The lower radiosensitivity of mouse kidney cells irradiated in vivo than in vitro: a cell contact effect phenomenon

Connexin-43 downregulation in G2/M phase enriched tumour cells causes extensive low-dose hyper-radiosensitivity (HRS) associated with mitochondrial apoptotic events

Enrichment of tumour cells in G2/M phases in vitro is known to be associated with low-dose hyper-radiosensitivity (HRS). These cell cycle phases also involve reduced expression of adhesion protein connexin-43 (Cx43). Therefore, we investigated the role of Cx43 in HRS. Asynchronous or G2/M enriched tumour cells (U87, BMG-1, HeLa) and normal primary fibroblasts (HDFn) were γ-irradiated at varying doses, with an asynchronous group separately subjected to Cx43-knockdown prior to irradiation. Cx43 level, gap junctional activity, clonogenic cell survival, cell growth/viability, mitochondrial alterations and other apoptosis-regulating events were studied. G2/M enrichment reduced Cx43 level by ~50% and caused considerable HRS at doses 10 cGy-30 cGy in all tumour cell lines. Cx43-knockdown to the same level (~60%) also elicited prominent HRS response in these cells. Quite important, radiosensitivity of primary HDFn cells remained unaltered by all these treatments. In Cx43-knockdown tumour cells, low-dose irradiation caused significant growth inhibition and apoptosis involving loss of MMP, cytochrome-c release and caspase-3 activation, thereby demonstrating the important cytoprotective role of Cx43. Therefore, this study significantly shows that Cx43 downregulation (a constitutive feature of G2/M phase) selectively renders tumour cells hypersensitive to low-dose radiation, and presents connexins as potential therapeutic targets.

Radiotherapy and the tumor microenvironment: mutual influence and clinical implications

Ionizing radiation has been employed in targeted cancer treatments for more than a century because of its cytotoxic effects on cancer cells. However, the responsiveness to radiation and the behavior of tumors in vivo may differ dramatically from observed behaviors of isolated cancer cells in vitro. While not fully understood, these discrepancies are due to a complex constellation of extracellular and intercellular factors that are together termed the tumor microenvironment. Radiation may alter or affect the components of the adjacent tumor microenvironment in significant ways, often with consequences for cancer cells beyond the direct effects of the radiation itself. Moreover, different microenvironmental states, whether induced or at baseline, can modulate or even attenuate the effects of radiation, with consequences for therapeutic efficacy. This chapter describes this bidirectional relationship in detail, exploring the role and clinical implications of the tumor microenvironment with respect to therapeutic irradiation.

Bystander-induced differentiation: a major response to targeted irradiation of a urothelial explant model

A ureter primary explant technique, using porcine tissue sections was developed to study bystander effects under in vivo like conditions where dividing and differentiated cells are present. Targeted irradiations of ureter tissue fragments were performed with the Gray Cancer Institute charged particle microbeam at a single location (2 microm precision) with 10 3He2+ particles (5 MeV; LET 70 keV/microm). After irradiation the ureter tissue section was incubated for 7 days allowing explant outgrowth to be formed. Differentiation was estimated using antibodies to Uroplakin III, a specific marker of terminal urothelial differentiation. Even although only a single region of the tissue section was targeted, thousands of additional cells were found to undergo bystander-induced differentiation in the explant outgrowth. This resulted in an overall increase in the fraction of differentiated cells from 63.5+/-5.4% to 76.6+/-5.6%. These changes are much greater than that observed for the induction of damage in this model. One interpretation of these results is that in the tissue environment, differentiation is a much more significant response to targeted irradiation and potentially a protective mechanism.

A proliferation-dependent bystander effect in primary porcine and human urothelial explants in response to targeted irradiation

The aim of this study was to test whether radiation-induced bystander effects are involved in the response of multicellular systems to targeted irradiation. A primary explant technique was used that reconstructed the in vivo microarchitecture of normal urothelium with proliferating and differentiated cells present. Sections of human and porcine ureter were cultured as explants and irradiated on day 7 when the urothelial outgrowth formed a halo around the tissue fragment. The Gray Cancer Institute charge particle microbeam facility allowed the irradiation of individual cells within the explant outgrowth with a predetermined exact number of (3)He(2+) ions (which have very similar biological effectiveness to alpha-particles). A total of 10 individual cell nuclei were irradiated with 10 (3)He(2+) ions either on the periphery, where proliferating cells are located, or at the centre of the explant outgrowth, which consisted of terminally differentiated cells. Samples were fixed 3 days after irradiation, stained and scored. The fraction of apoptotic and micronucleated cells was measured and a significant bystander-induced damage was observed. Approximately 2000-6000 cells could be damaged by the irradiation of a few cells initially, suggesting a cascade mechanism of cell damage induction. However, the fraction of micronucleated and apoptotic cells did not exceed 1-2% of the total number of the cells within the explant outgrowth. It is concluded that the bystander-induced damage depends on the proliferation status of the cells and can be observed in an in vitro explant model.

Low-dose radiation hypersensitivity in human tumor cell lines: effects of cell-cell contact and nutritional deprivation

The hyper-radiosensitivity at low doses recently observed in vitro in a number of cell lines is thought to have important implications for improving tumor radiotherapy. However, cell-cell contact and the cellular environment influence cellular radiosensitivity at higher doses, and they may alter hyper-radiosensitivity in vivo. To confirm this supposition, we investigated the effects of cell density, multiplicity and nutritional deprivation on low-dose hypersensitivity in vitro. Cell survival in the low-dose range (3 cGy to 2 Gy) was studied in cells of two human glioma (BMG-1 and U-87) and two human oral squamous carcinoma (PECA-4451 and PECA-4197) lines using a conventional macrocolony assay. The effects of cell density, multiplicity and nutritional deprivation on hyper-radiosensitivity/induced radioresistance were studied in cells of the BMG-1 cell line, which showed prominent hypersensitivity and induced radioresistance. The induction of growth inhibition, cell cycle delay, micronuclei and apoptosis was also studied at the hyper-radiosensitivity-inducing low doses. Hyper-radiosensitivity/induced radioresistance was evident in the cells of all four cell lines to varying extents, with maximum sensitivity at 10-30 cGy, followed by an increase in survival up to 50 cGy-1 Gy. Both the glioma cell lines had more prominent hyper-radiosensitivity than the two squamous carcinoma cell lines. Low doses inducing maximum hyper-radiosensitivity did not cause significant growth inhibition, micronucleation or apoptosis in BMG-1 cells, but a transient G(1)/S-phase block was evident. Irradiating and incubating BMG-1 cells at high density for 0 or 4 h before plating, as well as irradiating cells as microcolonies, reduced hyper-radiosensitivity significantly, indicating the role of cell-cell contact-mediated processes. Liquid holding of BMG-1 cells in HBSS + 1% serum during and after irradiation for 4 h significantly reduced hyper-radiosensitivity, suggesting that hyper-radiosensitivity may be due partly to active damage fixation processes at low doses. Therefore, our findings suggest that the damage-induced signaling mechanisms influenced by (or mediated through) cell-cell contact or the cellular environment, as well as the lesion fixation processes, play an important role in hyper-radiosensitivity. Further studies are required to determine the exact nature of the damage that triggers these responses as well as for evaluating the potential of low-dose therapy.

The selectivity and specificity of the actions of the lipido-sterolic extract of Serenoa repens (Permixon) on the prostate

PURPOSE

To investigate the effects of the phytotherapeutic agent, Permixon(R), on primary cultures of fibroblast and epithelial cells from the prostate, epididymis, testes, kidney, skin and breast and to determine the selectivity and specificity of the action of the drug.

MATERIALS AND METHODS

All primary cultures were examined by electron microscopy before and following treatment with Permixon(R) (10 microg./ml.). In addition the apoptotic index was assessed by flow cytometry employing propidium iodide as a fluorophore. The impact of the drug on 5alpha-reductase (5alphaR) isoenzymes was also tested utilizing a pH specific assay.

RESULTS

There were changes in the morphology of prostate cells after treatment including accumulation of lipid in the cytoplasm and damage to the nuclear and mitochondrial membranes; no similar changes were observed in other cells. Permixon(R) increased the apoptotic index for prostate epithelial cells by 35% and 12% in the prostate stromal/fibroblast. A lesser apoptotic effect was demonstrated in skin fibroblast (3%) whereas none of the other primary cultures showed any increase in apoptosis when compared with the controls. Permixon(R) was also an effective inhibitor of both 5alphaR type I and II isoenzymes in prostate cells, but other cells showed no inhibition of 5alphaR activity following treatment with the plant extract.

CONCLUSIONS

This investigation demonstrated the selectivity of the action of Permixon(R) for prostate cells. The morphological changes in the prostate are accompanied by an increase in the apoptotic index along with an inhibition in the activity of the nuclear membrane bound 5alphaR isoenzymes. No similar changes were observed in any of the other cells under investigation.

Differential radiosensitising effect of the scid mutation among tissues, studied using high and low dose rates: implications for prognostic indicators in radiotherapy

To assess whether radiation-sensitive or radiation-resistant individuals should in principle be predicted equally well using different cell types, the effect of the scid mutation on the radiosensitivity of colony-forming cells in different murine tissues was assessed using high and low dose-rates. At high dose-rate, the amount of radiosensitization due to the scid mutation was greater in epithelial cells of the intestine and the kidney than in haemopoietic and fibroblastoid cells in the bone marrow, when expressed as a dose reduction factor. However, this greater radiosensitization in intestine and kidney did not translate into bigger differences in SF2 (surviving fraction at 2 Gy) or SF3.5. This was because of the greater inherent radioresistance of the epithelial cells compared with the marrow cells, resulting in smaller changes in cell survival from a given dose. Reductions in cell survival due to the mutation increased with increasing dose as expected at high dose rate. The changes in SF2 and SF3.5 due to the scid mutation were not significantly increased by using low dose-rates, because of the tendency for the presence of some low dose-rate sparing in the scid cells as well as the marked amount observed in the wild-type cells. The implications for predictive testing in radiotherapy are that for genetic defects resulting in the same type of radiosensitization phenomenon shown here for scid cells, radiosensitive or radioresistant cell types may still give similar differentials in response due to the mutation when SF2 is used as an endpoint.(ABSTRACT TRUNCATED AT 250 WORDS)

Drug and radiation resistance in spheroids: cell contact and kinetics

Cells from multicellular spheroids are often more resistant than monolayers to drugs and radiation. While explanations for resistance can be based on differences in cell cycle distribution, inability of the drug to penetrate the spheroid, or the presence of hypoxic cells, these mechanisms do not adequately explain resistance to all agents. Small spheroids (containing about 25-50 cells) exposed to ionizing radiation, hyperthermia, photodynamic therapy, or topoisomerase II inhibitors, are more resistant to killing than monolayers; the close three-dimensional contact in spheroids has been implicated in this resistance. Proposed mechanisms for the 'contact effect' include gap junctional 'reciprocity', cell shape mediated changes in (repair-related) gene expression, and alterations in chromatin packaging which influence DNA repair. The consequences of the contact effect are especially important for multifraction exposures. Another form of resistance can be demonstrated during repetitive treatments; 'regrowth resistance' reflects the capacity of spheroid cells to proliferate more efficiently to compensate for cell killing.

The distribution of colony-forming cells in the kidney

A method is described for producing outgrowths of small nephron segments (average 24 cells) in culture. The method was used to estimate an overall colony-forming efficiency of 4.6% for cells constituting the segments. Efficiency was found to be lower for thick segments (1%) than for thin segments (6%) from Henle's loop. The latter higher level indicates that precursor cells are concentrated near the middle of the nephron. For comparison, a two-dose irradiation technique was used to calculate a mean number of 5 +/- 2 (SE) clonogens per segment producing outgrowths. This tended to be higher than the value of about 1 calculated from the 65% of segments producing outgrowths, as expected if the remaining segments contained no clonogens.

The dose-fractionation sensitivity of the kidney; assessment of viable tubule cross-sections at 19 months after X irradiation

The formation of viable tubule cross-sections was assessed in histological sections of murine kidneys at 19 months after fractionated bilateral X-ray doses with 12 h intervals between fractions. The data were analysed using the linear-quadratic model which provides values of alpha and beta characterizing the slope of the dose-response curve, and the ratio of alpha and beta indicative of the sparing effect of dose fractionation. The tubule data were characterized by alpha = 0.057 +/- 0.009 Gy-1, beta = 0.011 +/- 0.001 Gy-2, alpha/beta = 5.0 +/- 0.9 Gy. Also, the number of cells (per focus region of the nephron) calculated as being capable of producing a viable focus was 2.5 +/- 0.5, which was confirmed using a separate two-dose approach (2.1 +/- 0.3). Together with other data, of the order of 1000 regenerative cells per nephron (10(4) total cells) can be deduced. The values of the fractionation sensitivity parameters are similar to values measured previously for cells taken from irradiated kidneys up to a year after irradiation and forming colonies in primary culture, and also similar to values assessed using various functional measures of kidney injury.