Radiosensitivity and double-strand break rejoining in tumorigenic and non-tumorigenic human epithelial cell lines

Human embryonic stem cell responses to ionizing radiation exposures: current state of knowledge and future challenges

Human embryonic stem cells, which are derived from the inner cell mass of the blastocyst, have become an object of intense study over the last decade. They possess two unique properties that distinguish them from many other cell types: (i) the ability to self-renew indefinitely in culture under permissive conditions, and (ii) the pluripotency, defined as the capability of giving rise to all cell types of embryonic lineage under the guidance of the appropriate developmental cues. The focus of many recent efforts has been on the elucidating the signaling pathways and molecular networks operating in human embryonic stem cells. These cells hold great promise in cell-based regenerative therapies, disease modeling, drug screening and testing, assessing genotoxic and mutagenic risks associated with exposures to a variety of environmental factors, and so forth. Ionizing radiation is ubiquitous in nature, and it is widely used in diagnostic and therapeutic procedures in medicine. In this paper, our goal is to summarize the recent progress in understanding how human embryonic stem cells respond to ionizing radiation exposures, using novel methodologies based on “omics” approaches, and to provide a critical discussion of what remains unknown; thus proposing a roadmap for the future research in this area.

Modifications of the radiosensitivity of a renal cancer cell line as a consequence of stable TIMP-1 overexpression

PURPOSE

To investigate the potential effects of stable tissue inhibitor of matrix metalloproteinase-1 (TIMP-1) overexpression on DNA damage and cell killing following low-dose gamma-radiation and whether this up-regulation interfered with the activation of the matrix metalloproteinase -2 (MMP-2) and -9 (MMP-9) in a highly metastatic renal carcinoma cell line.

MATERIALS AND METHODS

Stable transfections were carried out using the cytomegalovirus expression plasmid pRc/CMV carrying TIMP-1 cDNA and LIPOFECTAMINE reagent. TIMP-1 expression in selected clones was determined by reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blot analysis. Exponentially growing Caki-1 cells were treated with sub lethal doses of ionizing radiation (0- 10Gy) either alone or following stable TIMP-1 transfection. DNA damage was assessed by the Alkaline Comet Assay and cell survival was determined by a clonogenic assay. Caki-1 cell cycle alterations following TIMP-1 transfection were assessed by fluorescence activated cell sorting (FACS) analysis of propidium iodide (PI)-stained cells. The interactions between TIMP-1 and MMP-2 and MMP-9 were analysed 24 hours post-irradiation by means of gelatin zymography.

RESULTS

Three clones with varying degrees of TIMP-1 expression were selected and used for further analysis. TIMP-1 transfected Caki-1 cells displayed significantly higher mean tail moment values (p < 0.05) following irradiation at doses between 5 and 10 Gy relative to that seen with radiation alone. The TIMP-1 radiosensitizing effect was accompanied by large decreases in the survival fraction of the parental Caki-1 cell line and significant increases in the alpha-parameter of the linear-quadratic fit. These effects were directly correlated to the degree of TIMP-1 gene expression detected in the selected clones. Interestingly, elevated levels of TIMP-2 protein were detected in the three TIMP-1 clones compared to TIMP-2 levels present in Caki-l cells. The three clones also displayed marked phenotypic alterations relative to their parental cell line. Significant increases in the percentage of cells arrested in the G2/M phase of the cell cycle were detected in the three clones under normal growth conditions and reduced serum conditions (p < 0.05). When the TIMP-1 clones were assessed for their MMP-2 activity, a marked decrease in the MMP-2 mean protein levels was detected in clone T1-3 following irradiation at doses between 2 and 6 Grays (Gy) (p < 0.01) and clone T1-2 at 2- 5Gy (p < 0.05). MMP-9 activity was differentially affected by ionizing radiation in the three TIMP-1 clones. T1-3 and TI-2 displayed significantly reduced MMP-9 levels at various dose points whereas T1-1 exhibited elevated levels of MMP-9 activity at higher doses of treatment (p < 0.05).

CONCLUSION

These results demonstrate a dual role for the TIMP-1 overexpression in this renal carcinoma cell line, both as radiosensitizing agents and effectors of MMP-2 and MMP-9 activity.

Age-related nonhomologous end joining activity in rat neurons

DNA double strand break (DSB) represents a potentially lethal form of DNA damage. Reports suggest that DSBs are introduced in neurons during the course of normal development, and repair of such DSBs is essential for neuronal survival. The molecular mechanisms of DSB repair by nonhomologous end joining (NHEJ) have been described in several cell types. The present study describes age-related NHEJ activity in the isolated neurons from rat cerebral cortex. Cell-free extracts prepared from rat cortical neurons support efficient NHEJ of linearized plasmid DNA in an in vitro DSB repair assay. End joining efficiency of young neurons is dependent on DNA end structure. A linear plasmid with blunt ends was joined less efficiently by the neuronal extracts than the cohesive or non-matching protruding DNA ends. NHEJ in neurons was blocked by the DNA-PKcs inhibitor wortmannin, and dNTP, and could occur in the absence of exogenously added ATP. The end joining process in young rat neurons is nonfaithful. In vitro NHEJ activity was considerably lower in adult brain, and neurons from old brain failed to support significant end joining. The age-dependent profile of neuronal NHEJ indicates that neurons in postnatal brain utilize error-prone NHEJ to repair DNA double strand breaks accumulated within the genome and this activity declines gradually with age.

Characterization of complex apurinic/apyrimidinic-site clustering associated with an authentic site-specific radiation-induced DNA double-strand break

Radiation lethality is largely attributed to radiation-induced DNA double-strand breaks (DSBs). A range of structural complexity is predicted for radiation-induced DSBs. However, this lesion has never been analyzed in isolation at the molecular level. To address this problem, we have created authentic site-specific radiation-induced DSBs in plasmid DNA by triplex-forming oligonucleotide-targeted 125I decay. No significant difference in DSB yield was observed after irradiation in the presence or absence of the radical scavenger DMSO, suggesting that DSB formation is a result of the direct effect of the radiation. A restriction fragment terminated by the DSB was isolated and probed with the Escherichia coli DNA repair enzyme endonuclease IV (endo IV), which recognizes apurinic/apyrimidinic (AP) sites. Enzymatic probing demonstrated clustering of AP sites within 10 bases of the 125I-targeted base in the DNA duplex. Our results suggest scavengeable radicals may not play a large role in the generation of AP sites associated with DSB formation, because at least 30% of all fragments have endo IV-sensitive sites, regardless of irradiation conditions. An internal control fragment recovered from the 125I linearized plasmid did not exhibit endo IV sensitivity in excess of that observed for a similar fragment recovered from an undamaged plasmid. Thus, AP site clustering proximal to the DSB resulted from the 125I decays responsible for DSB formation and was not due to untargeted background irradiation.

Potential use of the alkaline comet assay as a predictor of bladder tumour response to radiation

Bladder tumours show a variable response to radiotherapy with only about 50% showing good local control; currently there is no test to predict outcome prior to treatment. We have used five bladder tumour cell lines (T24, UM-UC-3, TCC-SUP, RT112, HT1376) to investigate the potential of the alkaline comet assay (ACA) to predict radiosensitivity. Radiation-induced DNA damage and repair were compared to clonogenic survival. When the five cell lines were irradiated and initial DNA damage was plotted against cell survival, at all doses (0-6 Gy), a significant correlation was found (r2=0.9514). Following 4 Gy X-irradiation, all cell lines, except T24, showed a correlation between SF2 vs half-time for repair and SF2 vs residual damage at 5, 10, 20 and 30 min. The T24 cell line showed radioresistance at low doses (0-2 Gy) and radiosensitivity at higher doses (4-6 Gy) using both cell survival and ACA end points, explaining the lack of correlation observed for this cell line. These data indicate that initial DNA damage and residual damage can be used to predict for radiosensitivity. Our data suggest that predictive tests of radiosensitivity, appropriate to the clinical situation, may require the use of test doses in the clinical range.

Induction and characterization of human glioma clones with different radiosensitivities

To facilitate investigation of the molecular mechanisms of tumor cell radiosensitivities, we have generated a set of clones with different radiosensitivities from a human glioma cell line U-251 MG-Ho. Forty-four colonies were isolated by subjecting parent cells to the mutagen N-methylnitrosourea and then irradiating these cells with increasing doses of x-rays. About half of the clones displayed different radiosensitivities than the parent cells. We selected one of the most sensitive clones (X3i) and one of the most resistant clones (Y6) for further study. Isoeffective doses for these two clones differed by about a factor of 1.7; the relative radiosensitivities of both clones were stable for at least 30 cell culture passages. These two clones do not differ significantly in either the induction or repair of radiation-induced DNA double-strand breaks as measured by pulsed field gel electrophoresis. Radiation-induced apoptosis measured by terminal deoxynucleotide transferase-mediated dUTP nick end labeling assay and micronucleus formation were similar in both clones. However, potentially lethal damage repair was greater in the radioresistant Y6 clone than in the radiosensitive X3i clone as determined by colony-forming efficiency assay.

Mouse testicular extracts process DNA double-strand breaks efficiently by DNA end-to-end joining

DNA double-strand break (DSB) processing was studied in mouse testicular extracts using a defined DSB created by cleaving supercoiled pUC12 DNA at a unique site as the substrate, and analysing the processed DNA by gel electrophoresis. Our results demonstrated that enzymatic activity in the extracts promoted multimerization of DNA and suppressed its circularization. This was distinctly different from T4 DNA ligase activity in the control and therefore the process must be more complex than simple ligation. Efficiency of this end-to-end joining was ATP and Mg(2+)-dependent and was much higher with cohesive (especially with 5') than with blunt ends. On recleaving, the joining was predominantly faithful, especially for cohesive ends; but a detectable fraction of DNA had undergone end-processed joining causing junctional deletions, mostly with blunt ends. Redigestion of end-joined products from time course experiments established that the end-deleted joining occurred concurrent to the faithful joining. Junctional segments were cloned and their restriction analysis confirmed the presence of large deletions from both the sides. These results suggested the association of an end-processing activity (exonuclease/helicase + flap endonuclease) along with the end-joining ligase(s). Suppression of end-edited joining on lowering the reaction temperature to 17 degrees or 14 degrees C, despite efficient faithful joining, indicated that this enzymatic activity is retarded at low temperature. Though the efficiency and fidelity of joining were termini-dependent, the orientation of joining was random. Lack of preference for homologous ends (H:H or T:T), as well as efficient joining of heterologous DNA (pUC12/pBR322) having two different blunt termini, showed that the end joining could occur independent of extensive/terminal homology. Retention of radioactivity on end joining of (alpha-32P)dCTP end-filled cohesive termini, and lack of their junctional cleavability, apparently due to restriction site duplication, suggested direct double strand ligation. Thus it is demonstrated that mouse male germ cells possess an efficient DNA end-joining activity, involving either a major pathway of precise joining, or a minor end-deleted joining, and it seems to be achieved by a multienzymatic complex as suggested also for somatic cells by others. These results show that mammalian male germ cells that are proficient in homologous recombination utilize nonhomologous end-joining (NHEJ) mechanism for DSB processing and therefore NHEJ is a conserved, universal pathway for the vital function of DSB repair.