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

The tumor suppressor gene TP53: implications for cancer management and therapy

The p53 protein is an inducible transcription factor with multiple anti-proliferative roles in response to genotoxic damage; unprogrammed proliferative stimuli; and deprivation of oxygen, nutrients, or ribonucleotides. Inactivation of the TP53 gene by mutation or deletion is the most common event in human cancer. Loss of p53 function compromises genetic homeostasis in cells exposed to mutagens and prevents normal cytotoxic responses to cancer therapies. Genetic and pharmacological approaches are being developed with the ultimate goal of restoring or controlling p53 functions in cancer patients. Genetic interventions aiming at expressing wild-type TP53 in cancer cells, either by retroviral or adenoviral transfer, have met limited clinical success. However, recently, the use of a defective adenovirus (ONYX-015) that selectively kills p53-incompetent cells has shown promising effects in pre-clinical and clinical studies. Pharmacological methods are under development to either stimulate wild-type p53 protein function or induce p53 mutant proteins to resume wild-type functions. These methods are based on small chemicals (CP-31388, PRIMA-1), peptides (CDB3), or single-chain Fv antibody fragments corresponding to defined p53 domains. In addition, detection of mutant TP53 may also serve as a marker for early cancer detection, prediction, and prognosis. In this review, we discuss the mechanisms underlying these approaches and their perspectives for cancer therapy.

Intestinal tumorigenesis in Min mice is enhanced by X-irradiation in an age-dependent manner

We examined the effect of X-irradiation on intestinal tumorigenesis in Min (multiple intestinal neoplasia) mice. Single whole-body irradiation was given to mice of various ages from newborn to young adults. On the C57BL/6J (B6) background, X-irradiation increased tumor multiplicity of the small intestine exposed at ages from 2-3 days to 24-25 days, with a peak of 2.7-fold increase at 10-12 days of age; exposure at later ages resulted in only a slight increase. X-irradiation also increased colonic tumors; however, the susceptible age period appeared earlier than that of the small intestine; the peak value of 4.6-fold increase was observed in the exposure at around 2-3 days of age. Irradiation at 24 days or later ages showed almost no effect on the colonic tumor induction. On the (B6 x MSM)F1 background, X-irradiation resulted in 2.7-fold increase in the small intestinal tumors, but no increase in the colonic tumors, and besides, the age dependency observed in the small intestinal tumors was much attenuated. Collectively, we conclude that tumorigenic efficacy of X-irradiation in Min mice was determined by the combination of the target organ, the age at exposure, and the genetic background.

Cell and tissue responses to genotoxic stress

Cancers arise as a consequence of the accumulation of multiple genetic mutations in a susceptible cell, resulting in perturbation of regulatory networks that control proliferation, survival, and cellular function. Here, the sources of cellular stress that can cause oncogenic mutations and the responses of cells to DNA damage are reviewed. The role of different repair pathways and the potential for cell- and tissue-specific reliance on individual repair mechanisms are discussed. Evidence for cell- and tissue-specific activation of p53-mediated growth arrest and apoptosis after exposure to an individual genotoxin is assessed and some of the potential mediators of these different responses are provided. These cell- and tissue-specific responses to particular forms of DNA damage are likely to be key determinants of tissue-specific tumour susceptibility, and there is good evidence for genetic variations in these responses. The role that genotoxic agents play in altering the microenvironment to produce indirect effects on tumourigenesis through altered production of free radicals and cytokines that are characteristic of inflammatory-type processes is also evaluated. Changes to the microenvironment as direct or indirect effects of genotoxic stress can be involved in both tumour initiation and progression and may even be a prerequisite for tumourigenesis. Therefore, tumour susceptibility after endogenous or exogenous genotoxic stress represents a balance between cell-intrinsic responses of target cells and changes to the microenvironment. A fuller understanding of cell- and tissue-specific responses, alterations to the microenvironment, and genetic modifiers of these responses could lead to novel prevention and therapeutic strategies for common forms of human malignancy.

Attenuation of the p53 response to DNA damage by high cell density

The p53 tumor suppressor is critical for preventing cancer progression. Numerous observations suggest that p53 function can be modulated by the cells' microenvironment. We addressed specifically the impact of cell crowding on the induction of p53 by DNA damage. We report that cell crowding attenuates markedly p53 upregulation, transcriptional activation and subsequent p53-dependent apoptosis following exposure to genotoxic stress. The p53 protein remains short-lived in confluent cultures regardless of the extent of DNA damage, even though it undergoes efficient phosphorylation on the mouse equivalent of human p53 serine 15. This inhibitory effect of cell crowding is not a secondary consequence of density-dependent cell cycle arrest (contact inhibition). Microscopic examination indicates that dense cultures display prominent cadherin-mediated cell-cell junctions, and only poor cell-matrix focal adhesions, whereas sparse cells possess conspicuous matrix adhesions and essentially no cell-cell contacts. High-density cell culture might recapitulate the microenvironment of cells in a living organism, where the response of p53 to DNA damage is reported to be low in some organs and ages. The impact of cell density on p53 activation may have important bearings on the involvement of p53 in tumor suppression and the cellular response to anticancer therapy.

Poly(ADP-ribose) polymerase-1 is a survival factor for radiation-exposed intestinal epithelial stem cells in vivo

Poly(ADP-ribose) polymerase-1 (PARP-1) is a key enzyme mediating the cellular response to DNA strand breaks. It plays a critical role in genomic stability and survival of proliferating cells in culture undergoing DNA damage. Intestinal epithelium is the most proliferative tissue in the mammalian body and its stem cells show extreme sensitivity to low-level genotoxic stress. We investigated the role of PARP-1 in the in vivo damage response of intestinal stem cells in crypts of PARP-1-/- and control mice following whole-body gamma-irradiation (1 Gy). In the PARP-1-/- mice there was a significant delay during the first 6 h in the transient p53 accumulation in stem cells whereas an increased number of cells were positive for p21(CIP1/WAF1). Either no or only marginal differences were noted in MDM2 expression, apoptosis, induction of or recovery from mitotic blockage, or inhibition of DNA synthesis. We further observed a dose-dependent reduction in crypt survival measured at 4 days post-irradiation in control mice, and this crypt-killing effect was significantly potentiated in PARP-1-/- mice. Our results thus establish that PARP-1 acts as a survival factor for intestinal stem cells in vivo and suggest a functional link with early p53 and p21(CIP1/WAF1) responses.

Differential action of growth hormone in irradiated tumoral and nontumoral intestinal tissue

Growth hormone (GH) protects the intestines from antitumoral therapy, but it is not known whether or not the tumor is also protected in vivo. The aim of the present work was to determine whether GH administration modifies the response by a colonic adenocarcinoma to radiation in vitro and in vivo. BDIX rats were implanted with a colonic adenocarcinoma and two weeks later GH treatment was started. Animals were then irradiated, and four days later samples from the intestines and tumor were taken for analysis. In vitro assays were performed in parallel to confirm the effects observed in vivo. GH reduced radiation-induced intestinal injury by improving proliferation and reducing apoptosis and p53 expression. However, tumor proliferation was reduced by GH while apoptosis and p53 expression remained unchanged. A similar response was observed in vitro. Thus, GH administration before radiotherapy protects the intestines but not the implanted adenocarcinoma in the rat.

Sepsis from Pseudomonas aeruginosa pneumonia decreases intestinal proliferation and induces gut epithelial cell cycle arrest

OBJECTIVES

To evaluate whether the up-regulation in sepsis-induced gut epithelial apoptosis is balanced by an increase in intestinal proliferation and to assess mechanisms affecting the gut's regenerative response to overwhelming infection.

DESIGN

Prospective, randomized, controlled study.

SETTING

Animal laboratory in a university medical center.

INTERVENTIONS

Mice were subjected to intratracheal injection of Pseudomonas aeruginosa and killed between 1.5 and 24 hrs after induction of pneumonia-induced sepsis to assess for gut epithelial proliferation and cell division and for apoptosis. Animals were compared with sham-operation controls, septic transgenic mice that overexpress Bcl-2 throughout their small intestinal epithelium, and septic p53-/- mice.

MEASUREMENTS AND MAIN RESULTS

Proliferation and cell division were assessed by measuring S-phase and M-phase cells in intestinal crypts. The number of S-phase cells showed a progressive decline at all time points measured, with a 5-fold decrease in proliferation between control animals and septic mice 24 hrs after intratracheal injection of pathogenic bacteria (p <.0001). In contrast, cells in M-phase remained constant for the first 12 hrs after the onset of sepsis, but increased nearly 50% at 24 hrs after instillation of P. aeruginosa (p <.005). Both the decrease in S-phase cells and the increase in M-phase cells were partially suppressible in Bcl-2 overexpressors, but cellular proliferation and division were similar between septic p53-/- and p53+/+ mice. Crypt apoptosis was increased at all time points, with maximal death occurring between 12 and 24 hrs.

CONCLUSIONS

Sepsis from P. aeruginosa pneumonia induces a p53-independent decrease in gut epithelial proliferation. Despite an increase in sepsis-induced intestinal apoptosis, there is no compensatory increase in intestinal epithelial proliferation, and there is evidence of a cell cycle block with an accumulation of cells in M-phase. Decreasing gut apoptosis by overexpression of Bcl-2 is associated with a partial reversal of the effect of sepsis on the cell cycle.

Progastrin stimulates murine colonic epithelial mitosis after DNA damage

BACKGROUND & AIMS

Transgenic mice that overexpress progastrin are more susceptible than either wild-type mice or mice that overexpress amidated gastrin to chemical carcinogen-induced colonic adenomas. We have investigated whether alterations in the regulation of apoptosis or mitosis after DNA damage contribute to the effects of progastrin on murine colonic epithelium.

METHODS

Apoptosis and mitosis were assessed on a cell positional basis in murine intestinal epithelium after gamma-irradiation. Mice analyzed were progastrin overexpressing, gastrin overexpressing, gastrin knockout, and their wild-type counterparts. The expression of cell cycle regulators was analyzed by gene array and Western blotting.

RESULTS

Apoptosis was induced to similar levels in the small intestinal and colonic crypts of all mice 4.5 hours after 8 Gy gamma-radiation. Colonic mitosis was inhibited to almost undetectable levels by 8Gy gamma-radiation in wild-type, gastrin-knockout, and gastrin-overexpressing mice. However, significant colonic mitosis persisted in progastrin-overexpressing mice up to 24 hours after 8Gy gamma-radiation. Increased levels of cdk4 and cyclin D1 proteins were found in the colonic epithelium of progastrin-overexpressing mice relative to wild-type animals after gamma-radiation.

CONCLUSIONS

After DNA damage by gamma-radiation, mice with elevated progastrin exhibit significantly higher levels of colonic mitosis than wild-type or gastrin-overexpressing mice. Persistently elevated cdk4 and cyclin D1 in progastrin overexpressing mice accounts for the capacity of colon cells to continue with the cell cycle after DNA damage.

Tissue-specific p53 responses to ionizing radiation and their genetic modification: the key to tissue-specific tumour susceptibility?

Although little is understood of the underlying mechanisms, there are tissue-specific responses to tumourigenic and therapeutic agents and these responses are influenced by genetic factors. Ionizing radiation is an important tumourigenic and therapeutic agent for which there is substantial evidence for such tissue-dependent and genotype-dependent responses. Because the p53 tumour suppressor protein is a major determinant of cellular responses to radiation, the present study has investigated whether modification of the p53 pathway contributes to tissue-dependent and genotype-dependent responses using inbred strains of mice. Comparison of responses in haemopoietic and epithelial cells in irradiated C57BL/6 and DBA/2 mice revealed significant differences in p53 and apoptotic responses in different cell types and in different cells of the same type, reflecting the complexity of damage responses operating in the whole organism. The data suggest that p53-mediated up-regulation of Bax is a major determinant of apoptosis in the spleen, but not in the intestine, whereas p53-mediated induction of p21(waf1) plays an anti-apoptotic role in the spleen, but not in the intestine. It is also shown that p53 stabilization and differential transactivational activities towards Bax or p21(waf1) are influenced by genetic factors that act in a tissue-specific manner. Analysis of ATM, a potential mediator of differential p53 activation, indicates that this key regulator of radiation responses is preferentially induced in epithelial cells, but is unlikely to account for genetic modification of p53 or apoptotic responses in the mouse strains studied. Polymorphisms in the p53 or DNA-PKcs genes are also unlikely to account for the genetic modifications that are reported here. There are numerous further potential modifiers of the p53 pathway, but analysis of backcross and inter-cross mice demonstrates that genes responsible for the complex modification of these in vivo responses can be identified by linkage analysis. This approach has the potential to reveal new or unexpected interactions involving the p53 pathway that determine both short-term and long-term effects of radiation exposure and the basis of tissue-specific responses and tumour susceptibility.

Estrogen stimulates expression of p21Waf1/Cip1 in mouse uterine luminal epithelium

p21Waf1/Cip1 was originally identified as an inhibitor of the cell cycle. Recent evidence suggests that it can act as a positive regulator of the cell cycle under the influence of some growth stimulators. We investigated the effects of ovarian steroids on the expression of p21, DNA synthesis, and mitosis in the uterus. Capsules containing 17beta-estradiol (E2) were subcutaneously implanted in ovariectomized mice that were sacrificed on different days. Their uteri were collected for p21 immunohistochemical staining. To study mitosis and DNA synthesis, colchicine and bromodeoxyuridine (BrdU) were injected into mice 3 or 5 h before sacrifice. The results showed that p21 expression, BrdU incorporation, and the mitotic index in uterine luminal epithelium increased 1 to 2 d after E2 stimulation and then declined to basal levels between d 3 and 6. Furthermore, cotreatment with progesterone (P4) and E2 suppressed both p21 expression and the DNA synthesis stimulated by E2 alone in uterine epithelial cells. Our results show that estrogen stimulates p21 expression and cell proliferation in uterine luminal epithelium and that cotreatment with P4 prevents both effects, suggesting that p21 may act as a positive cell-cycle regulator.

Early cellular events in colorectal carcinogenesis

Colorectal cancer develops through a multistage process recognizable at a histopathological level by progression from normal mucosa to invasive carcinoma (the adenoma-carcinoma sequence). For many years, it has been hypothesized that increased cell proliferation in the colonic crypt represents the earliest recognizable stage in this sequence. This perspective is now changing. While several human studies have reported increased crypt cell proliferation in samples from at-risk patients, there are many inconsistencies and paradoxes in their conclusions. In addition, it is appreciated that the process of apoptosis (programmed cell death) is vital for normal crypt homeostasis and its impairment may be an early event in the neoplastic process. It is now believed that aberrant crypt foci (ACFs) represent the earliest step in colorectal carcinogenesis. Two ACF types are identifiable: hypercellular and dysplastic. Increased proliferative activity may be seen in both, but the dysplastic entity is most relevant to carcinogenesis. Animal and human studies support the notion that ACFs grow by crypt fission leading to the formation of microadenomas. Adenomas are monoclonal expansions of an altered cell, but very early lesions may be polyclonal. There are outward and inward theories of polypoid growth, and evidence to support both mechanisms. The ACF assay has become a useful tool to detect carcinogens in animal studies but has been less frequently used in human studies. For future cancer chemopreventive and risk assessment studies in humans, the identification and quantification of ACFs should be considered a more effective intermediate marker of risk than the determination of crypt cell proliferation alone.

p53 expression in human rectal tissue after radiotherapy: upregulation in normal mucosa versus functional loss in rectal carcinomas

PURPOSE

In vitro, ionizing radiation of epithelial cells leads to upregulation of wild-type p53 and subsequent induction of p21(waf1). The effect of radiotherapy (RT) on the expression of these proteins in patients is unknown. We assessed the influence of RT on the expression of p53 and p21(waf1) in normal mucosa and rectal carcinomas in vivo.

METHODS

Tumor and normal tissue samples were derived from rectal cancer patients randomized in a clinical trial in which the value of preoperative RT was evaluated. p53 and p21(waf1) expression was determined in 51 irradiated and 52 nonirradiated patients using immunohistochemistry.

RESULTS

In normal mucosa, both p53 and p21(waf1) were strongly upregulated after RT compared with the expression in unirradiated normal tissue (p <0.001). In tumor cells, no significant difference in the expression of p53 or p21(waf1) was found in the irradiated vs. nonirradiated group. In the few rectal tumors with wild-type p53, induction of p53 after RT did not necessarily lead to upregulation of p21(waf1).

CONCLUSION

These findings demonstrate that in normal mucosa, a functional p53-p21(waf1) pathway is present, whereas in tumor cells it is defective in almost all cases because of either p53 mutation or down- or upstream disruption in tumors with wild-type p53. Therefore, we believe that the role of p53 expression as a single prognostic marker in rectal cancer needs reconsideration.

Morphological aspects of ionizing radiation response of small intestine

Knowledge of the acute and late ionizing radiation exposure damage to the gastrointestinal tract, particularly injury of the small intestine, is of great significance in radiotherapy, as is management of accidental radiation exposure. Irradiation (X-ray, neutron, cobalt gamma) induces a series of events in this rapidly renewing tissue resulting in the well-known symptoms of the gastrointestinal (GI) radiation syndrome, such as GI haemorrhage, endotoxemia, bacterial infection, anorexia, nausea, vomiting, diarrhoea, and loss of electrolytes and fluid. In spite of the significant advances that have occurred in research on underlying mechanisms over the last two decades, the overall etiology and pathogenesis of the GI-syndrome still remains unclear. Currently, to our knowledge, these symptoms are probably due to a rapid modification of the intestinal motility and to the structural alteration of the intestinal mucosa (cell loss and altered crypt integrity). Several evidences suggest that radiation-induced dysfunctions and structural changes of this organ (either changes in subcellular, cellular, and histological structure) are mediated by concerted and interrelated changes of a plethora of various extracellular mediators and their intracellular messengers. The aim of this review is to summarize our current knowledge about the pathomorphology and cell biology of the ionizing radiation response of the GI tract with a focus on the small intestine.

Genotoxic and non-genotoxic pathways of p53 induction

Since the initial concept of p53 as a sensor of DNA-damage, the picture of the role of p53 has widened to include the sensing of much more diverse forms of stress, including hypoxia and constitutive activation of growth-promoting cascades. The pathways by which these processes regulate p53 are partially overlapping, but imply different patterns of post-translational modifications. In this review, we summarize current knowledge on post-translational modifications of p53, and we discuss how hypoxia and oncogene activation stresses may induce p53 independently of DNA damage.

Small bowel review: diseases of the small intestine

In the past year there have been many advances in the area of small bowel physiology and pathology and therapy. In preparation for this review, over 1500 papers were assessed. The focus is on presenting clinically useful information for the practicing gastroenterologist. Selected important clinical learning points include the following: (1) glutamine may restore the AIDs-associated increased intestinal permeability to normal; (2) substance P is a major mediator of diarrhea caused by Costridium difficile toxin A, acting by binding to a G-protein-coupled receptor, and represents a possible 2therapeutic target; (3) the serological diagnosis of celiac disease has been greatly enhanced with the use of anti-endomysial antibody testing, and the recent antitransglutaminase; (4) a quarter of patients with celiac disease may have secondary pancreatic insufficiency and require enzyme replacement therapy; (5) in the patient with unexplained elevation in the serum transaminase concentration, consider celiac disease as an obscure possibility; (6) bosentan and endothelin receptor agonist may prove to be useful in reducing gut ischemia in patients with septic shock; and (7) the administration of recombinant human fibroblast growth factor-2 may prove to be useful to prevent radiation damage to the gastrointestinal tract.

Differential post-translational modification of the tumour suppressor proteins Rb and p53 modulate the rates of radiation-induced apoptosis in vivo

Ionizing radiation induces p53-dependent apoptosis in the spleen, providing a model system to study p53 regulated events in a normal cell type. We have developed an in vivo model that identifies genetic differences in the regulation of p53-mediated apoptosis and addresses whether altered post-translational events in the p53-p21/Rb axis modulate the sensitivity of cells to radiation-induced cell death in vivo. Splenocytes from mice with distinct genetic backgrounds (DBA/2 and C57BL/6) exhibit differences in the rate of apoptosis. Whilst no obvious strain differences in protein levels of Bcl-2 or the cyclin-CDKs were observed, early post-translational regulatory events in the p53-p21/Rb axis showed striking differences in the two mouse strains. Cells from C57BL/6 animals undergo more rapid apoptosis after irradiation resulting from elevated levels and rapid induction of p53, pronounced Rb-cleavage, and the absence of a sustained induction of p21. In contrast, cells from DBA/2 animals have a reduced rate of apoptosis following irradiation with elevated levels of hyperphosphorylated Rb and a sustained induction of the p21 protein that is coincident with the C-terminal phosphorylation of p53. These data suggest that quantitative differences in the level of p21 protein can affect the rate of apoptosis in vivo, consistent with the view that p21 is an anti-apoptotic effector of p53. However, striking differences in the Rb protein-caspase cleavage or hyperphosphorylation-in the same cell type, but in different genetic backgrounds, demonstrates that p53-dependent apoptosis can be modulated in vivo by genetic factors that impinge upon the pro- or anti-apoptotic potential of Rb. In addition, we show that Rb cleavage is p53-dependent and that its phosphorylation status can be uncoupled from p21 expression. This study highlights the possibility that genetic factors can be identified that affect differential sensitivity of cells to ionizing radiation in vivo.

Gastrointestinal cells of IL-7 receptor null mice exhibit increased sensitivity to irradiation

IL-7 is a critical cytokine in the development of T and B cells but little is known about its activity on nonhematopoietic cells. An unexpected finding was noted in allogeneic bone marrow transplant studies using IL-7 receptor null (IL-7R alpha(-/-)) mice as recipients. These mice exhibited a significantly greater weight loss after total body irradiation compared with wild type, IL-7R alpha(+/+), mice. Pathological assessment indicated greater intestinal crypt damage in IL-7R alpha(-/-) recipients, suggesting these mice may be predisposed to gut destruction. Therefore, we determined the effect of the conditioning itself on the intestinal tract of these mice. IL-7R alpha(-/-) mice and IL-7R alpha(+/+) mice were irradiated and examined for lesions and apoptosis within the small intestine. In moribund animals, IL-7R alpha(-/-) mice had extensive damage in the small intestine, including marked ablation of the crypts and extreme shortening of villi following 1500 cGy total body irradiation. In contrast, by 8 days after irradiation, the small intestines of IL-7R alpha(+/+) mice had regenerated as distinguished by normal villus length and hyperplastic crypts. Following 750 cGy irradiation, IL-7R alpha(-/-) mice had a higher proportion of apoptotic cells in the crypts and an accompanying increase in the pro-apoptotic protein Bak was expressed in intestinal epithelial cells. These results demonstrate the increased radiosensitivity of intestinal stem cells within the crypts in IL-7R alpha(-/-) mice and a role for IL-7 in the protection of radiation-induced apoptosis in these same cells. This study describes a novel role of IL-7 in nonhematopoietic tissues.

Thermal injury effects on intestinal crypt cell proliferation and death are cell position dependent

We examined the effects of thermal injury on intestinal epithelial cell proliferation and death. We recorded histologically identifiable mitotic and apoptotic crypt cells in relation to cell position after a 60% full thickness cutaneous thermal injury in the rat. The injury significantly reduced mitosis (0.53 +/- 0.11 vs. 1. 50 +/- 0.70, P < 0.05) at cell positions 4-6, stem cells, 6 h after injury. A similar reduction in mitosis (1.13 +/- 0.59 vs. 3.50 +/- 0. 80, P < 0.05) was observed at higher cell positions 7-9 12 h after injury, indicating a positional cell shift. In addition, a significant increase in the number of apoptotic bodies occurred at cell positions 7-9 (2.32 +/- 0.87 vs. 0.13 +/- 0.22, P < 0.05) and 10-12 (2.2 +/- 0.12 vs. 0.00, P < 0.05) 6 h after injury. Thermal injury-induced alterations in mitotic and apoptotic activities were transient since crypts recovered with a moderate increase in mitotic activity 24 h after injury. In control and thermal-injury rats 24 h after injury, crypt cell mitosis and apoptosis did not differ significantly. This demonstrates that cutaneous thermal injury causes a transient suppression of mitosis as well as induction of apoptosis in a cell position-dependent manner in the small intestinal crypt.

Cell proliferation and death in the gastric epithelium of developing rats after glucocorticoid treatments

Glucocorticoids take part in the intense morphofunctional modifications that occur in the gastric mucosa during fetal and postnatal development. Two studies were designed to evaluate corticoids role in gastric cell proliferation and apoptosis in developing rats: in vivo, using suckling animals; in vitro, using gastric explants obtained from 20-day fetuses. These explants were cultured in DMEM/F12 medium treated or not with 50 ng/ml of corticosterone; after 22 hr, vincristine was added to the medium for 2 hr to block mitosis. The metaphasic index decreased significantly after the 24-hr treatment (controls: 1.52 +/- 0.53; treated: 0.40 +/- 0.21) and apoptotic cells were visualized under light and electron microscopy. Fifteen-day-old rats were treated with hydrocortisone (25 mg/Kg) for 3 days, and injected with BrDU (100 mg/Kg) 1 hr before sacrifice on the 18th day. BrDu-labeled and non-labeled cells were counted to determine the labeling index of epithelial cells. As apoptotic cells are rapidly eliminated, other animals were treated for only 2-3 hr. Sections were investigated for the presence of apoptotic cells, using morphological criteria and TUNEL labeling. Hydrocortisone significantly reduced the labeling index (controls: 15.6 +/- 1.6 vs. treated: 11.7 +/- 1.1), besides altering the body weight gain. Hydrocortisone treatment doubled the number of apoptotic cells after 2 hr, and quadruplicated it after 3 hr. The results demonstrated that glucocorticoids inhibit cell proliferation in the gastric epithelium of fetuses and suckling rats and increase apoptotic rates, suggesting the exit from cell cycle.

Clustering of apoptotic cells via bystander killing by peroxides

Clustering of apoptotic cells is a characteristic of many developing or renewing systems, suggesting that apoptotic cells kill bystanders. Bystander killing can be triggered experimentally by inducing apoptosis in single cells and may be based on the exchange of as yet unidentified chemical cell death signals between nearby cells without the need for cell-to-cell communication via gap junctions. Here we demonstrate that apoptotic cell clusters occurred spontaneously, after serum deprivation or p53 transfection in cell monolayers in vitro. Clustering was apparently induced through bystander killing by primary apoptotic cells. Catalase, a peroxide scavenger, suppressed bystander killing, suggesting that hydrogen peroxide generated by apoptotic cells is the death signal. Although p53 expression increased the number of apoptoses, clustering was found to be similar around apoptotic cells whether or not p53 was expressed, indicating that there is no specific p53 contribution to bystander killing. Bystander killing through peroxides emitted by apoptotic cells may propagate tissue injury in different pathological situations and be relevant in chemo-, gamma-ray, and gene therapy of cancer.

Different impact of p53 and p21 on the radiation response of mouse tissues

Mammalian tissues differ dramatically in their sensitivity to genotoxic stress, although the mechanisms determining these differences remain largely unknown. To analyse the role of p53 and p21 in determination of tissue specificity to DNA damage in vivo, we compared the effects of gamma radiation on DNA synthesis on whole-body sections of wild type, p53-deficient and p21-deficient mice. A dramatic reduction in 14C-thymidine incorporation after gamma irradiation was observed in the majority of rapidly proliferating tissues of wild type and p21-/- but not in p53-/- mice, confirming the key role of p53 in determination of tissue response to genotoxic stress in vivo and suggesting that p53-mediated inhibition of DNA synthesis does not depend on p21. Rapid radiation induced p53-dependent apoptosis was mapped to the areas of high levels of p53 mRNA in radiation sensitive tissues analysed (white pulp in the spleen and bases of crypts in small intestine), indicating that p53 regulation at the mRNA level is a determinant of cellular sensitivity to genotoxic stress. High p53 mRNA expression is inherited as a recessive trait in cell-cell hybrids suggesting the involvement of a negative control mechanism in the regulation of p53 gene expression.

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.

Enhanced cell kinetics, p53 accumulation and high p21WAF1 expression in chronic cholecystitis: comparison with background mucosa of gallbladder carcinomas

AIMS

Since neoplasia resulting from chronic inflammation has recently attracted increasing attention, we have investigated surgically removed gallbladders to examine the relationship between chronic cholecystitis and carcinogenesis.

METHODS AND RESULTS

The mucosa of 108 cholecystectomy specimens without gallbladder cancer and 54 surgically resected gallbladder carcinomas were classified into three groups according to the degree of lymphocytic infiltration, and assessed immunohistochemically for Ki67, p53, p21WAF1 and apoptosis. In gallbladder mucosa without carcinoma, all four parameters tended to increase with the inflammation score (IS). Significantly positive correlations were revealed between Ki67 and p53, Ki67 and p21WAF1, and p53 and p21WAF1. However, in gallbladder carcinoma cases, values of p53 and p21WAF1 for background mucosa were elevated as compared to the mucosa of cholecystitis with low IS, but there was no correlation between their expression and IS, except for Ki67.

CONCLUSIONS

Severe chronic cholecystitis is associated with acceleration of epithelial cell turnover, damaged cells being eliminated by apoptosis. The background mucosa of gallbladder carcinomas showed similar cell proliferative activity (Ki67) to that in cholecystitis, with no parallel changes of p53 and p21WAF1 expression, suggesting the possibility of unknown cofactors causing genomic damage.

Different responses of epidermal and hair follicular cells to radiation correlate with distinct patterns of p53 and p21 induction

Different parts of the skin respond to ionizing radiation with different sensitivities. To examine the mechanisms underlying these different responses, we investigated various cellular parameters in the skin after exposure of mice to 5 Gy of ionizing radiation. Epidermal cells responded to radiation by undergoing growth arrest, whereas the cells in the matrix of hair follicles underwent apoptosis but not growth arrest. These distinct responses correlated with differential increases in p53 and p21 proteins in these two populations of cells; whereas an increase in p53 protein levels was observed in both epidermis and hair follicular matrix, especially in the latter, the induction of p21 was strong in the epidermis but absent in the follicular matrical cells. Studies using p53-null and p21-null mice demonstrated that the radiation-induced apoptosis in the hair follicles was fully dependent on p53, and growth arrest in the epidermis was only partially dependent on p53 but fully dependent on p21. These results indicate that two epithelial cell types respond to radiation by different pathways that are governed in part by the differential p53- and p21-dependent responses of these cells; high-level induction of p53 in the absence of p21 induction led to apoptosis, whereas intermediate induction of both p53 and p21 led to growth arrest.

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.

Intestinal adaptation and enterocyte apoptosis following small bowel resection is p53 independent

Adaptation following small bowel resection (SBR) signals enterocyte proliferation and apoptosis. Because p53-induced p21(waf1/cip1) may be important for apoptosis in many cells, we hypothesized that these genes are required for increased enterocyte apoptosis during adaptation. Male C57BL/6 (wild-type) or p53-null mice underwent 50% proximal SBR or sham operation (bowel transection-reanastomosis). Adaptation (DNA-protein content, villus height-crypt depth, enterocyte proliferation), appearance of apoptotic bodies, and p53 and p21(waf1/cip1) protein expression were measured in the ileum after 5 days. Adaptation was equivalent after SBR in both wild-type and p53-null mice as monitored by significantly increased ileal DNA-protein content, villus height, and enterocyte proliferation. The number of crypt apoptotic bodies increased significantly after SBR evenly in both wild-type and p53-null mice. In the p53-null mice, SBR substantially induced the expression of p21(waf1/cip1) protein in villus enterocytes. The p53-independent induction of p21(waf1/cip1) may account for the similar intestinal response to SBR between wild-type and p53-null mice. Intestinal adaptation and increased enterocyte apoptosis following intestinal resection occur via a p53-independent mechanism.

Apoptosis and therapy

The dogma that antineoplastic treatments kill tumour cells by damaging essential biological functions has been countered by the notion that treatment itself initiates a programmed cellular response. This response often produces the morphological features of apoptosis and is determined by a network of proliferation and survival genes, some of which are differentially expressed in normal and malignant cells. Correspondingly, mutations that interfere with the initiation or execution of apoptosis may produce tumour-cell drug resistance. Remarkably, many of the genes that modulate apoptosis in response to cytotoxic drugs also affect apoptosis during tumour development; hence, the process of apoptosis provides a conceptual framework for understanding how cancer genes can influence the outcome of cancer therapy. Although the relative contribution of apoptosis to radiation and drug-induced cell death remains controversial, clinical studies have associated anti-apoptotic mutations with treatment failure. While careful preclinical and clinical studies will be necessary to resolve this point, our current understanding of apoptosis should facilitate the design of rational new therapies.

Chemically-induced apoptosis: p21 and p53 as determinants of enterotoxin activity

The relationship between toxin-induced apoptosis and longer-term (> 72 h) intestinal toxicity was investigated in vivo using p53 wild type (+/+) and 'knockout' (-/-) mice. The enterotoxic antimetabolite 5-fluorouracil (5-FU) induced acute p53-dependent apoptosis in the crypts of both small intestine and midcolon. Although the amount of apoptosis was the same order of magnitude at its peak (24 h) at both 40 and 400 mg/kg 5-FU, only 400 mg/kg 5-FU brought about changes in the integrity of the gut after 96 h. These were characterised by the loss of epithelial cells from crypts and villi. Only after 400 mg/kg 5-FU were mitotic index and DNA synthesis significantly suppressed in both small intestinal and midcolonic crypts. This correlated with a prolonged, p53-dependent expression of p21(waf-1/cip1). In p53 null (-/-) mice significant reductions in 5-FU-induced apoptosis and relief from the inhibition of cell cycle progression permitted retention of crypt integrity after 5-FU. Thus, quantitative measures of acute apoptosis in vivo did not accurately predict subsequent pathological changes in the gut. Rather, p53-dependent inhibition of cell cycle progression together with cell loss by apoptosis caused a loss of crypt integrity. Importantly, the tissue toxicity of 5-fluorouracil was genetically determined at a locus (p53) separate from that directly associated with toxin action. The selectivity of toxin action is therefore also determined by events 'downstream' of those associated with the direct mode of action of the toxin.