Feeder cells and cell survival in spheroids and monolayers

Increased radiosensitivity in cells of two human cell lines treated with bystander medium from irradiated repair-deficient cells

Radiation-induced bystander factors have been shown to be more toxic if they are from medium harvested from irradiated repair-deficient cells. The aim of this study was to test the hypothesis that the radiosensitivity of repair-proficient cells can be increased by exposing them to medium-borne factors harvested from sensitive cells and vice versa. Cells from a mismatch repair (MMR)-deficient cell line (Raji 10) with a sensitive response to radiation or the wild-type parent cell line were irradiated to 0.5 Gy gamma rays and then monitored for growth rate in their own medium or in the alternative conditioned medium. In other experiments, cells or conditioned medium were added to reporter cells (HPV-G, which are relatively sensitive keratinocytes, or highly radioresistant HT29 cells). The subsequent responses of the two cell lines to a 0.5-Gy dose of (60)Co gamma rays were measured. The results show that prior exposure of resistant cells to medium from irradiated sensitive cells reduced the clonogenic survival of the subsequently irradiated resistant cells. The reverse is also true. Measurement of the apoptosis index and BCL2 expression confirmed that the harvested medium was capable of modulating apoptosis after irradiation. This may have important applications in tumor therapy and also in the understanding of mechanisms involved in induction of adaptive responses.

Cell proximity is a prerequisite for the proliferative response of bystander cells co-cultured with cells irradiated with ?-rays

BACKGROUND

In a recent study, we showed that unirradiated cells, when they are in the presence of cells irradiated with gamma-rays, are characterized by enhanced cell growth (Cytometry 2003;54A:1-7). However, the mechanisms and factors involved in the proliferative response of bystander cells are largely unknown. The aim of the current work was to investigate the possible role of spatial proximity of cells, including gap junctional intercellular communication (GJIC), in transmitting proliferation signals from cells irradiated with gamma-rays to unirradiated cells (bystander cells).

METHODS

Confluent monolayers of rat liver epithelial cells (WB-F344) were irradiated with 137Cs gamma-rays at doses ranging between 0.5 and 10 Gy. The impact of cell proximity on the proliferative response of bystander cells was determined by co-culturing different densities of irradiated and unirradiated cells (ratio 1:1) followed 24 h later by two-scheme flow cytometric analysis of their proliferation. To determine whether soluble extracellular factors play a role in this response, irradiated cells were plated together with unirradiated cells at the same ratio (1:1), but they were not allowed to directly contact unirradiated cells by using porous low-protein binding polyethylene terephthalate membranes. After 24 h, the numbers of unirradiated cells in the co-culture with irradiated cells were compared with the numbers of unirradiated cells in the control (unirradiated cells co-cultured with unirradiated cells). To investigate the possible involvement of GJIC in mediating the proliferative response in the bystander cells, the proliferation status of unirradiated GJIC-incompetent WB-aB1 cells was compared with that of unirradiated WB-F344 cells that were GJIC competent. In all of the protocols, the two populations of co-cultured cells were distinguished by labeling one population with fluorescent cell tracers, such as membrane-resident or CFDA SE (carboxy) fluorescein diacetate, succinimidyl ester that metabolizes intracellularly.

RESULTS

Unirradiated cells that were co-cultured with irradiated cells, but were not allowed to directly contact them, did not show any changes in the proliferation rate compared with that of unirradiated cells in the control. Unirradiated cells that were co-cultured as a mixture with irradiated cells showed enhanced and statistically significant proliferation, particularly when they were plated together more densely. The proliferation rate of bystander WB-aB1 cells was apparently higher than that of bystander WB-F344 cells; however, the difference was statistically insignificant.

CONCLUSIONS

In the present experimental model, the spatial proximity of cells is a crucial element for transmitting growth stimulation signals from irradiated cells to neighboring unirradiated cells. Direct cell-to-cell contact appears to be required for transmitting these signals. Neither functional GJIC nor soluble extracellular factors released by irradiated cells into the culture medium appear to play significant roles in this process under the present experimental conditions.

Flow cytometry as a strategy to study radiation-induced bystander effects in co-culture systems

BACKGROUND

There is growing interest in the role of bystander effects in the biological response of mammalian cells to ionizing radiation. Qualitative and quantitative assessment of bystander effects requires quick and reliable methodologies. The present work uses a flow cytometric approach to study proliferation of bystander cells co-cultured with irradiated cells.

METHODS

Confluent monolayers of rat liver epithelial cells (WB-F344) were irradiated with 137Cs gamma-rays at dose ranges 0.5, 1, 5, 10, and 20 Gy. Irradiated cells were mixed with unirradiated cells (i.e., bystander cells) at a ratio of 1:1 and cultured together for 24 h followed by a flow cytometry (FCM) study of their proliferation. In order to discriminate the two populations of co-cultured cells, one of the cell populations (unirradiated cells) was stained with the lipophilic fluorescent dye DiI.

RESULTS

Unirradiated cells, in the presence of cells irradiated at doses above 0.5 Gy, showed an enhanced cell growth by approximately 14-17%.

CONCLUSIONS

Cells irradiated with gamma rays can induce increased proliferation in neighboring unirradiated bystander cells. FCM provides an excellent basis for characterization of these and other bystander effects in co-culture systems.

A review of human cell radiosensitivity in vitro

The survival curves of 694 human cell lines irradiated in exponentially growing phase in vitro were collected from the literature. Among them, 271 were derived from tumors, 423 were nontransformed fibroblasts and other normal cell strains from healthy people or people with some genetic disorders. Seventy-six different cell types are identified, and a specific radiosensitivity could be associated with each, using D and surviving fraction at 2 Gy. Technical factors such as culture medium, feeder cells, and scoring method were found to affect intrinsic radiosensitivity. In particular, the cell type is not a discriminating factor when cells are studied in agar. Results obtained with cells irradiated in agar must be used cautiously, depending on how the cells were prepared for the experiments. The use of feeder cells narrows the range of radiosensitivity of human cells. For cells irradiated as monolayer, it was possible to build a scale of radiosensitivity according to cell type, ranging, in terms of D from 0.6 Gy for the most sensitive cell lines to more than 4 Gy for the most resistant. Considering that, in most cases, we could estimate the variation of radiosensitivity within each cell type, our classification among cell types can be used by researchers to place their results in the context of the literature.

Evidence that the feeder effect in mammalian cells is mediated by a diffusible substance

Chinese hamster ovary (CHO), baby hamster kidney (BHK), and NG108-15 neuroblastoma cells were heated and then plated for the colony-formation assay either with or without feeder cells present. Besides these cell lines, four other cell types were used as feeders. All cell lines functioned equally well as feeders for each of the heated cell lines. In some experiments the heated and feeder cells were separated by semipermeable membranes. This separation had no effect on the feeder effect, indicating that cell-to-cell contact was not requisite. The feeder effect appears to be mediated by a low molecular weight, diffusible substance produced by the feeder cells.

Multicellular spheroids. A review on cellular aggregates in cancer research

Cellular aggregates have been used in developmental biology and in experimental cancer research for several decades. Spherical aggregates of malignant cells, i.e. multicellular tumor spheroids, may serve as in vitro models of tumor microregions and of an early, avascular stage of tumor growth. The similarities between the original tumor and the respective spheroids include volume growth kinetics, cellular heterogeneity, e.g. the induction of proliferation gradients and quiescence, as well as differentiation characteristics, such as the development of specific histological structures or the expression of antigens. Research using cell aggregates has been focussed on mechanisms involved in the control of proliferation, invasion and metastasis. Immunological studies with spheroids have resulted in the characterization of defense cells which are responsible for specific host-versus-tumor reactions. The vast majority of investigations on spheroids concerns the simulation of therapy with regard to various treatment modalities, combination treatments and systematic analyses of using various endpoints in predictive assays. Only a few pathophysiological studies on the interrelationship among tumor-specific micromilieu, cellular metabolism, proliferative status, and cellular viability have been undertaken with the spheroid model up to now. Since these studies are indicative of a large influence of the cellular microenvironment on basic biological properties of cancer cells, investigations of these epigenetic mechanisms should be intensified in future research on cell aggregates. Similarly, the molecular basis of the biological peculiarities found in malignant cells grown as three-dimensional aggregates has to be investigated more intensively.

Cell-cell contact affects cellular sensitivity to hyperthermia

The influence on heat sensitivity of interactions between HT29 human colon adenocarcinoma cells grown in two- or three-dimensional contact was investigated. No evidence for intercellular cooperation affecting cellular sensitivity to hyperthermia was found. Cells grown in monolayer were found to be more heat sensitive than those in aggregates, probably due to the different physical properties of the membrane of flat cells attached to the substratum. Cell-cell contact appears to affect the heat sensitivity of HT29 cells possibly by means of inducing arrest in a heat resistant phase of the cell cycle.

Molecular and cellular radiobiology of heavy ions

Quantitative studies at the BEVALAC have demonstrated some of the physical and radiobiological factors that promise to make accelerated heavy ions important for the therapy of cancer. The measured physical dose-biological effect relationships allow the safe and effective delivery of therapeutic schedules of heavy ions. Among the charged particle beams available, carbon, neon and helium ions in the "extended Bragg peak mode" have optimal physical and biological effectiveness for delivery of therapy to deep seated tumors. The depth-dose profiles of these beams protect intervening and adjacent tissues as well as tissues beyond the range of the particles. For the treatment of hypoxic tumors, silicon and argon beams are being considered because they significantly depress the radiobiological oxygen effect in the region of the extended Bragg ionization peak. The depth-effectiveness of the argon beam is somewhat limited, however, because of primary particle fragmentation. Silicon beams have a depth-dose profile which is intermediate between that of neon and argon, and are candidates to become the particle of choice for maximizing high LET particle effects. Heavy accelerated ions depress enzymatic repair mechanisms, decrease variations of radiosensitivity during the cell division cycle, cause greater than expected delays in cell division, and decrease the protective effects of neighboring cells in organized systems. Near the Bragg peak, enhancement of heavy particle effects are observed in split dose schedules. Late and carcinogenic effects are being studied. With the newly developed Repair-Misrepair theory we can quantitatively model most observations.