Connexins and cyclooxygenase-2 crosstalk in the expression of radiation-induced bystander effects

Background:

Signalling events mediated by connexins and cyclooxygenase-2 (COX-2) have important roles in bystander effects induced by ionising radiation. However, whether these proteins mediate bystander effects independently or cooperatively has not been investigated.

Methods:

Bystander normal human fibroblasts were cocultured with irradiated adenocarcinoma HeLa cells in which specific connexins (Cx) are expressed in the absence of endogenous Cx, before and after COX-2 knockdown, to investigate DNA damage in bystander cells and their progeny.

Results:

Inducible expression of gap junctions composed of connexin26 (Cx26) in irradiated HeLa cells enhanced the induction of micronuclei in bystander cells (P<0.01) and reduced the coculture time necessary for manifestation of the effect. In contrast, expression of connexin32 (Cx32) conferred protective effects. COX-2 knockdown in irradiated HeLa Cx26 cells attenuated the bystander response due to connexin expression. However, COX-2 knockdown resulted in enhanced micronucleus formation in the progeny of the bystander cells (P<0.001). COX-2 knockdown delayed junctional communication in HeLa Cx26 cells, and reduced, in the plasma membrane, the physical interaction of Cx26 with MAPKKK, a controller of the MAPK pathway that regulates COX-2 and connexin.

Conclusions:

Junctional communication and COX-2 cooperatively mediate the propagation of radiation-induced non-targeted effects. Characterising the mediating events affected by both mechanisms may lead to new approaches that mitigate secondary debilitating effects of cancer radiotherapy.

Regulation of normal cell cycle progression by flavin-containing oxidases

Mechanisms underlying the role of reactive oxygen species (ROS) generated by flavin-containing oxidases in regulating cell cycle progression were examined in human and rodent fibroblasts. Incubation of confluent cell cultures with nontoxic/nonclastogenic concentrations of the flavoprotein inhibitor, diphenyleneiodonium (DPI), reduced nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidase activity and basal ROS levels, but increased proteolysis of cyclin D1, p21(Waf1) and phospho-p38(MAPK). When these cells were allowed to proliferate by subculture in DPI-free medium, an extensive G(1) delay was observed with concomitant activation of p53/p21(Waf1) signaling and reduced phosphorylation of mitogen-activated kinases. Compensation for decreased oxidant generation by simultaneous exposure to DPI and nontoxic doses of the ROS generators, gamma-radiation or t-butyl-hydroperoxide, attenuated the G(1) delay. Whereas the DPI-induced G(1) checkpoint was completely dependent on PHOX91, ATM and WAF1, it was only partially dependent on P53. Interestingly, G(1) to S progression was not affected when another flavin-containing enzyme, nitric oxide synthase, was inhibited nor was it associated with changes in mitochondrial membrane potential. Proliferating cells treated with DPI also experienced a significant but attenuated delay in G(2). We propose that ATM performs a critical function in mediating normal cellular proliferation that is regulated by nonphagocytic NAD(P)H oxidase enzymes activity, which may serve as a novel target for arresting cancer cells in G(1).

Stress signaling from irradiated to non-irradiated cells

Evidence accumulated over the past two decades has indicated that exposure of cell populations to ionizing radiation results in significant biological effects occurring in both the irradiated and non-irradiated cells in the population. This phenomenon, termed the "bystander response", has been shown to occur both in vitro and in vivo. Experiments have indicated that genetic alterations, changes in gene expression and lethality occur in bystander cells that neighbor directly irradiated cells. Furthermore, cells recipient of growth medium harvested from irradiated cultures exhibit responses similar to those of the irradiated cells. Several mechanisms involving secreted soluble factors, gap-junction intercellular communication and oxidative metabolism have been proposed to regulate the radiation-induced bystander effect. In this review, our current knowledge of this phenomenon and its potential impact both on the estimation of risks of exposure to low doses/low fluences of ionizing radiation and on radiotherapy is discussed.

Bystander effects: intercellular transmission of radiation damage signals

Biological effects were examined in confluent cultures of fibroblasts and epithelial cells exposed to very low mean doses of alpha radiation, doses by which only 1-2% of the cells were actually traversed by an alpha particle. Enhanced frequencies of sister chromatid exchanges and HPRT mutations occurred in the non-irradiated, 'bystander' cells associated with a similar increase in the frequency of micronuclei, indicating the induction of DNA damage in these cells. In order to gain information concerning molecular pathways, changes in gene expression were examined in bystander cells by western analysis and in situ immunofluorescence staining. The expression levels of p53, p21 and MDM2 were significantly modulated in bystander cells; the damage signals leading to these changes were transmitted from irradiated to bystander cells by gap junction mediated intercellular communication. The bystander response was suppressed by incubation with superoxide dismutase as well as an inhibitor of NADPH oxidase, suggesting the effect may be mediated by oxidative stress. To examine other signalling pathways responsive to oxidative stress, the activation of stress-related kinases and their downstream transcription factors were analysed in bystander cells by western blotting and electrophoretic mobility shift assays; a 2-4-fold increase in the phosphorylation levels of JNK, ERK1/2, p90RSK, Elk-1 and ATF2 was observed. These changes were detected by 15 min after irradiation and persisted for at least 1 h. These findings indicate the activation of multiple signal transduction pathways in bystander cells, involving signals arising from the plasma membrane as well as from DNA damage.

Direct evidence for the participation of gap junction-mediated intercellular communication in the transmission of damage signals from -particle irradiated to nonirradiated cells

It has generally been considered that important biological effects of ionizing radiation arise as a direct consequence of DNA damage occurring in irradiated cells. We have examined this hypothesis by exposing cells to very low fluences of alpha-particles, similar to those emitted by radon gas, such that as few as 1% of the cells in a population are traversed by a particle and thus receive any radiation exposure. By using the endpoints of changes in gene expression and induction of DNA damage, we show that nonirradiated "bystander" cells participate in the overall response of confluent density-inhibited populations of cultured fibroblast and epithelial cells. By in situ immunofluorescence techniques and the use of cells genetically compromised in their ability to perform gap junction intercellular communication, we present direct evidence for the involvement of connexin43-mediated intercellular communication in the transmission of damage signals to nonirradiated cells. Induction of the stress-inducible p21(Waf1) protein in aggregates of neighboring cells far exceeding the fraction of cells whose nucleus has been traversed occurred in gap junction-competent cells only. These changes in p21(Waf1) expression correlated with both the induction of DNA damage (as measured by micronucleus formation) as well as increased Ser-15 phosphorylation of p53.

ATM complexes with HDM2 and promotes its rapid phosphorylation in a p53-independent manner in normal and tumor human cells exposed to ionizing radiation

To further understand the mechanism(s) by which DNA damage activates p53, we analysed the expression levels of p53 and HDM2 (the human homolog of murine MDM2) in various human diploid fibroblast and tumor cell strains during the period that precedes activation of known downstream effectors of p53. In X-irradiated human cells, HDM2 protein was rapidly phosphorylated in serine/threonine residues in a p53, p14ARF and p73-independent manner. In p53 wild-type cells, HDM2 phosphorylation precedes a detectable increase in the levels of p53 and is not observed in ataxia telangiectasia (AT) fibroblasts. The transfection of AT cells with a vector expressing ATM restored the ability to rapidly phosphorylate HDM2 following X-irradiation, confirming a role for ATM in its phosphorylation. We also show that ATM complexes with HDM2. The DNA lesions signaling the early rapid phosphorylation of HDM2 are a result of X-ray and not UV-type damage. The ATM-promoted early covalent modification of HDM2 in X-irradiated human cells may provide a mechanism to activate p53.

High and low fluences of alpha-particles induce a G1 checkpoint in human diploid fibroblasts

The effects of exposure to high and very low fluence alpha-particles on the G1 checkpoint were investigated in human diploid fibroblasts irradiated and released from density-inhibited confluent cultures by the use of the cumulative labeling index method. Transient and permanent arrests in G1 occurred in fibroblast populations exposed to mean doses as low as 1 cGy, suggesting that nontraversed bystander cells may contribute to the low dose response. In cells exposed to high fluences, the G1 checkpoint is at least as extensive as in gamma-irradiated cells. In contrast to gamma-irradiated cells, neither repair of potentially lethal damage nor a reduction in the fraction of cells transiently or permanently arrested in G1 were observed in cells held in confluence for 6 h after alpha-particle irradiation. Studies with isogenic wild-type, p53-/-, and p21Waf1-/- mouse embryo fibroblasts exposed to either gamma or alpha-particle radiation revealed a total lack of G1 arrest in either p53-/- or p21waf1-/- cells, indicating that the G1 checkpoint in wild-type cells is p53-dependent and that p21Wf1 fully mediates the role of p53 in its induction. In contrast to human cells, mouse embryo fibroblasts do not undergo a permanent G1 arrest. Except under conditions favoring potentially lethal damage repair, a comparable expression pattern of p53, p21Waf1, and other cell cycle-regulated proteins (pRb, p34cdc2, and cyclin B1) was observed in alpha-particle or gamma-irradiated human fibroblasts.

Regulation by ionizing radiation of CDC2, cyclin A, cyclin B, thymidine kinase, topoisomerase IIalpha, and RAD51 expression in normal human diploid fibroblasts is dependent on p53/p21Waf1

Induced cell cycle delays were among the first described cellular responses to ionizing radiation (IR). To understand the sensitivity and the molecular events involved in the response to low doses of IR and to examine the role of p53 and its downstream effector p21Waf1, we measured changes in expression of genes postulated to be involved in the cellular response to IR. Expression levels were examined in normal human diploid fibroblasts irradiated and maintained in quiescent density-inhibited growth up to 24-48 h after exposure to X-ray doses as low as 0.1-0.3 Gy, which have negligible effects on cell survival. Among 31 genes analyzed, we observed down-regulation in response to IR of the mRNA levels of CDC2, cyclin A, cyclin B, thymidine kinase, topoisomerase IIalpha, and RAD51. A similar reduction in the expression levels of these genes occurred when irradiated cells were released from confluence and allowed to proliferate. This was not observed in cells in which p53 function was defective and up-regulation of p21Waf1 levels either did not occur (E6 transfected normal human fibroblasts and Li-Fraumeni fibroblasts) or was delayed (ataxia telangiectasia fibroblasts) after irradiation. Down-regulation was also absent in p21Waf1-null mouse embryo fibroblasts (MEFs) but occurred at a lower level in p53-null MEFs, due to slight increases in p21Waf1 levels by a p53-independent pathway. These findings indicate that the down-regulation of these cell cycle regulated genes in irradiated cells is p53-dependent and involves its effector p21Waf1. Although no down-regulation in the expression of genes involved in G2-M was observed in p53 or in p21Waf1-null MEFs, these cells showed a G2-M delay after irradiation, indicating that the expression levels of these genes does not regulate the G2-M delay.

Intercellular communication is involved in the bystander regulation of gene expression in human cells exposed to very low fluences of alpha particles

We demonstrate by western analysis that the expression levels of TP53 (formerly known as p53), CDKN1A (formerly known as p21Waf1), CDC2 (formerly known as p34cdc2), CCNB1 (cyclin B1) and RAD51 are significantly modulated in confluent, density-inhibited human diploid cell populations exposed to doses where only a small fraction of the nuclei are actually traversed by an alpha-particle track. The extent of modulation of TP53 and CDKN1A is significantly reduced in the presence of the gap junction inhibitor lindane and in irradiated low-density cell populations. In situ immunofluorescence studies show that at doses where about 2% of the nuclei would be traversed by an alpha particle, induction of CDKN1A occurs in more cells than predicted. Furthermore, the induced cells are present in isolated aggregates of neighboring cells. Therefore, our studies at the gene expression level indicate that similar signaling pathways are induced in bystander cells that are not traversed by an alpha particle as in traversed cells, and that biological effects in cell populations are not restricted to the response of individual cells to the DNA damage they receive.

CDC2 is down-regulated by ionizing radiation in a p53-dependent manner

The mammalian cellular response to ionizing radiation results in delays in progression through the cell cycle at several checkpoints and includes alterations in the activity of cyclin-dependent kinases. The product of the CDC2 gene is a key kinase involved in cell cycle progression. The signaling events that regulate its expression after exposure to DNA-damaging agents are not known. We show that cdc2 mRNA and protein are down-regulated after irradiation of normal human and mouse fibroblasts with doses as low as 0.5 Gy. This down-regulation is preceded by induction of p53 and p21Waf1 proteins. In human cells in which p53 was nonfunctional and in p53-/- or p21-/- mouse embryo fibroblasts, no effect of ionizing radiation on p34cdc2 expression levels was observed. These findings indicate that CDC2 down-regulation after irradiation is p53-dependent and involves the cyclin-dependent kinase inhibitor p21Waf1 as a negative factor in the control of CDC2 expression. Correspondence between the delay in initiation of DNA synthesis in irradiated cells and the down-regulation of CDC2 is described.

Low-dose ionizing radiation decreases the frequency of neoplastic transformation to a level below the spontaneous rate in C3H 10T1/2 cells

We have previously shown that chronic exposure of plateau-phase C3H 10 T1/2 cells to (60)Co gamma radiation at doses as low as 10 cGy protected the cells against neoplastic transformation by a subsequent large acute radiation exposure. We have also shown that this induced resistance to neoplastic transformation correlated with an increased ability to repair radiation-induced chromosome breaks. We now show that a single exposure of quiescent cells to doses as low as 0.1 cGy also reduces the risk of neoplastic transformation, from the spontaneous level to a rate three- to fourfold below that level. Higher doses, up to 10 cGy at the same dose rate (0.24 cGy/min), did not reduce the neoplastic transformation frequency further. This protective effect was seen only in irradiated cells that were allowed to incubate at 37 degrees C before release from contact inhibition. Cells released into low-density subcultures immediately after irradiation had unchanged neoplastic transformation frequencies. These results demonstrate that low or chronic exposure to radiation can induce processes which protect the cell against naturally occurring as well as radiation-induced alterations that lead to cell transformation. If similar processes are induced in human cells, the results also suggest that a single low dose, at background or occupational exposure levels, may in some circumstances reduce rather than increase cancer risk, a conclusion inconsistent with the linear no-threshold model of cancer risk from radiation.

Use of semiquantitative reverse transcription-polymerase chain reaction to study gene expression in normal human skin fibroblasts following low dose-rate irradiation

One way to study the effect of radiation on gene expression is to monitor changes in the levels of specific messenger RNAs. We describe the use of reverse transcription-polymerase chain reaction (RT-PCR) analysis, a faster and more sensitive procedure than the traditional techniques to monitor RNA levels. Using RT-PCR, we confirmed previous results showing increased levels of GADD45 transcripts after high dose-rate X-irradiation in normal human fibroblasts. No differences were observed in the transcript levels of beta-ACTIN, beta-MICROGLOBULIN, Cu-Zn SUPEROXIDE DISMUTASE (SOD-1) and CATALASE. In cells exposed to 3-6 Gy low dose-rate gamma-irradiation we observed increased levels of the GADD45 transcript and lower transcript levels of the genes TOPOISOMERASE II alpha, FACC, CYCLIN A and CYCLIN B. No differences were detected in the transcript levels of beta-ACTIN, beta-MICROGLOBULIN, SOD-1, URACYL-DNA GLYCOSYLASE, CYCLIN C, CYCLIN E, CYCLIN D1, CYCLIN D2, CYCLIN D3, TOPOISOMERASE I and TOPOISOMERASE II beta.

Pituitary hormones regulate c-myc and DNA synthesis in lymphoid tissue

Hypophysectomy of Fischer 344 rats of both sexes led to a rapid involution of the thymus and spleen which was associated with a profound decrease in spontaneous DNA synthesis in these organs. The proportion of B lymphocytes in the spleen, of T cells and their subsets (CD4+/CD8+) in spleen and thymus, and the histological structure of the involuted organs remained normal. Treatment of hypophysectomized animals with growth hormone (GH) or prolactin (PRL) stimulated the expression of the c-myc proto-oncogene and DNA synthesis and reversed the involution in these organs. Replacement doses of adrenocorticotrophic hormone, follicle-stimulating hormone, luteinizing hormone, or thyroid-stimulating hormone had no influence on thymus or spleen size and DNA synthesis. A rapid expression of c-myc was also observed in thymuses and spleens of intact rats after the injection of GH or PRL. In vitro physiological concentrations (2.5 ng/ml) of either ovine or rat PRL or GH stimulated the incorporation of [3H]thymidine by thymus and spleen cells. These results indicate that GH and PRL regulate lymphocyte growth. This regulatory role is likely to serve as the principal mechanism of immunoregulation by these hormones.

Pituitary hormones regulate c-myc and DNA synthesis in lymphoid tissue

Hypophysectomy of Fischer 344 rats of both sexes led to a rapid involution of the thymus and spleen which was associated with a profound decrease in spontaneous DNA synthesis in these organs. The proportion of B lymphocytes in the spleen, of T cells and their subsets (CD4+/CD8+) in spleen and thymus, and the histological structure of the involuted organs remained normal. Treatment of hypophysectomized animals with growth hormone (GH) or prolactin (PRL) stimulated the expression of the c-myc proto-oncogene and DNA synthesis and reversed the involution in these organs. Replacement doses of adrenocorticotrophic hormone, follicle-stimulating hormone, luteinizing hormone, or thyroid-stimulating hormone had no influence on thymus or spleen size and DNA synthesis. A rapid expression of c-myc was also observed in thymuses and spleens of intact rats after the injection of GH or PRL. In vitro physiological concentrations (2.5 ng/ml) of either ovine or rat PRL or GH stimulated the incorporation of [3H]thymidine by thymus and spleen cells. These results indicate that GH and PRL regulate lymphocyte growth. This regulatory role is likely to serve as the principal mechanism of immunoregulation by these hormones.