Cell kinetic studies using a monoclonal antibody to bromodeoxyuridine

Prostate-specific natural health products (dietary supplements) radiosensitize normal prostate cells

PURPOSE

Prostate-specific health products (dietary supplements) are taken by cancer patients to alleviate the symptoms linked with poor prostate health. However, the effect of these agents on evidence-based radiotherapy practice is poorly understood. The present study aimed to determine whether dietary supplements radiosensitized normal prostate or prostate cancer cell lines.

METHODS AND MATERIALS

Three well-known prostate-specific dietary supplements were purchased from commercial sources available to patients (Trinovin, Provelex, and Prostate Rx). The cells used in the study included normal prostate lines (RWPE-1 and PWR-1E), prostate tumor lines (PC3, DU145, and LNCaP), and a normal nonprostate line (HaCaT). Supplement toxicity was assessed using cell proliferation assays [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide] and cellular radiosensitivity using conventional clonogenic assays (0.5-4Gy). Cell cycle kinetics were assessed using the bromodeoxyuridine/propidium iodide pulse-labeling technique, apoptosis by scoring caspase-3 activation, and DNA repair by assessing gammaH2AX.

RESULTS

The cell growth and radiosensitivity of the malignant PC3, DU145, and LNcaP cells were not affected by any of the dietary prostate supplements (Provelex [2 microg/mL], Trinovin [10 microg/mL], and Prostate Rx [50 microg/mL]). However, both Trinovin (10 microg/mL) and Prostate Rx (6 microg/mL) inhibited the growth rate of the normal prostate cell lines. Prostate Rx increased cellular radiosensitivity of RWPE-1 cells through the inhibition of DNA repair.

CONCLUSION

The use of prostate-specific dietary supplements should be discouraged during radiotherapy owing to the preferential radiosensitization of normal prostate cells.

Control of the G2/M checkpoints after exposure to low doses of ionising radiation: implications for hyper-radiosensitivity

Two molecularly distinct G2/M cell cycle arrests are induced after exposure to ionising radiation (IR) depending on the cell cycle compartment in which the cells are irradiated. The aims of this study were to determine whether there are threshold doses for their activation and investigate the molecular pathways and possible links between the G2 to M transition and hyper-radiosensitivity (HRS). Two human glioblastoma cell lines (T98G-HRS(+) and U373-HRS(-)) unsynchronized or enriched in G2 were irradiated and flow cytometry with BrdU or histone H3 phosphorylation analysis used to assess cell cycle progression and a clonogenic assay to measure radiation survival. The involvement of ATM, Wee1 and PARP was studied using chemical inhibitors. We found that cells irradiated in either the G1 or S phase of the cell cycle transiently accumulate in G2 in a dose-dependent manner after exposure to doses as low as 0.2Gy. Only Wee1 inhibition reduced this G2 accumulation. A block of the G2 to M transition was found after irradiation in G2 but occurs only above a threshold dose, which is cell line dependent, and requires ATM activity after exposure to doses above 0.5Gy. A failure to activate this early G2/M checkpoint correlates with low dose radiosensitization. These results provide evidence that after exposure to low doses of IR two distinct G2/M checkpoints are activated, each in a dose-dependent manner, with distinct threshold doses and involving different damage signalling pathways and confirm links between the early G2/M checkpoint and hyper-radiosensitivity.

Transition in survival from low-dose hyper-radiosensitivity to increased radioresistance is independent of activation of ATM Ser1981 activity

PURPOSE

The molecular basis of low-dose hyper-radiosensitivity (HRS) is only partially understood. The aim of this study was to define the roles of ataxia telangiectasia mutated (ATM) activity and the downstream ATM-dependent G(2)-phase cell cycle checkpoint in overcoming HRS and triggering radiation resistance.

METHODS AND MATERIALS

Survival was measured using a high-resolution clonogenic assay. ATM Ser1981 activation was measured by Western blotting. The role of ATM was determined in survival experiments after molecular (siRNA) and chemical (0.4 mM caffeine) inhibition and chemical (20 microg/mL chloroquine, 15 microM genistein) activation 4-6 h before irradiation. Checkpoint responsiveness was assessed in eight cell lines of differing HRS status using flow cytometry to quantify the progression of irradiated (0-2 Gy) G(2)-phase cells entering mitosis, using histone H3 phosphorylation analysis.

RESULTS

The dose-response pattern of ATM activation was concordant with the transition from HRS to radioresistance. However, ATM activation did not play a primary role in initiating increased radioresistance. Rather, a relationship was discovered between the function of the downstream ATM-dependent early G(2)-phase checkpoint and the prevalence and overcoming of HRS. Four cell lines that exhibited HRS failed to show low-dose (<0.3-Gy) checkpoint function. In contrast, four HRS-negative cell lines exhibited immediate cell cycle arrest for the entire 0-2-Gy dose range.

CONCLUSION

Overcoming HRS is reliant on the function of the early G(2)-phase checkpoint. These data suggest that clinical exploitation of HRS could be achieved by combining radiotherapy with chemotherapeutic agents that modulate this cell cycle checkpoint.

Cell kinetics

Cell kinetic concepts have pervaded radiation therapy since the early part of the 20th century and have been instrumental in the development of modern radiotherapy. In this review, the fundamental radiobiological concepts that have been developed based on cell kinetic knowledge will be revisited and discussed in the context of contemporary radiation therapy. This will include how the proliferation characteristics, variation in sensitivity during the cell cycle and the extent of radiation-induced cell cycle delay translate into a variable time for the expression of damage, how cell kinetics interacts with hypoxia and how the response to fractionated radiation schedules is influenced by cell kinetics in terms of repair, redistribution, reoxygenation and repopulation. The promise of combining radiation with new biologically targeted agents and the potential of non-invasive positron emission tomography imaging of proliferation are areas where cell kinetics will continue to influence radiotherapy practice.

Cell cycle analysis and synchronization of the Xenopus laevis XL2 cell line: study of the kinesin related protein XlEg5

Cell free extracts prepared from Xenopus eggs are one of the most powerful in vitro systems to analyze cell cycle-regulated mechanisms such as DNA replication, nuclear assembly, chromosome condensation, or spindle formation. Xenopus embryos can complete several synchronous cell cycles in the absence of transcription, consequently Xenopus extracts are very helpful to study the molecular level of cellular mechanisms. Many key cell cycle regulators like p34cdc2 and cdk2 have been discovered and characterized using those extracts, but their regulation during somatic cell cycles have only been studied in mammalian cultured cells. In this paper, we describe optimized conditions to obtain cell cycle arrested Xenopus XL2 cultured cells. Synchronization of XL2 cells at different stages of the cell cycle was achieved by serum starvation and drug treatments such as aphidicolin, nocodazole, and ALLN. The degree of synchronization was assessed by indirect fluorescence microscopy and FACS analysis. This method was used to study the cell cycle expression of the Xenopus kinesin-related protein, XlEg5, a microtubule-based motor protein involved in movement and cell division in early development. We found that the expression of the protein was maximum in mitosis and minimum in G1, which correlated with the expression of its messenger RNA. XL2 cultured cells were also used to analyze the ultrastructural sub-cellular localization of XlEg5. During mitosis, the protein was found around the centrosome in prophase, on the spindle microtubules in metaphase, and, interestingly, around the minus end of the midbody microtubules in telophase.