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

A thixotropic nanocomposite gel for three-dimensional cell culture

Thixotropic materials, which become less viscous under stress and return to their original state when stress is removed, have been used to deliver gel-cell constructs and therapeutic agents. Here we show that a polymer-silica nanocomposite thixotropic gel can be used as a three-dimensional cell culture material. The gel liquefies when vortexed--allowing cells and biological components to be added--and resolidifies to trap the components when the shear force from spinning is removed. Good permeability of nutrients and gases through the gel allows various cell types to proliferate and be viable for up to three weeks. Human mesenchymal stem cells cultured in stiffer gels developed bone-like behaviour, showing that the rheological properties of the gel can control cell differentiation. No enzymatic, chemical, or photo-crosslinking, changes in ionic strength or temperature are required to form or liquefy the gel, offering a way to sub-culture cells without using trypsin-a protease commonly used in traditional cell culture techniques.

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.

Ultrasound permeabilizes CHO cells for the endonucleases AluI and benzon nuclease

Ultrasound permeabilizes Chinese hamster ovary (CHO) cells for the endonucleases AluI and benzon nuclease which leads to the induction of chromosomal aberrations by these enzymes. A few aberrant cells were observed when trypsinized cells or adherent cells were exposed to the enzymes in the absence of ultrasound. Our data show that sonication can be used to introduce endonucleases into CHO cells. We further demonstrate that few cells can internalize endonucleases without previous permeabilization.

Induction of DNA strand breaks by ethylene oxide in human diploid fibroblasts

In vitro exposure of normal human diploid fibroblasts (strain VH-10) to ethylene oxide (EtO) induced DNA strand breaks in the dose range of 2.5-30 mMh of EtO. Alkaline DNA unwinding (ADU), neutral filter elution (NFE), pulsed field gel electrophoresis (PFGE), and the comet assay were used to measure DNA single (SSBs) and double strand breaks (DSBs). Different induction rates of SSBs and DSBs, depending on applied method and also on treatment conditions (cells in monolayer or in suspension were used), were found. A dose-dependent increase of DNA strand breaks was found by the ADU method in the dose range of 2.5-20 mMh of EtO when treatment was performed in monolayer and in suspension. DSBs were detected by NFE only when the cells were treated with EtO in suspension (doses 10-30 mMh). The highest induction rate of DSBs (about 4 DSBs per 100 Mbp per 1 mMh of EtO) was detected in suspension with PFGE applied. We have shown that heat-labile sites are formed by EtO. Presumably, the different DSB levels detected by PFGE and NFE result from the conversion of these sites to DSBs during cell lysis at elevated temperature in the PFGE method. The results of the comet assay confirmed that apoptotic processes are not involved in the formation of DSBs in our experimental conditions (less than 1% of apoptotic cells were observed at all doses studied). Possible mechanisms for the induction of DNA strand breaks by EtO-treatment are discussed. The capacity to repair DSBs in EtO-exposed (5-7.5 mMh) cells was studied, and it was found that a considerable part of the damage (about 50%) could be repaired during 18 hr of incubation.

Significance and measurement of DNA double strand breaks in mammalian cells

Techniques for measuring DNA double strand breaks in mammalian cells are being used increasingly by researchers studying both physiological processes, such as recombination, replication, and apoptosis, as well as pathological processes, such as clastogenesis induced by ionizing radiation, chemotherapeutic drugs, and chemical toxicants. In this review we evaluate commonly used assays for measuring DNA double strand breaks, focusing on neutral filter elution and pulsed field gel electrophoresis, and explore the advantages and limitations of applying these techniques to problems of current interest in carcinogenesis and genetic toxicology.