Comparison of HepG2 feeder cells generated by exposure to gamma-rays, X-rays, UV-C light or mitomycin C for ability to activate 7,12-dimethylbenz[a]anthracene in a cell-mediated Chinese hamster V79/HGPRT mutation assay

An optimization protocol for Swiss 3T3 feeder cell growth-arrest by Mitomycin C dose-to-volume derivation strategy

Feeder cell functionality following growth-arrest with the cost-effective Mitomycin C vis-à-vis irradiation is controversial due to several methodological variables reported. Earlier, we demonstrated variability in growth arrested Swiss 3T3 feeder cell life-span following titration of feeder cell densities with Mitomycin C concentrations which led to the derivation of doses per cell. Alternatively, to counter the unexpected feeder regrowth at high exposure cell density, we proposed titration of a fixed density with arithmetically derived volumes of Mitomycin C solution that corresponded to permutations of specific concentrations and doses per cell. We now describe an experimental procedure of inducing differential feeder cell growth-arrest by titrating with such volumes and validating the best feeder batch through target cell growth assessment. A safe cell density of Swiss 3T3 tested for the exclusion of Mitomycin C resistant variants was titrated with a range of volumes of a Mitomycin C solution. The differentially growth-arrested feeder batches generated were tested for short-term and long-term viability and human epidermal keratinocyte growth supporting ability. The feeder cell extinction rate was directly proportional to the volume of Mitomycin C solution within a given concentration per se. The keratinocyte colony forming efficiency and the overall growth in mass cultures were maximal with a median extinction rate produced by an intermediate volume, while the faster and slower extinction rates by high and low volumes, respectively, were suboptimal. The described method could counter the inadequacies of growth-arrest with Mitomycin C.

Exposure cell number during feeder cell growth-arrest by Mitomycin C is a critical pharmacological aspect in stem cell culture system

INTRODUCTION

Growth-arrested feeder cells following Mitomycin C treatment are instrumental in stem cell culture allowing development of regenerative strategies and alternatives to animal testing in drug discovery. The concentration of Mitomycin C and feeder cell type was described to affect feeder performance but the criticality of feeder cell exposure density was not addressed. We hypothesize that the exposure cell density influences the effectiveness of Mitomycin C in an arithmetic manner.

METHODS

Three different exposure cell densities of Swiss 3T3 fibroblasts were treated with a range of Mitomycin C concentrations for 2h. The cells were replaced and the viable cells counted on 3, 6, 9, 12 and 20days. The cell extinctions were compared with doses per cell which were derived by dividing the product of concentration and volume of Mitomycin C solution with exposure cell number.

RESULTS

The periodic post-treatment feeder cell extinctions were not just dependent on Mitomycin C concentration but also on dose per cell. Analysis of linearity between viable cell number and Mitomycin C dose per cell derived from the concentrations of 3 to 10μg/ml revealed four distinct categories of growth-arrest. Confluent cultures exposed to low concentration showed growth-arrest failure.

DISCUSSION

The in vitro cell density titration can facilitate prediction of a compound's operational in vivo dosing. For containing the growth arrest failure, an arithmetic volume derivation strategy is proposed by fixing the exposure density to a safe limit. The feeder extinction characteristics are critical for streamlining the stem cell based pharmacological and toxicological assays.

An evaluation of the choice of feeder cell growth arrest for the production of cultured epidermis

Growth arrested 3T3 cells have been used as feeder cells in human epidermal keratinocyte cultures to produce cultured epidermal autografts for the treatment of burns. The feeder cells were ideally growth-arrested by gamma-irradiation. Alternatively, growth arrest by mitomycin C treatment is a cost effective option. We compared the functional efficacy of these two approaches in keratinocyte cultures by colony forming efficiency, the net growth area of colonies, BrdU labeling and histological features of cultured epidermal sheets. The growth area estimation involved a semi-automated digital technique using the Adobe Photoshop and comprised of isolation and enumeration of red pixels in Rhodamine B-stained keratinocyte colonies. A further refinement of the technique led to the identification of critical steps to increasing the degree of accuracy and enabling its application as an extension of colony formation assay. The results on feeder cell functionality revealed that the gamma irradiated feeders influenced significantly higher colony forming efficiency and larger growth area than the mitomycin C treated feeders. The BrdU labeling study indicated significant stimulation of the overall keratinocyte proliferation by the gamma irradiated feeders. The cultured epidermal sheets produced by gamma feeders were relatively thicker than those produced by mitomycin C feeders. We discussed the clinical utility of mitomycin C feeders from the viewpoint of cost-effective burn care in developing countries.

Further examination of the effects of nitrosylation on Alternaria alternata mycotoxin mutagenicity in vitro

Previously, Alternaria extract and metabolite mutagenicities+/-nitrosylation were characterized using Ames Salmonella strains TA98 and TA100, which are both reverted at GC sites. To examine other targets for mutation, the metabolites Altertoxin I (ATX I), Altenuene (ALT), Alternariol (AOH), Alternariol monomethyl ether (AME), Tentoxin (TENT), Tenuazonic acid (TA) and Radicinin (RAD) were reexamined+/-nitrosylation, using Ames Salmonella strain TA97, sensitive to frameshift mutations at a run of C's, as well as strains TA102 and TA104, reverted by base pair mutations at AT sites and more sensitive to oxidative damage. ATX I was also assessed for mammalian mutagenicity at the Hprt gene locus in Chinese hamster V79 lung fibroblasts and rat hepatoma H4IIE cells. When tested from 1 to 100 microg/plate without nitrosylation, ATX I was mutagenic in TA102+/-rat liver S9 for activation and weakly mutagenic in TA104+/-S9, demonstrating direct-acting AT base pair mutagenicity. AOH was also directly mutagenic at AT sites in TA102+/-S9 while AME was weakly mutagenic in TA102+/-S9 and TA104+S9. Nitrosylation of ATX I enhanced mutagenicity at AT sites in TA104+/-S9 but produced little change in TA102+/-S9 compared to native ATX I. However, nitrosylated ATX I generated a potent direct-acting frameshift mutagen at C sites in TA97+/-S9. While ATX I was not mutagenic in either V79 cells or H4IIE cells, 5 and 10 microg/ml nitrosylated ATX I produced a doubling of 6-thioguanine resistant V79 colonies and 0.5 and 1 microg/ml were mutagenic to H4IIE cells, becoming toxic at higher concentrations. These results suggest ATX I, AME and AOH induce mutations at AT sites, possibly through oxidative damage, with nitrosylation enhancing ATX I frameshift mutagenicity at runs of C's. Nitrosylated ATX I was also directly mutagenic in mammalian test systems.

Increased efficiency of gamma-irradiated versus mitomycin C-treated feeder cells for the expansion of normal human cells in long-term cultures

Several normal human cells, such as hematopoietic stem cells, dendritic cells, and B cells, can be cultured in vitro in defined optimal conditions. Several ex vivo culture systems require the use of feeder cells to support the growth of target cells. In such systems, proliferation of feeder cells has to be stopped, so that they can be used as nonreplicating viable support cells. Because feeder cells need to provide one or few active signals, it is important to maintain them in an metabolically active state, allowing continued expression of specific ligands or cytokines. Mitomycin C and gamma-irradiation treatments are commonly used to prepare nonproliferating feeder cells and are usually considered to be equivalent. Normal human B lymphocytes can be expanded in vitro in the presence of feeder cells expressing the CD40 ligand CD154. Here we compared the ability of gamma-irradiation- and mitomycin C-treated feeder cells to support the expansion of normal human B lymphocytes. The results indicate that expansion of B cells during a long-term culture was 100 times more potent using gamma-irradiated feeder cells compared to mitomycin C-treated cells. This difference could be related to a significant reduction in both cellular metabolism and level of CD154 expression observed in mitomycin C-treated feeder cells, but not in gamma-irradiated cells nor in control untreated cells. These results indicate that mitomycin C-treated feeder cells are metabolically altered, and consequently less efficient at maintaining cell expansion in the long-term cell culture system used.