Trypsinization and the radiosensitivity of mitotic and log phase Chinese hamster V79 cells exposed to 250 kVp X-rays

Improved alkaline comet assay protocol for adherent HaCaT keratinocytes to study UVA-induced DNA damage

The comet assay is one of the well-accepted tests to measure radiation-induced DNA damage. The most commonly used protocols require single-cell suspensions that are embedded in agarose in order to perform electrophoresis. For adherently growing cells such as human HaCaT skin keratinocytes this method bears several problems. We show that trypsinization required for maintaining single-cell suspensions is prolonged after UV radiation and thereby reduces cell viability and allows partial repair, with the consequence of reduced damage detection after irradiation. Therefore, we here introduce a modified version of the comet assay where HaCaT cells are seeded onto comet slides 24h before the assay and overlaid with agarose immediately after irradiation. Using this modification we are now able to reproducibly measure high DNA-damage levels (13-fold increase compared with controls) following irradiation with 60J/cm(2) UVA as well as a dose-dependent increase of DNA damage after 10, 20 and 60J/cm(2) UVA. Thus, by maintaining the cells in their natural configuration, i.e. adherently growing, we exclude several artefacts that are likely to influence the damage responses. These include: (i) trypsinization-dependent changes in cell morphology and polarity (clear lateral, i.e. adherent, and apical side of keratinocytes) which are likely of consequence for the gene-expression pattern, (ii) trypsin- and dislodgement-induced damage reducing cell viability, and (iii) the time delay between damage induction and damage evaluation to unpredictable results due to partial repair. Since these advantages pertain to all adherently growing cells, this improved protocol is not restricted to HaCaT cells but offers great potential also with all non-haematopoietic cells for obtaining accurate results and for studying repair processes in a highly reproducible manner.

Trypsin-induced changes in cell shape and chromatin structure result in radiosensitization of monolayer Chinese hamster V79 cells

Trypsin is the enzyme commonly used to prepare single cell suspensions from monolayer and spheroid cultures, both to determine survival and to assay DNA damage. Trypsin induces rounding, dissociation and radiosensitization of anchorage-dependent cells. Radiosensitivity and chromatin structure were compared between trypsin-treated (0.05%) round V79 cells from monolayers and spheroids vs. untreated spread monolayer cells in situ. The fluorescent halo technique was used to measure the changes in DNA supercoiling in nucleoids isolated from control and irradiated round and spread cells. Maximal halo diameters, the amount of initial and residual radiation-induced DNA damage (estimated from nucleoid halo diameter changes), and the radiosensitivity were higher in round cells than in spread monolayer V79 cells. The effects on cellular radiosensitivity and maximal halo diameter of other agents which also round and dissociate cells, e.g. 0.25% trypsin, pronase E and a non-enzymatic cell-dissociation solution, were similar to those of 0.05% trypsin. In LY-S cells, which are anchorage-independent, DNA loop size, the initial amount of DNA damage and radiosensitivity were not affected by trypsin. We suggest that the higher radiosensitivity of anchorage-dependent cells under immediate trypsinization and plating conditions, compared to cells with postirradiation in situ repair incubation, is due to correlated changes in cell shape and chromatin structure.

Epidermal growth factor modification of radioresistance related to cell-cell interactions

The effect of epidermal growth factor on radiosensitivity of cells in spheroids and its relationship to radioresistance associated with cell-cell interactions was examined. A human squamous carcinoma cell lines, CaSki, was used. Epidermal growth factor present for 48 hr before irradiation reduced the plating efficiency but did not affect the radiosensitivity of cells. However, epidermal growth factor present after irradiation, that is, during the period of colony formation, reduced the plating efficiency and increased the radiosensitivity of cells from spheroids. Both effects were maximum at 10 ng/ml epidermal growth factor. The enhancement in radiation response was not related to epidermal growth factor effects on potentially lethal and sublethal damage repair. In the absence of cell-cell interactions, such as monolayer cultures and spheroids disaggregated for 15 hr before irradiation, radiosensitivity enhancement by epidermal growth factor was associated with reduced shoulder of the cell survival curve. However, in the presence of cell-cell interactions, such as intact spheroids and spheroids disaggregated immediately before irradiation, in addition to reduced shoulder, epidermal growth factor treatment increased the slope compared to that of the monolayer cultures. The results indicate that epidermal growth factor enhances cellular radiosensitivity and modifies effects of cell-cell interactions.

Effects of trypsin on X-ray-induced cell killing, chromosome abnormalities and kinetics of DNA repair in mammalian cells

When cells are trypsinized before irradiation a potentiation of X-ray damage may occur. This is known as the 'trypsin effect'. Potentiation of X-ray damage on cell killing was seen in V79 Chinese hamster cells but was marginal in Chinese hamster ovary (CHO K1) cells and not evident in murine Ehrlich ascites tumour (EAT) cells. Trypsinization did however increase the number of X-ray-induced chromosomal abnormalities in all 3 lines. To investigate the possibility that trypsin acts by digestion of proteins in chromatin, further experiments were performed to monitor DNA damage and repair. Induction of DNA breaks by X-rays was unaffected by trypsin but trypsinized EAT (suspension) cells repaired single-strand breaks (ssb) less rapidly than controls indicating an inhibitory effect of trypsin on ssb repair. However double-strand break (dsb) repair was unaffected by trypsin. It was also found that the EDTA solution in which the trypsin was dissolved also contributes to the inhibition of dsb repair. The results show that trypsinization can enhance X-ray-induced cell killing, chromosomal damage and DNA repair, the effect varying between cell lines.

Similarities in the repair kinetics of sublethal and potentially lethal X-ray damage in log phase Chinese hamster V79 cells

We have compared the repair kinetics of X-ray-induced sublethal damage (SLD) and potentially lethal damage (PLD) in V79 cells during postirradiation incubation in growth medium at 37 degrees C. Kinetics of SLD repair were studied by dose fractionation (D1 + D2 = 2 + 2, 4 + 2, 4 + 4, 10 + 4 and 10 + 7 Gy). Kinetics of PLD repair were studied by incubating cells with 0.5 M hypertonic saline (HS) for 20 min at various times after irradiation. The repair of SLD, after 2, 4 and 10 Gy, was complete in about 11, 23 and 62 min, respectively. This repair time was independent of the magnitude of dose D2 (test dose). The repair of PLD, after 2, 4 and 10 Gy, was also complete in about 9, 23 and 61 min, respectively. These data indicate that: (1) the kinetics of repair of SLD for a given dose are independent of the magnitude of the test dose D2; (2) the time taken to complete the repair (repair time) is dependent on dose D1, i.e. the larger the dose D1 the longer the repair time; (3) for a given dose, the repair times assayed either by dose fractionation or by HS treatment are similar; and (4) similarities in the repair times may imply that SLD and PLD are the same. The relationship between the repair kinetics and the shape of the shoulder on fractionated survival curves is discussed.