ROS) in PC, leading to adaptation to radiation-induced cytotoxic oxidative stress. Here, we show that androgens increase the expression of p22^phox and gp91^phox subunits of NADPH oxidase (NOX) and ROS production by NOX2 and NOX4 in PC. Pre-radiation treatment of 22Rv1 human PC cells with NOX inhibitors sensitize the cells to radiation similarly to ADT, suggesting that their future usage may spare the need for adjuvant ADT in PC patients undergoing radiation.

Targeting homologous recombination using imatinib results in enhanced tumor cell chemosensitivity and radiosensitivity

RAD51 is a key protein in the homologous recombination (HR) pathway of DNA double-strand break repair, and HR represents a novel target for cancer therapy. Because imatinib (Gleevec) has been reported to reduce RAD51 protein levels, we tested the clonogenic survival for RT112, H1299, PANC1, and PC3 tumor cell lines of varying p53 status and normal GM05757 normal fibroblasts after exposure to single agent imatinib (0-20 micromol/L; 0-72 hours). We also combined imatinib with DNA damaging agents that are toxic to RAD51-deficient cells, including ionizing radiation, gemcitabine, and mitomycin C. We observed decreased nuclear expression and chromatin binding of RAD51 protein following imatinib treatment. Imatinib also resulted in decreased error-free HR as determined by a flow cytometry-based integrated direct repeat-green fusion protein reporter system; this correlated to reduced RAD51 expression. Clonogenic survival experiments revealed increased cell kill for imatinib-treated cells in combination with ionizing radiation, gemcitabine, and mitomycin C, due in part to mitotic catastrophe. In experiments using imatinib and gemcitabine, tumor cell lines were sensitized to a greater extent than normal fibroblasts. This preservation of the therapeutic ratio was confirmed in vivo using PC3 xenograft growth delay and intestinal crypt cell clonogenic assays. HR inhibition may be an additional mechanism of action for the chemosensitization and radiosensitization of solid tumors with imatinib with preservation of the therapeutic ratio.

The extreme radiosensitivity of the squamous cell carcinoma SKX is due to a defect in double-strand break repair

PURPOSE

Squamous cell carcinomas (SCCs) are characterized by moderate radiosensitivity. We have established the human head & neck SCC cell line SKX, which shows an exceptionally high radiosensitivity. It was the aim of this study to understand the underlying mechanisms.

MATERIALS & METHODS

Experiments were performed with SKX and FaDu, the latter taken as a control of moderate radiosensitivity. Cell lines were grown as xenografts as well as cell cultures. For xenografts, radiosensitivity was determined via local tumour control assay, and for cell cultures using colony assay. For cell cultures, apoptosis was determined by Annexin V staining and G1-arrest by BrdU labelling. Double-strand breaks (DSBs) were detected by both constant-field gel electrophoresis (CFGE) and gammaH2AX-foci technique; DSB rejoining was also assessed by in vitro rejoining assay; chromosomal damage was determined by G01-assay.

RESULTS

Compared to FaDu, SKX cells are extremely radiosensitive as found for both xenografts (TCD(50) for 10 fractions 46.0Gy [95% C.I.: 39; 54 Gy] vs. 18.9 Gy [95% C.I.: 13; 25Gy]) and cell cultures (D(0.01); 7.1 vs. 3.5Gy). Both cell lines showed neither radiation-induced apoptosis nor radiation-induced permanent G1-arrest. For DSBs, there was no difference in the induction but for repair with SKX cells showing a higher level of both, slowly repaired DSBs and residual DSBs. The in vitro DSB repair assay revealed that SKX cells are defective in nonhomologous endjoining (NHEJ), and that more than 40% of DSBs are rejoined by single-strand annealing (SSA). SKX cells also depicted a two-fold higher number of lethal chromosomal aberrations when compared to FaDu cells.

CONCLUSIONS

The extreme radiosensitivity of the SCC SKX seen both in vivo and in vitro can be ascribed to a reduced DNA double-strand break repair, resulting from a defect in NHEJ. This defect might be due to preferred usage of other pathways, such as SSA, which prevents efficient endjoining.

Critical role of glutathione in melatonin enhancement of tumor necrosis factor and ionizing radiation-induced apoptosis in prostate cancer cells in vitro

The role of antioxidants in reducing cancer initiation and progression has been highlighted in recent years. Not only antioxidants limit cancer cell growth but also, in some situations, they promote the effectiveness of conventional treatments. Melatonin, an endogenously synthesized antioxidant, reduces cell growth of several tumor types both in vivo and in vitro. Additionally, the indole limits the collateral damage induced by many chemotherapeutic agents. By using a cellular model of human prostate cancer, we studied the ability of melatonin to enhance apoptosis induced by tumor necrosis factor or gamma radiation. It has been reported that melatonin reduces prostate cancer cell growth and, more recently, it promotes cell differentiation. In this work, we also show that melatonin elevates p21 protein levels and increases antioxidant capacity of prostate cancer cells. In addition, melatonin significantly enhances hrTNFalpha induced cell death by decreasing NFkappaB activation. Bcl-2 and survivin down-regulation appears to be associated to apoptosis stimulation under NFkappaB inhibition. On the contrary, melatonin does not promote irradiation-induced cell death due to an increment in intracellular glutathione content. In conclusion, prevention of NFkappaB activation by melatonin enhances the effectiveness of cytokine treatment in prostate cancer cells but it is not sufficient to enhance cell death triggered by other therapies which generate free radicals. A crucial role of glutathione in survival mechanisms of prostate cancer cells should be carefully considered.

Nutlin-3 radiosensitizes hypoxic prostate cancer cells independent of p53

Nutlin-3 is a small-molecule inhibitor that acts to inhibit MDM2 binding to p53 and subsequent p53-dependent DNA damage signaling. Whether Nutlin-3 alters cell toxicity following DNA damage under oxic versus hypoxic conditions has not been studied. The potential radiosensitization (0-10 Gy) properties of Nutlin-3 (dose range, 2-10 micromol/L for up to 24 h) were investigated in vitro using three prostate cancer cell lines, 22RV1 [wild-type p53 (WTp53)], DU145 (mutated p53), and PC-3 (p53-null) under oxic (21% O(2)), hypoxic (0.2% O(2)), and anoxic (0% O(2)) conditions. As a single agent, Nutlin-3 (2-10 micromol/L) stabilized p53 and p21(WAF) levels and was toxic to WTp53-22RV1 cells (IC(50), 4.3 micromol/L) but had minimal toxicity toward p53-deficient cells (IC(50), >10 micromol/L). When combined with radiation under oxic conditions, Nutlin-3 decreased clonogenic survival in all three cell lines: 22RV1 [sensitizing enhancement ratio (SER), 1.24], DU145 (SER, 1.27), and PC-3 (SER, 1.12). Anoxia induced p53 protein expression in 22RV1 cells and this was augmented by Nutlin-3 treatment. Furthermore, Nutlin-3 was more effective as a radiosensitizer under hypoxic conditions particularly in WTp53-expressing cells: 22RV1 (SER, 1.78), DU145 (SER, 1.31), and PC-3 (SER, 1.28). The decrease in clonogenic survival with Nutlin-3 was not correlated to altered levels of radiation-induced apoptosis within the three cell lines. Our results indicate that Nutlin-3 can act as a radiosensitizer via p53-independent mechanisms under low O(2) levels. Nutlin-3 may be a useful adjunct to improve the therapeutic ratio using precision radiotherapy targeted to hypoxic cells and warrants further study in vivo.

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.

Chronic hypoxia decreases synthesis of homologous recombination proteins to offset chemoresistance and radioresistance

Hypoxic and/or anoxic tumor cells can have increased rates of mutagenesis and altered DNA repair protein expression. Yet very little is known regarding the functional consequences of any hypoxia-induced changes in the expression of proteins involved in DNA double-strand break repair. We have developed a unique hypoxic model system using H1299 cells expressing an integrated direct repeat green fluorescent protein (DR-GFP) homologous recombination (HR) reporter system to study HR under prolonged chronic hypoxia (up to 72 h under 0.2% O(2)) without bias from altered proliferation, cell cycle checkpoint activation, or severe cell toxicity. We observed decreased expression of HR proteins due to a novel mechanism involving decreased HR protein synthesis. Error-free HR was suppressed 3-fold under 0.2% O(2) as measured by the DR-GFP reporter system. This decrease in functional HR resulted in increased sensitivity to the DNA cross-linking agents mitomycin C and cisplatin but not to the microtubule-interfering agent, paclitaxel. Chronically hypoxic H1299 cells that had decreased functional HR were relatively radiosensitive [oxygen enhancement ratio (OER), 1.37] when compared with acutely hypoxic or anoxic cells (OER, 1.96-2.61). Using CAPAN1 cells isogenic for BRCA2 and siRNA to RAD51, we confirmed that the hypoxia-induced radiosensitivity was due to decreased HR capacity. Persistent down-regulation of HR function by the tumor microenvironment could result in low-fidelity DNA repair and have significant implications for response to therapy and genetic instability in human cancers.

Homologous recombination and prostate cancer: a model for novel DNA repair targets and therapies

Using elegant targeting techniques such as IMRT, radiation oncology has improved the therapeutic ratio of prostate cancer radiotherapy through increased physical precision (e.g. increased local control through dose-escalation without increased normal tissue toxicity). The therapeutic ratio might be further improved by the addition of "biologic precision and escalation" pertaining to the use of molecular inhibitors of DNA damage sensing and repair. Indeed, proteins involved in the ATM-p53 damage signaling axis and the homologous (HR) and non-homologous end-joining (NHEJ) pathways of DNA double-strand break (DNA-dsb) rejoining pathways may be attractive candidates to elucidate cancer risk, prognosis, prediction of response and to develop sensitizers towards oxic and hypoxic prostate tumor cells. This review highlights DNA-dsb in prostate cancer research in terms of novel molecular inhibitors, the role of the microenvironment in DNA-dsb repair and potential DNA-dsb biomarkers for clinical trials.