p53, mutation frequency and apoptosis in the murine small intestine

The Protective Effects of Sour Orange (Citrus aurantium L.) Polymethoxyflavones on Mice Irradiation-Induced Intestinal Injury

Sour orange (Citrus aurantium L.) is one of the biological sources of polymethoxyflavones (PMFs), which are often used to deal with gastrointestinal diseases. The intestine is highly sensitive to irradiation damage. However, limited certain cures have been released for irradiation-induced gastrointestinal injury, and the potentials of sour orange PMFs as radio-resistance agents have not been fully discussed yet. The present study aims to (1) investigate the PMF components in 12 sour orange cultivars, (2) determine the protective effects of PMFs on irradiation-induced intestinal injury by treating mice that received 12 Gy abdominal irradiation with different doses of PMFs and observing the changes in organ indexes and pathological sections and (3) test cytotoxicity of PMFs by CCK-8 method. The results showed that sour orange PMFs appeared to have high intraspecies similarity. Besides, PMFs protected mice from irradiation-induced injury by alleviating body weight loss, reliving organ index changing and maintaining the intestinal structure. Finally, IC₅₀ concentrations to cell line CCD 841 CoN of PMFs and nobiletin were calculated as 42.23 μg/mL and 51.58 μg/mL, respectively. Our study uncovered PMF contents in 12 sour orange materials and determined the protective effects on irradiation-induced intestinal injuries, providing guidance for the utilization of sour orange resources.

New insights into negative effects of lithium on sea urchin Paracentrotus lividus embryos

The diffuse use of lithium in a number of industrial processes has produced a significant contamination of groundwater and surface water with it. The increased use of lithium has generated only scarce studies on its concentrations in ambient waters and on its effects on aquatic organisms. Only few contributions have focused on the toxicity of lithium in marine organisms (such as marine animals, algae and vegetables), showing that the toxic effect depends on the animal species. In the present study we describe the morphological and the molecular effects of lithium chloride (LiCl), using the sea urchin Paracentrotus lividus as a model organism. We show that LiCl, if added to the eggs before fertilization, induces malformations in the embryos in a dose-dependent manner. We have also followed by RT qPCR the expression levels of thirty seven genes (belonging to different classes of functional processes, such as stress, development, differentiation, skeletogenesis and detoxifications) to identify the molecular targets of LiCl. This study opens new perspectives for the understanding of the mechanism of action of lithium on marine organisms. The findings may also have relevance outside the world of marine organisms since lithium is widely prescribed for the treatment of human bipolar disorders.

Toxic Diatom Aldehydes Affect Defence Gene Networks in Sea Urchins

Marine organisms possess a series of cellular strategies to counteract the negative effects of toxic compounds, including the massive reorganization of gene expression networks. Here we report the modulated dose-dependent response of activated genes by diatom polyunsaturated aldehydes (PUAs) in the sea urchin Paracentrotus lividus. PUAs are secondary metabolites deriving from the oxidation of fatty acids, inducing deleterious effects on the reproduction and development of planktonic and benthic organisms that feed on these unicellular algae and with anti-cancer activity. Our previous results showed that PUAs target several genes, implicated in different functional processes in this sea urchin. Using interactomic Ingenuity Pathway Analysis we now show that the genes targeted by PUAs are correlated with four HUB genes, NF-κB, p53, δ-2-catenin and HIF1A, which have not been previously reported for P. lividus. We propose a working model describing hypothetical pathways potentially involved in toxic aldehyde stress response in sea urchins. This represents the first report on gene networks affected by PUAs, opening new perspectives in understanding the cellular mechanisms underlying the response of benthic organisms to diatom exposure.

Obesity and intestinal epithelial deletion of the insulin receptor, but not the IGF 1 receptor, affect radiation-induced apoptosis in colon

Current views suggest that apoptosis eliminates genetically damaged cells that may otherwise form tumors. Prior human studies link elevated insulin and reduced apoptosis to risk of colorectal adenomas. We hypothesized that hyperinsulinemia associated with obesity would lead to reduced colon epithelial cell (CEC) apoptosis after radiation and that this effect would be altered by deletion of the insulin-like growth factor (IGF) 1 receptor (IGF1R) or the insulin receptor (IR). Mice with villin-Cre-mediated IGF1R or IR deletion in CECs and floxed littermates were fed a high-fat diet to induce obesity and hyperinsulinemia or control low-fat chow. Mice were exposed to 5-Gy abdominal radiation to induce DNA damage and euthanized 4 h later for evaluation of apoptosis by localization of cleaved caspase-3. Obese mice exhibited decreased apoptosis of genetically damaged CECs. IGF1R deletion did not affect CEC apoptosis in lean or obese animals. In contrast, IR loss increased CEC apoptosis in both diet groups but did not prevent antiapoptotic effects of obesity. Levels of p53 protein were significantly reduced in CECs of obese mice with intact IR but increased in both lean and obese mice without IR. Levels of mRNAs encoding proapoptotic Perp and the cell cycle inhibitor Cdkn1b/p27 were reduced in CECs of obese mice and increased in lean mice lacking IR. Together, our studies provide novel evidence for antiapoptotic roles of obesity and IR, but not IGF1R, in colonic epithelium after DNA damage. However, neither IR nor IGF1R deletion prevented a reduction in radiation-induced CEC apoptosis during obesity and hyperinsulinemia.

Zinc-induced G2/M blockage is p53 and p21 dependent in normal human bronchial epithelial cells

The involvement of p53 and p21 signal pathway in the G2/M cell cycle progression of zinc-supplemented normal human bronchial epithelial (NHBE) cells was examined using the small interferring RNA (siRNA) approach. Cells were cultured for one passage in a different concentration of zinc: <0.4 microM (ZD) as zinc deficient; 4 microM as normal zinc level (ZN) in culture medium; 16 microM (ZA) as normal human plasma zinc level; and 32 microM (ZS) as the high end of plasma zinc attainable by oral supplementation. Nuclear p21 protein and mRNA levels as well as promoter activity in ZS cells, but not in ZD cells, were markedly elevated to almost twofold compared with ZN control cells. G2/M blockage in ZS cells was coupled with the observation of elevated p21 gene expression. In ZS cells, the abrogation of p21 protein induction by the transfection of p21 siRNA was shown to alleviate the G2/M blockage, demonstrating the positive linkage of p21 elevation and G2/M blockage. Abolishment of the increase in p53 protein in ZS cells with transfection of p53 siRNA normalized the elevated p21 protein to a similar level as in ZN control cells, which demonstrated that the p21 induction is p53 dependent. Furthermore, the normalization of p53 protein by siRNA treatment in ZS cells alleviated cell growth depression and G2/M blockage, which demonstrated that p53 was involved in the high zinc status-induced G2/M blockage and growth depression. Thus high zinc status in NHBE cells upregulates p53 expression which in turn elevates p21 that eventually induces G2/M blockage.

53BP1 deficiency in intestinal enterocytes does not alter the immediate response to ionizing radiation, but leads to increased nuclear area consistent with polyploidy

The p53-binding protein 53BP1 has been implicated in the DNA damage response and genomic instability. Previous reports have highlighted these roles in vivo in haematopoietic lineages. To investigate the importance of 53BP1 to the DNA damage response in epithelial cells in vivo, we have investigated the role of 53BP1 in mediating apoptosis and proliferation within the murine small intestine following gamma-irradiation. 53BP1 deficiency does not affect the immediate response to gamma-irradiation with normal levels of apoptosis, proliferation and p53 and p21 accumulation. However, 48 h post-gamma-irradiation there was a significant accumulation of cells with much larger nuclei marked by p53 and p21 accumulation. These data reflect increases in polyploidy observed 53BP1-/- deficient fibroblasts following gamma-irradiation. At 72 h post-irradiation both the 4N and 8N populations were significantly increased in 53BP1-/- MEFS. Taken together, these results show that following in vivo exposure to gamma-irradiation, 53BP1 is dispensable for signalling apoptosis and cell-cycle arrest in the intestinal epithelium. However, it is important for prevention of genomic instability within this epithelial cell population.

DNA damage-induced apoptosis: insights from the mouse

The availability of murine models with precisely defined genetic lesions has greatly increased our understanding of the genetic control of cell death, with functional dependence established for a wide range of genes including (amongst others) the p53 and Bcl-2 gene family members, the mismatch repair (MMR) genes and the methyl binding domain family member Mbd4. These studies raised the attractive hypotheses that tumour predisposition may be explained in terms of failed cell death, and also that tumour regression may be initiated through activation of an apoptotic programme. The studies that have addressed these notions have revealed complex consequences of a failed death programme, such that these simple hypotheses have not always been supported. Remarkably, however, some tissues show more predictable responses than others, most apparent in the contrast between the intestine and the haematopoietic system. This review will focus upon a discussion of these relationships, and will also consider the relevance of some of these findings to tumour predisposition and regression.

Exposure to ultraviolet radiation causes apoptosis in developing sea urchin embryos

Laboratory exposures of embryos from the sea urchin Strongylocentrotus droebachiensis to ultraviolet B radiation (UV-B, 290-320 nm), equivalent to a depth of 1-3 m in the Gulf of Maine, resulted in significant damage to DNA measured as cyclobutane pyrimidine dimer formation. Cells with DNA damage caused by ultraviolet radiation (UVR, 290-400 nm) and oxidative stress can survive, but are often retained in the G1/S phase of the cell cycle to repair DNA as a result of the expression of cell cycle genes such as p53 and p21, and the subsequent inhibition of the activity of cyclin-dependent kinases such as cdc2; if DNA cannot be repaired it can lead to programmed cell death or apoptosis. Sea urchin embryos exposed to UV-B radiation exhibit significantly higher protein concentrations of the antioxidant enzyme superoxide dismutase, and the transcriptional activators p53 and p21. The downstream activator of the cell cycle, cdc2, showed significantly lower protein concentrations with exposure to increasingly shorter wavelengths of UVR. Decreases in cdc2 could have been caused directly by exposure to UV-B or as a result of downregulation via the p53, p21 cascade, or both. These cellular events lead to apoptosis, as shown by the significant increase in DNA strand breaks observed in the nuclei of developing embryos exposed to UVR using the TUNEL assay. Cellular death, and a decrease in sea urchin embryo survivorship, are caused by the indirect and direct effects of exposure to UVR that leads to apoptosis in these laboratory experiments.

p73alpha is a candidate effector in the p53 independent apoptosis pathway of cisplatin damaged primary murine colonocytes

AIMS

Colonocytes were derived from wild-type (wt) and p53 deficient mice to investigate p53 dependent and independent death pathways after cisplatin treatment, and the role of p53 in growth regulation of primary, untransformed epithelial cells.

METHODS

Wt and p53 null colonocytes were exposed to cisplatin and DNA synthesis, apoptosis, and p53, p21, and p73 expression were investigated after six, 12, and 24 hours. Major p73 isoforms were identified by reverse transcription polymerase chain reaction (RT-PCR).

RESULTS

Cisplatin treated wt cells exhibited cell cycle arrest, whereas p53 null cells continued to synthesise DNA, although both cell types died. Apoptosis was significantly higher in cisplatin treated wt and p53 null colonocytes than in controls at all timepoints, although apoptosis was lower in cisplatin treated p53 null colonocytes than in wt cells. p53 expression was upregulated in cisplatin treated wt colonocytes. p21 expression was high and remained unchanged in cisplatin treated wt cells, although it was reduced in the absence of p53. p73 was investigated because it could account for p53 independent p21 expression and p53 independent death. RT-PCR detected full length p73alpha. p73 transcript levels remained unchanged, whereas p73 protein accumulated in the nucleus of cisplatin treated cells, irrespective of genotype.

CONCLUSIONS

p53 is essential for cell cycle arrest, but not apoptosis in primary murine colonocytes. Apoptosis is reduced in cisplatin treated p53 null cells. Nuclear accumulation of endogenous p73 after cisplatin treatment suggests a proapoptotic role for p73alpha in the absence of p53 and collaboration with p53 in wt colonocytes.

Mouse models with modified p53 sequences to study cancer and ageing

Experiments with p53 transgenic and p53 gene-targeted mouse strains have substantiated, and in some cases challenged, a number of hypotheses on the biology of the p53 protein. New questions have emerged regarding similarities and differences between murine and human genetic networks in various tissues. Mouse models with targeted p53 alleles are now applied not only to investigate tumour susceptibility, but also to address questions pertinent to molecular epidemiology, chemoprevention, development of anticancer p53-specific pharmaceuticals, and ageing.

Heterozygosity for p53 promotes microsatellite instability and tumorigenesis on a Msh2 deficient background

In colorectal tumorigenesis, loss of function of the mismatch repair genes is closely associated with genomic instability at the nucleotide level whereas p53 deficiency has been linked with gross chromosomal instability. We have addressed the contribution of these two forms of genetic instability to tumorigenesis using mice mutant for Msh2 and p53. As previously reported, deficiency of both genes leads to rapid lymphomagenesis Here we show that heterozygosity for p53 also markedly reduces survival on an Msh2 null background. We characterized the patterns of genomic instability in a small set of tumours and showed that, as predicted p53 deficiency predisposes to aneuploidy and Msh2 deficiency leads to microsatellite instability (MSI). However, heterozygosity for p53 in the absence of Msh2 resulted in increased MSI and not aneuploidy. This implied role for p53 in modulating MSI was confirmed using a large cohort of primary fibroblast clones. The differences observed were highly significant (P<0.01) in both the fibroblast clones (which all retained p53 functionality) and the tumours, a proportion of which retained p53 functionality. Our results therefore demonstrate a dose sensitive role for p53 in the maintenance of genomic integrity at the nucleotide level.

The ability to engage enterocyte apoptosis does not predict long-term crypt survival in p53 and Msh2 deficient mice

Apoptosis and long term enterocyte survival were examined in vivo after exposure to three cytotoxic agents (Cisplatin, Nitrogen Mustard and N-methyl-N-nitrosourea (NMNU/MNU)) within mice either singly or doubly mutant for p53 and Msh2. P53 deficiency caused abrogation of the immediate apoptotic response to each agent, but only led to increased survival after cisplatin treatment. Msh2 deficiency reduced the apoptotic response to each agent, but only led to increased crypt survival after NMNU treatment. Following cisplatin treatment, the response of (Msh2(-/-), p53(-/-)) mice paralleled that of the p53(-/-) mice. A delayed wave of apoptosis was observed in both p53(-/-) and (Msh2(-/-), p53(-/-)) mice demonstrating this phenomenon to be independent of functional Mismatch repair (MMR). We conclude that loss of either p53 or Msh2 dependent apoptosis does not predict long-term crypt survival in vivo, however genetic status clearly can modulate survival for some agents such as cisplatin.

Radiation-induced genetic instability in vivo depends on p53 status

In response to ionizing radiation and other agents that damage DNA, the p53 tumor suppressor protein activates multiple cellular processes including cell cycle checkpoints and programmed cell death. Although loss of p53 function is associated with radiation-induced genetic instability in cell lines, it is not clear if this relationship exists in vivo. To study the role of p53 in maintenance of genetic stability in normal tissues following irradiation, we have measured mutant frequencies at the adenine phosphoribosyltransferase (Aprt) and hypothanine-guanine phosphoribosyltransferase (Hprt) loci and examined mechanisms of loss of heterozygosity (LOH) in normal T cells of p53-deficient, Aprt heterozygous mice that were subjected to whole-body irradiation with a single dose of 4Gy X-rays. The radiation-induced mutant frequency at both the Aprt and Hprt loci was elevated in cells from mice with different p53 genotypes. The radiation-induced elevation of p53-/- mice was significantly greater than that of p53+/- or p53+/+ mice and was caused by several different kinds of mutational events at the both chromosomal and intragenic levels. Most significantly, interstitial deletion, which occurs rarely in unirradiated mice, became the most common mechanism leading to LOH in irradiated p53 null mice. These observations support the idea that absence or reduction of p53 expression enhances radiation-induced tumorigenesis by increasing genetic instability at various loci, such as those for tumor suppressor genes.

Oxidative stress, DNA damage and p53 expression in the larvae of atlantic cod (Gadus morhua) exposed to ultraviolet (290–400 nm) radiation

Decreases in stratospheric ozone levels from anthropogenic inputs of chlorinated fluorocarbons have resulted in an increased amount of harmful ultraviolet-B (UVB, 290–320 nm) radiation reaching the sea surface in temperate latitudes (30–50 degrees N). In the Gulf of Maine, present-day irradiances of ultraviolet-A (UVA, 320–400 nm) radiation can penetrate to depths of 23 m and UVB radiation can penetrate to depths of 7–12 m, where the rapidly developing embryos and larvae of the Atlantic cod (Gadus morhua) are known to occur. Laboratory exposures of embryos and larvae of Atlantic cod to ultraviolet radiation (UVR) equivalent to a depth of approximately 10 m in the Gulf of Maine resulted in significant mortality of developing embryos and a decrease in standard length at hatching for yolk-sac larvae. Larvae at the end of the experimental period also had lower concentrations of UVR-absorbing compounds and exhibited significantly greater damage to their DNA, measured as cyclobutane pyrimidine dimer formation, after exposure to UVB radiation. Larvae exposed to UVB radiation also exhibited significantly higher activities and protein concentrations of the antioxidant enzyme superoxide dismutase and significantly higher concentrations of the transcriptional activator p53. p53 is expressed in response to DNA damage and can result in cellular growth arrest in the G1- to S-phase of the cell cycle or to programmed cell death (apoptosis). Cellular death caused by apoptosis is the most likely cause of mortality in embryos and larvae in these laboratory experiments, while the smaller size at hatching in those larvae that survived is caused by permanent cellular growth arrest in response to DNA damage. In addition, the sub-lethal energetic costs of repairing DNA damage or responding to oxidative stress may also contribute to poor individual performance in surviving larvae that could also lead to increases in mortality. The irradiances of UVB radiation that elicit these responses in cod larvae can occur in many temperate latitudes, where these ecologically and commercially important fish are known to spawn, and may contribute to the high mortality of cod embryos and larvae in their natural environment.

P53 null mice: damaging the hypothesis?

P53 is extremely well characterised as a tumour suppressor gene, and many activities have been attributed to it which are consistent with this function. However, despite being the subject of intense study it still remains unclear precisely which of these functions is crucial to its in vivo role as a tumour suppressor gene. This is particularly true of its role in the induction of apoptosis. The original observation of p53-dependent apoptosis gave rise to the following hypothesis: namely, that p53 deficiency leads to a persistence of DNA damaged cells which are the potential founders of malignancy. This review summarises the data for and against this hypothesis, with specific emphasis on data obtained from studies of the murine intestine. What emerges from these studies is a complex picture, where data can be obtained in support of this hypothesis, but there are many circumstances which exist where it is not supported. Taken together this collection of data suggests that the abrogation of p53-dependent apoptosis may indeed impact upon carcinogenesis and neoplastic progression, but that the simplistic notion of p53 as the single gatekeeper of this pathway is untenable.

Chromosome instability contributes to loss of heterozygosity in mice lacking p53

The p53 tumor suppressor protein participates in multiple cellular processes including cell cycle checkpoints and programmed cell death. In cell lines, loss of p53 function is associated with increased genetic instability including aneuploidy, gene amplification, and point mutation. Although similar genetic instability often accompanies the progression of malignancy in tumors, its role in tumor initiation in normal cells is not clear. To study whether or not loss of p53 leads to genetic instability in normal cells in vivo, we have examined mechanisms of loss of heterozygosity (LOH) at the Aprt (adenine phosphoribsyltransferase) and flanking loci in normal fibroblasts and T lymphocytes of p53-deficient mice. Somatic cell variants that arose in vivo as a consequence of genetic or epigenetic alterations abolishing Aprt function were selected and expanded in vitro by virtue of their resistance to 2,6-diaminopurine (DAP). We observed that p53 null mice produced about three times as many DAP-resistant fibroblast colonies than wild-type mice, but the frequency of DAP-resistant T lymphocyte colonies was not significantly changed. Mitotic recombination, but not point mutation, partly accounted for the increase in the frequency of DAP-resistant fibroblasts. Most significantly, chromosome loss/duplication and interstitial deletion, which were extremely rare events in the wild-type mice, represented a significant proportion of LOH events in both fibroblasts and T lymphocytes of p53 null mice. Also, increased interstitial deletion was observed in fibroblasts of p53 heterozygous mice. These data suggest that increased genetic variation, including chromosome instability, starts at the initiation stage of tumorigenesis when functional p53 is absent or reduced.

In vivo administration of O(6)-benzylguanine does not influence apoptosis or mutation frequency following DNA damage in the murine intestine, but does inhibit P450-dependent activation of dacarbazine

Clinically relevant cancer chemotherapeutic alkylating agents such as temozolomide and dacarbazine induce apoptosis and are mutagenic via the formation of O(6)-alkylguanine adducts in DNA. The DNA repair protein O(6)-alkylguanine-DNA alkyltransferase (AGT) functions by dealkylating such adducts and can thus prevent apoptosis and mutagenesis. In attempts to maximize the clinical effectiveness of these alkylating agents, inhibitors of AGT such as O(6)-benzylguanine (BeG) have been developed. We show here that within murine small intestinal crypt cells, BeG administration does not alter the apoptotic response to the direct-acting methylating agents N-methyl-N-nitrosurea (MNU), temozolomide and N-methyl-N'-nitro-N-nitrosoguanidine. Furthermore, we show that BeG pretreatment fails to elevate the mutation frequency at the murine Dlb-1 locus following exposure to MNU. Consistent with these results, we show that intestinal AGT activity is effectively abolished by administration of 100 mg/kg temozolomide, even in the absence of BeG. In contrast, pretreatment with BeG transiently abolished the apoptotic response to the methylating prodrug dacarbazine. Activation of dacarbazine to its reactive intermediate has previously been shown to be cytochrome P450 dependent and we show here that pretreatment of mice with the cytochrome P450 inhibitor metyrapone also inhibits dacarbazine-induced apoptosis. Thus BeG increases neither the prevalence of apoptosis nor mutation frequency in the murine small intestine, but is capable of inhibiting P450-dependent prodrug activation. The positive implication from this study is that BeG treatment may not exacerbate the toxic and mutagenic effects of methylating agents within normal cells, although it may engender other adverse reactions through the suppression of cytochrome P450-dependent processes.

Stem cells: the intestinal stem cell as a paradigm

Stem cell research provides a foundation for therapeutic advancement in oncology, clinical genetics and a diverse array of degenerative disorders. For example, the elucidation of pathways governing proliferative regulation and differentiation within cellular systems will result in medical strategies aimed at the root cause of cancer. At present the characterization of reliable stem cell markers is the immediate aim in this particular field. Over the past 30 years investigators have determined many of the physical and functional properties of stem cells through careful and imaginative experimentation. Intestinal stem cells reside at the crypt base and give rise to all cell types found within the crypt. They readily undergo altruistic apoptosis in response to toxic stimuli although their progeny are hardier and will regain stem cell function to repopulate the tissue compartment, giving rise to the concept of a proliferative hierarchy. Contention exists when deciding whether the full complement of cells within a crypt is derived from either a single or multiple stems. Evidence has also arisen to challenge the long held view that colorectal tumours arise from a single mutated stem cell, as early adenomas from a human XO/XY mosaic contained distinct clones. Mechanisms governing the stem cell cycle and subsequent proliferative activity largely remain obscure. The adenomatous polyposis coli gene product has, however, been shown to promote the degradation of beta-catenin, an enhancer of cell proliferation, thereby downregulating this activity in healthy individuals.

Lessons from genetically engineered animal models. VII. Apoptosis in intestinal epithelium: lessons from transgenic and knockout mice

Apoptosis plays an important role in homeostasis of intestinal epithelia and is also a stress response to toxic stimuli. Transgenic and knockout mice have provided insights into the regulation of intestinal epithelial apoptosis that could not have been obtained by cell culture techniques. Two broad types of apoptosis have been characterized: spontaneous apoptosis, which occurs continuously at low levels in the normal, unstressed intestine, and stress-induced apoptosis, which occurs after genotoxic insult such as exposure to gamma radiation or DNA-damaging drugs. Spontaneous apoptosis occurs at the base of the crypt at or near the position of epithelial stem cells. Knockout studies have shown that spontaneous apoptosis is independent of p53 and Bax in both small and large intestine, whereas Bcl2 only regulates spontaneous apoptosis in the colon. Little is known about the regulation of the specialized form of cell death at the villus tip. In contrast, knockout studies have demonstrated that both p53 and Bcl2 are important regulators of stress-induced apoptosis but that there are significant differences between early and late time points. Bax plays only a minor role in the regulation of stress-induced apoptosis. The cumulative effect of stress-induced apoptosis on tissue architecture is not straightforward, and cell cycle arrest also plays a critical role. Nevertheless, p53 is an important determinant of the histopathological damage induced by 5-fluorouracil in murine intestinal epithelium. These studies have important implications for the development of more effective treatment for inflammatory bowel disease and cancer.

Damage-induced apoptosis in intestinal epithelia from bcl-2-null and bax-null mice: investigations of the mechanistic determinants of epithelial apoptosis in vivo

The influence of bcl-2 and bax expression on apoptotic cell death in mouse intestinal epithelia was assessed using homozygously null mice. Apoptosis was induced in vivo by the enterotoxin 5-fluorouracil (5FU) or by gamma-irradiation and its cell positional incidence was assessed. 5FU and gamma-radiation treated bax-null mice surprisingly showed no reductions in apoptotic yield in the small intestine or midcolon at 4.5 h at cell positions in which both agents had previously been shown to strongly induce p53 protein expression. The colonic epithelia of 5FU treated bcl-2-null mice showed elevated levels of apoptosis at 4.5 h: from 48 apoptotic events in wild-type mice to 273 in the nulls, scoring 200 half crypts. The increase occurred specifically in the cell positions considered to harbour colonic stem cells, at the base of crypts, where there is selective expression of bcl-2. There was a modest but significant increase in apoptosis in the small intestine of the bcl-2-null mice although the epithelia of wild-type mice here are not immunohistochemically positive for bcl-2 protein. These findings show that bcl-2 plays a key role in determining the sensitivity of colonic stem cells to damage-induced death but that bax is not responsible for the p53-dependent induction of apoptosis in this context.

Genotoxicity of 3'-azido-3'-deoxythymidine in the human lymphoblastoid cell line, TK6: relationships between DNA incorporation, mutant frequency, and spectrum of deletion mutations in HPRT

Perinatal treatment with 3'-azido-3'-deoxythymidine (AZT) has been found to reduce the rate of maternal-infant transmission of HIV; however, AZT is genotoxic in mammalian cells in vitro and induces tumors in the offspring of mice treated in utero. The purpose of the present study was to investigate the relationships between incorporation of AZT into DNA, and the frequency and spectrum of mutations at the HPRT locus of the human lymphoblastoid cell line, TK6, following in vitro exposures to AZT. Cells were cultured in medium containing 0 or 300 microM AZT for 1, 3, or 6 day(s) (n = 5/group). The effects of exposure duration on incorporation of AZT into DNA and HPRT mutant frequency were determined using an AZT radioimmunoassay and a cell cloning assay, respectively. AZT accumulated in DNA in a supralinear manner, approaching a plateau at 6 days of treatment (101.9 +/- 14.7 molecules AZT/10(6) nucleotides). After 3 days of AZT exposure, HPRT mutant frequency was significantly increased (1.8-fold, p = 0.016) compared to background (mutant frequency = 3.78 x 10(-6)). Multiplex PCR amplification of genomic DNA was used to determine the frequency of exon deletions in HPRT mutant clones from untreated cells versus AZT-treated cells. Molecular analyses of AZT-induced mutations revealed a significant difference in the frequency of total gene deletions (44/120 vs. 18/114 in controls, p = 0.004 by the Mann-Whitney U-statistic). In fact, the Chi-square test of homogeneity demonstrate that the differences between the control and AZT-treatment groups is attributed mainly to this increase in total gene deletion mutations (p = 0.00001). These data indicate that the primary mechanism of AZT mutagenicity in human TK6 cells is through the production of large deletions which occur as a result of AZT incorporation into DNA and subsequent chain termination. The data imply that perinatal chemoprophylaxis with AZT may put children of HIV-infected women at potential risk for genetic damage.

The role of p53 in bleomycin-induced DNA damage in the lung. A comparative study with the small intestine

To elucidate the role of p53 and apoptosis in the pathogenesis of lung injury, we examined histological changes, expressions of p53 and p21waf1/cip1 (p21), apoptosis, DNA double strand breaks, cell kinetics, and DNA synthesis in C57/BL6 mice (p53+/+) and mice deficient for p53 (p53-/-) at 2 hours to 7 days after a single intravenous administration of bleomycin. We also compared these parameters between the lung cells and small intestinal epithelial cells to explore potential differences in their response to DNA damage. Bleomycin induced p21 expression in a p53-dependent manner in p53+/+ mice but neither p53 nor p21 expression in p53-/- mice. In the lung of both groups of mice, focal inflammation followed by fibrosis was observed, but there was no evidence of apoptosis. Cells with DNA breaks and those undergoing DNA synthesis were unequivocally increased, but the cycling cell fraction remained unchanged, suggesting that the DNA synthesis detected in the lung reflected unscheduled DNA synthesis for repair of damaged DNA. DNA breaks and unscheduled DNA synthesis were prolonged in p53-/- mice compared to p53+/+ mice. By contrast, in the small intestine, marked cell cycle arrest and extensive apoptosis were evoked in the cycling crypt cells of both groups of mice, but these changes were milder and DNA breaks remained detectable for a longer time in p53-/- mice than in p53+/+ mice. Among the resting enterocytes in the villi, apoptosis was observed almost equally in both groups, but repair of DNA breaks was significantly delayed in the p53-/- mice. These observations imply that apoptosis is mediated largely by the p53-dependent pathway in the crypts but exclusively by the p53-independent pathway in the villi, that this pathway is particularly important in DNA repair in the villi, and that despite this difference in the significance of apoptosis, p53 plays an important role in DNA repair in both the crypts and villi. Our results suggest that the lung cells and small intestinal cells respond to the bleomycin treatment in different ways in terms of the induction of apoptosis and that p53 carries out an essential role in the early response to and repair of DNA damage by a non-apoptotic mechanism which appears to be crucial in the noncycling lung cells and enterocytes. Importantly, the p53-p21 pathway and apoptosis are unlikely to be essential for bleomycin-induced tissue injury in the lung.

The p53 tumor suppressor protein reduces point mutation frequency of a shuttle vector modified by the chemical mutagens (+/-)7, 8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene, aflatoxin B1 and meta-chloroperoxybenzoic acid

p53 has been postulated to be the guardian of the genome. However, results supporting the prediction that point mutation frequencies are elevated in p53-deficient cells either have not been forthcoming or have been equivocal. To analyse the effect of p53 on point mutation frequency, we used the supF gene of the pYZ289 shuttle vector as a mutagenic target. pYZ289 was treated in vitro by ultraviolet irradiation, aflatoxin B1, (+/-)7,8-dihydroxy-9, 10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene and meta-chloroperoxybenzoic acid and then transfected into p53-deficient cells with or without a p53 expression vector. p53 reduced the mutant frequency up to fivefold when pYZ289 was treated with aflatoxin B1, (+/-)7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene or meta-chloroperoxybenzoic acid but not when it was ultraviolet-irradiated. The p53-dependent mutation frequency reduction was higher at a higher level of premutational lesions for aflatoxin B1 and (+/-)7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene and at a lower level of lesions for meta-chloroperoxybenzoic acid. This suggests that the chemical mutagens produce, in a dose-dependent fashion, two kinds of DNA damage, one subject to p53-dependent mutation frequency reduction and the other not. These results indicate that p53 can reduce the point mutation frequency in a shuttle vector treated by chemical mutagens and suggest that p53 can act as guardian of the genome for at least some kinds of point mutations.

The role of nucleotide excision repair in protecting embryonic stem cells from genotoxic effects of UV-induced DNA damage

In this study the role of nucleotide excision repair (NER) in protecting mouse embryonic stem (ES) cells against the genotoxic effects of UV-photolesions was analysed. Repair of cyclobutane pyrimidine dimers (CPD) in transcribed genes could not be detected whereas the removal of (6-4) photoproducts (6-4PP) was incomplete, already reaching its maximum (30%) 4 h after irradiation. Measurements of repair replication revealed a saturation of NER activity at UV doses >5 J/m2 while at a lower dose (2.5 J/m2) the repair kinetics were similar to those in murine embryonic fibroblasts (MEFs). Cytotoxic and mutagenic effects of photolesions were determined in ES cells differing in NER activity. ERCC1-deficient ES cells were hypermutable (10-fold) compared to wild-type cells, indicating that at physiologically relevant doses ES cells efficiently remove photolesions. The effect of the NER deficiency on cytoxicity was only 2-fold. Exposure to high UV doses (10 J/m2) resulted in a rapid and massive induction of apoptosis. Possibly, to avoid the accumulation of mutated cells, ES cells rely on the induction of a strong apoptotic response with a simultaneous shutting down of NER activity.

Mitotic recombination produces the majority of recessive fibroblast variants in heterozygous mice

Mice heterozygous at Aprt (adenine phosphoribosyltransferase) were used as a model to study in vivo loss of heterozygosity (LOH) in normal fibroblasts. Somatic cell variants that exhibited functional loss of the wild-type Aprt in vivo were recovered as APRT-deficient cell colonies after culturing in selection medium containing 2, 6-diaminopurine (DAP), an adenine analog that is toxic only to cells with APRT enzyme activity. DAP-resistant (DAP(r)) fibroblast variants were recovered at a median frequency of 12 x 10(-5) from individual ears from progeny of crosses between mouse strains 129/Sv and C3H/HeJ. The frequency of DAP(r) variants varied greatly among individual ears, suggesting that they preexisted in vivo and arose at various times during development. Polymorphic molecular markers and a cytological marker on the centromere of chromosome 8 made it possible to discriminate between each of six possible mechanistic pathways of LOH. The majority (about 80%) of the DAP(r) variants were a consequence of mitotic recombination. The prevalence of mitotic recombination in regions proximal to Aprt did not correlate with meiotic map distances. In particular, there was a higher than expected frequency of crossovers within the interval 59 cM to 67 cM. The high spontaneous frequency of Aprt LOH, mediated primarily by mitotic recombination, is fully consistent with our previous results with human peripheral T cells from individuals known to be heterozygous at APRT. Thus, this Aprt heterozygote mouse is a valid model for studying somatic mutagenesis and mitotic recombination in vivo.

Molecular mechanisms of ionizing radiation-induced apoptosis

Ionizing radiation activates not only signalling pathways in the nucleus as a result of DNA damage, but also signalling pathways initiated at the level of the plasma membrane. Proteins involved in DNA damage recognition include poly(ADP ribose) polymerase (PARP), DNA-dependent protein kinase, p53 and ataxia- telangiectasia mutated (ATM). Many of these proteins are inactivated by caspases during the execution phase of apoptosis. Signalling pathways outside the nucleus involve tyrosine kinases such as stress-activated protein kinase (SAPK)/c-Jun N-terminal kinase (JNK), protein kinase C, ceramide and reactive oxygen species. Recent evidence shows that tumour cells resistant to ionizing radiation-induced apoptosis have defective ceramide signalling. How these signalling pathways converge to activate the caspases is presently unknown, although in some cell types a role for calpain has been suggested.

Msh2 status modulates both apoptosis and mutation frequency in the murine small intestine

Deficiency in genes involved in DNA mismatch repair increases susceptibility to cancer, particularly of the colorectal epithelium. Using Msh2 null mice, we demonstrate that this genetic defect renders normal intestinal epithelial cells susceptible to mutation in vivo at the Dlb-1 locus. Compared with wild-type mice, Msh2-deficient animals had higher basal levels of mutation and were more sensitive to the mutagenic effects of temozolomide. Experiments using Msh2-deficient cells in vitro suggest that an element of this effect is attributable to increased clonogenicity. Indeed, we show that Msh2 plays a role in the in vivo initiation of apoptosis after treatment with temozolomide, N-methyl-N'-nitro-N-nitrosoguanidine, and cisplatin. This was not influenced by the in vivo depletion of O6-alkylguanine-DNA-alkyltransferase after administration of O6-benzylguanine. By analyzing mice mutant for both Msh2 and p53, we found that the Msh2-dependent apoptotic response was primarily mediated through a p53-dependent pathway. Msh2 also was required to signal delayed p53-independent death. Taken together, these studies characterize an in vivo Msh2-dependent apoptotic response to methylating agents and raise the possibility that Msh2 deficiency may predispose to malignancy not only through failed repair of mismatch DNA lesions but also through the failure to engage apoptosis.

P53-dependent and -independent links between DNA-damage, apoptosis and mutation frequency in ES cells

The hypothesis that p53 deficiency enhances the survival of DNA-damage bearing cells was investigated in wild-type and p53 mutant embryonic stem (ES) cells. Following UV-C irradiation, p53 is rapidly induced in wild-type cells and p53-dependent apoptosis follows within 8 h, resulting in the death of the majority of cells within 36 h. Increasing doses of UV-irradiation resulted in enhanced clonogenic survival of null cells as compared to wild-type. Amongst surviving clones, the Hprt mutation frequency was found to be dependent upon UV dose and influenced by p53 status. Treatment with ionizing radiation led to enhanced expression of p53 but resulted in little induction of apoptosis irrespective of p53 status. However, clonogenic potential was considerably reduced, particularly in wild-type cells which showed a tenfold lower survival than null cells. In contrast to the effects of UV-irradiation, the incidence of Hprt mutation did not differ significantly between wild-type and p53 null survivors. The data confirm that p53 restricts the numbers of cells bearing mutations that survive DNA damage induced by either agent, albeit by different mechanisms.

Life and death in the genesis of the tumour cell

Most tumours arise because of an aberrant response of cells following exposure to chemicals deliberately ingested, for example cigarette smoke, or present as an environmental pollutant, for example dietary aflatoxin. Recent evidence has highlighted the importance of tumour suppressor genes and oncogenes in determining the response of a cell to potentially mutagenic or growth disrupting events. Many toxicants in vivo can cause apoptosis in a dose dependent manner. At low dose apoptosis is engaged, but with high exposure cells may undergo necrosis as cellular metabolism is catastrophically overwhelmed preventing the ordered set of events that constitute apoptosis from occurring. Mutations in genes that control deletion of potentially damaged cells result in overriding of death signals and may result in survival of a cell that otherwise should have been deleted. This gave rise to the concept of the 'undead' cell--the aberrant cell that has escaped normal growth controls taking the first step towards cancer. However, not all cell lineages respond to injury in the same ways, and even the same gene may have quite varied effects depending on the cellular and tissue environment.

Radiation-induced p53 and p21WAF-1/CIP1 expression in the murine intestinal epithelium: apoptosis and cell cycle arrest

p53-dependent expression of p21WAF-1/CIP1 has been studied in murine intestinal epithelium after exposure to ionizing radiation. In un-irradiated small intestine, neither p53 nor p21WAF-1/CIP1 could be detected by immunohistochemistry. After irradiation (8 Gy), there was a time- and dose-dependent increase in the expression of both proteins. In the small bowel, the positional expression of p53 and p21WAF-1/CIP1 was similar but not coincident. Both proteins could be observed throughout the crypts with greatest frequency of expression over the first 15 cell positions, which includes the stem cell population (approximately positions 3 to 5) and the proliferating, transit cell population (approximately positions 5 to 15). p53-positive cells were primarily distributed toward the base of the crypt relative to p21WAF-1/CIP1. Subdivision of the p53-positive cell population revealed that the cells with strongest p53 immunoreactivity were positioned farther toward the base of the crypt, and their distribution was approximately coincident with the frequency distribution of apoptotic cells. Cells that were either weakly or moderately immunoreactive for p53 were located toward the middle of the crypt and were approximately coincident with the distribution of p21WAF-1/CIP1. The numbers of both p53- and p21WAF-1/CIP1-positive cells declined steadily with time, and by 6 days after irradiation there were very few immunoreactive cells to observe. Radiation-induced increase in p53 and p21WAF-1/CIP1 expression was not detected in mice homozygously null for p53. Expression of p21WAF-1/CIP1 and incorporation of tritiated thymidine were found to be mutually exclusive. In the large bowel, p21WAF-1/CIP1 and p53 expression were observed along the entire length of the colonic crypts after irradiation (8 Gy), and, unlike in the small intestine, this expression was not only maintained but increased over 72 hours. p21WAF-1/CIP1 immunoreactivity was detected in large intestine epithelium up to 6 days after irradiation. The differential expression of p21WAF-1/CIP1, observed between the large and small bowel and within the small intestinal crypts, is discussed.