Impact of carbohydrate restriction in the context of obesity on prostate tumor growth in the Hi-Myc transgenic mouse model.

BACKGROUND: Previously, we showed that carbohydrate restriction with calorie restriction slowed tumor growth in xenograft mouse prostate cancer models. Herein, we examined the impact of carbohydrate restriction without calorie restriction on tumor development within the context of diet-induced obesity in the Hi-Myc transgenic mouse model of prostate cancer. METHODS: Mice were randomized at 5 weeks of age to ad libitum western diet (WD; 40% fat, 42% carbohydrate; n=39) or ad libitum no carbohydrate ketogenic diet (NCKD; 82% fat, 1% carbohydrate; n=44). At age 3 or 6 months, mice were killed, prostates weighed and prostate histology, proliferation, apoptosis and macrophage infiltration evaluated by hematoxylin and eosin, Ki67, TUNEL and F4/80 staining, respectively. Body composition was assessed by DEXA, serum cytokines measured using multiplex, and Akt/mTOR signaling assessed by Western. RESULTS: Caloric intake was higher in the NCKD group, resulting in elevated body weights at 6 months of age, relative to the WD group (45 g vs 38g; P=0.008). Despite elevated body weights, serum monocyte chemoattractant protein (MCP)-1 and interleukin (IL)-1α levels were lower in NCKD versus WD mice (P=0.046 and P=0.118, respectively), and macrophage infiltration was reduced in prostates of NCKD versus WD mice (P=0.028). Relative Akt phosphorylation and phospho-S6 ribosomal protein levels were reduced in prostates of NCKD versus WD mice. However, while mice randomized to NCKD had smaller prostates after adjustment for body weight at 3 and 6 months (P=0.004 and P=0.002, respectively), NCKD mice had higher rates of adenocarcinoma at 6 months compared to WD mice (100 vs 80%, P=0.04). CONCLUSIONS: Despite higher caloric intake and elevated body weights, carbohydrate restriction lowered serum MCP-1 levels, reduced prostate macrophage infiltration, reduced prostate weight, but failed to slow adenocarcinoma development. Together, these data suggest that although carbohydrate restriction within the context of obesity may reduce obesity-associated systemic inflammation and perhaps slow tumor growth, it is not sufficient to counteract obesity-associated tumor development.

Loss of p27 Associated with Risk for Endometrial Carcinoma Arising in the Setting of Obesity.

Endometrial carcinoma (EC) exhibits the strongest association with obesity of all cancers. Growth of these tumors is driven by PI3K/AKT activation, and opposed by tumor suppressors, including the tuberous sclerosis complex 2 (TSC-2) and p27, with inactivation of TSC2 and loss or cytoplasmic mislocalization of p27 both being linked to PI3K/AKT activation. However, little is known about the involvement of p27 in the development of EC arising in the setting of obesity, especially its role early in disease progression. Using a panel of EC cell lines, in vitro studies using PI3K inhibitors provided evidence that p27 rescue contributes to the efficacy of interventions that inhibit endometrial cell growth. In "at risk" obese patients, and in an animal model of obesity-associated EC (Tsc2-deficient Eker rats), p27 was moderately-to-severely reduced in both "normal" endometrial glands as well as in endometrial complex atypical hyperplasia (obese women), and endometrial hyperplasia (obese rats). In obese Eker rats, an energy balance intervention; caloric restriction from 2-4 months of age, reduced weight, increased adiponectin and lowered leptin to produce a favorable leptin:adiponectin ratio, and reduced circulating insulin levels. Caloric restriction also increased p27 levels, relocalized this tumor suppressor to the nucleus, and significantly decreased hyperplasia incidence. Thus, dietary and pharmacologic interventions that inhibit growth and decrease risk for development of endometrial lesions are associated with increased expression and nuclear (re)localization of p27. These data suggest that p27 levels and localization may be useful as a biomarker, and possible determinant, of risk for EC arising in the setting of obesity.

Obesity, Diabetes and Cancer: A Mechanistic Perspective.

Nearly 35% of adults and 20% of children in the United States are obese, defined as having a body mass index (BMI) ≥ 30 kg/m2. Obesity is an established risk factor for many cancers, and obesity-associated metabolic perturbations often manifest in Type 2 diabetes mellitus and/or the metabolic syndrome. As part of the growth-promoting, proinflammatory microenvironment of the obese and/or diabetic state, crosstalk between macrophages, adipocytes, and epithelial cells occurs via metabolically-regulated hormones, cytokines, and other mediators to enhance cancer risk and/or progression. This review synthesizes the evidence on key biological mechanisms underlying the associations between obesity, diabetes and cancer, with particular emphasis on enhancements in growth factor signaling, inflammation, and vascular integrity processes. These interrelated pathways represent mechanistic targets for disrupting the obesity-diabetes-cancer link, and several diabetes drugs, such as metformin and rosiglitazone, are being intensely studied for repurposing as cancer chemopreventive agents.

Obesity and resistance to cancer chemotherapy: interacting roles of inflammation and metabolic dysregulation.

The prevalence of obesity, an established risk factor for many chronic diseases, including several types of cancer, has risen steadily over the past four decades in the United States and worldwide. To date, research in this area has focused on the epidemiologic associations between obesity and cancer risk, as well as on the mechanisms underlying those associations. However, an emerging but understudied issue of clinical importance is the diminution of chemotherapeutic efficacy in obese cancer patients. The mechanisms underlying the negative impact of obesity on therapeutic responses are likely multifactorial. The effects of obesity on chemotherapy drug pharmacokinetics and dosage have been extensively reviewed elsewhere, so this review will focus on the interplay among obesity, increased inflammation, metabolic perturbations, and chemoresistance. The ultimate goal of this review is to delineate areas for future research that could lead to the identification of new targets and strategies for improved cancer outcomes in obese patients.

Obesity-related systemic factors promote an invasive phenotype in prostate cancer cells.

BACKGROUND: Obesity is associated with larger tumors, shorter time to PSA failure, and higher Gleason scores. However, the mechanism(s) by which obesity promotes aggressive prostate cancer remains unknown. We hypothesize that circulating factors related to obesity promote prostate cancer progression by modulating components of the metastatic cascade. METHODS: Male C57BL/6 mice (6 weeks) were fed an ad libitum diet-induced obesity (60% fat) or control diet (10% fat) for 12 weeks. Serum was collected, metabolic and inflammatory proteins were measured by an antibody array. Sera were used to measure, in vitro, characteristics of a metastatic phenotype. RESULTS: Comparable to obese men, obese sera contained higher levels or leptin, vascular endothelial growth factor, PAI-1, interleukin-6 (IL-6) and lower levels of testosterone. In prostate cells, serum was used to assess: proliferation, invasion, migration, epithelial-mesenchymal-transition (EMT) and matrix metalloproteinase (MMP) activity. LNCaP and PacMetUT1 cells exposed to obese sera increased proliferation, whereas PrEC and DU145 were unaffected. LNCaP, PacMetUT1 and DU145 cancer cells exposed to obese sera resulted in increased invasion, migration and MMP-9 activity. Prostate cancer cells exposed to obese sera showed increased vimentin, dispersion of e-cadherin and ß-catenin from the plasma membrane. CONCLUSION: We report, prostate cancer cells exposed to sera from obese mice increases proliferation, invasion, migration, MMP activity and induces changes in proteins critical for EMT.

Diet-induced adiposity alters the serum profile of inflammation in C57BL/6N mice as measured by antibody array.

Morbid obesity is considered a systemic inflammatory state. The objective of this project was to characterize the adipokine, cytokine and chemokine protein profile in serum from control, lean and obese mice. We hypothesized that chemokines and cytokines are altered by caloric restriction and diet-induced obesity as a function of changes in body composition. Six-week-old female C57BL/6N mice (n = 12 per group) were randomized to one of three diets: control (fed ad libitum); lean (30% calorie-restricted regimen relative to control) and diet-induced obese (DIO; high calorie diet, fed ad libitum). Body weight, body composition and food intake were monitored throughout the study. After 10 weeks on the diets, blood samples were collected, and adipokine/cytokine/chemokine serum profiles were measured by antibody array. Lean mice, relative to the control group, displayed increased concentrations of insulin-like growth factor (IGF) binding protein-3, -5 and -6 and adiponectin and decreased IGF-1. These mice also showed increased concentrations of interleukin (IL)-10, IL-12 p40/p70, eotaxin, monocyte chemoattractant protein-5 and SDF-1. In contrast, DIO mice displayed increased leptin, IL-6 and LPS-induced chemokine and decreased concentrations of all chemokines/cytokines measured relative to control mice. As such, these data indicate that DIO may lead to an inflammatory state characterized as a shift towards a T helper lymphocyte type 1-skewed responsiveness. The demonstration of differential adipokine, cytokine and chemokine protein profile in control, lean and DIO mice may have implications for immune responsiveness and risk of disease.

Nutrition and cancer prevention: a multidisciplinary perspective on human trials.

More than one million Americans were expected to be diagnosed with cancer in 2003 (7a). Compelling experimental, epidemiological, and clinical evidence indicates that many cancers are preventable, especially because diet and nutrition are key factors in the modulation of cancer risk. The road to nutritional intervention in cancer prevention has led to successful trials as well as trials that did not reach their intended endpoints. This chapter reviews four case studies of trials, with two ending in success and two ending in null findings or adverse effects. The goal is to identify lessons learned from all four case studies and from the investigations of the complexities inherent to nutritional intervention trials. Additional insights are presented by the research addressing potential mechanisms underlying the endpoints of human trials. Future progress in nutrition and cancer prevention will require expertise from multidisciplinary teams to develop new knowledge about specific nutrients and dietary modifications within a framework of interaction between animal and human research.

Genotoxicity and carcinogenicity studies of soy isoflavones.

The potential cancer preventive efficacy of soy isoflavones is being investigated in preclinical and phase 1 clinical studies sponsored by the U.S. National Cancer Institute. Although 90-day oral toxicity studies with PTI G-2535 (an investigational soy isoflavone drug product) in rats and dogs, as well as teratology studies, indicated no signs of toxicity, there remains a mechanistic concern associated with the ability of isoflavones (i.e., genistein) to inhibit topoisomerase, possibly leading to DNA strand breaks. The present report describes results from two in vitro genotoxicity studies, one in vivo genotoxicity study, and a single carcinogenicity study conducted in p53 knockout mice. Bacterial mutagenesis experiments using six tester strains without metabolic activation revealed no evidence that PTI G-2535 was mutagenic. In similar experiments with exogenous metabolic activation there were statistically significant increases in revertants, but less than twofold, in a single (Salmonella typhimurium TA100) tester strain. Mouse lymphoma cell mutagenesis experiments conducted with and without metabolic activation revealed statistically significant increases in mutation frequency at PTI G-2535 concentrations > or = 0.8 and 12 microg/ml, respectively; such increases were dose related and increases in the frequency of both small and large colonies were observed. A statistically significant increase in the frequency of micronucleated polychromatic erythrocytes was also seen 24 hours after treatment in male, but not female, mice who received 500 and 1000 mg/kg body weight PTI G-2535; however,such increases were small, were not dose related, and were not observed 48 hours after treatment. In contrast, dietary genistein had no effect on survival, weight gain, or the incidence or types of tumors that developed in cancer-prone rodents lacking the p53 tumor suppressor gene, p53 knockout mice. The apparent risk/benefit of isoflavone ingestion may ultimately depend on the dose and developmental timing of exposure.

Diet and cancer prevention studies in p53-deficient mice.

Progress in mechanism-based cancer prevention research may be facilitated by the use of animal models displaying specific genetic susceptibilities for cancer such as mice deficient in the p53 tumor suppressor gene, the most frequently altered gene in human cancer. We observed in p53-knockout (p53-/-) mice that calorie restriction (CR; 60% of the control group's intake of carbohydrate energy) increased the latency of spontaneous tumor development (mostly lymphomas) approximately 75%, decreased serum insulin-like growth factor (IGF)-1 and leptin levels, significantly slowed thymocyte cell cycle traverse and induced apoptosis in immature thymocytes. In heterozygous p53-deficient (p53+/-) mice, CR and 1 d/wk of food deprivation each significantly delayed spontaneous tumor development (a mix of lymphomas, sarcomas and epithelial tumors) and decreased serum IGF-1 and leptin levels even when begun late in life. We have also developed a rapid and relevant p53+/- mouse mammary tumor model by crossing p53-deficient mice with MMTV-Wnt-1 transgenic mice, and found that CR and 1 d/wk food deprivation significantly increased mammary tumor latency (greater than twofold) and reduced the mean serum IGF-1 and leptin levels to <50% of that of control mice (P < 0.0001). In addition, fluasterone, fenretinide and soy each delayed tumor development but had little effect on IGF-1 or leptin levels. We have capitalized on the susceptibility of p53+/- mice to chronic, low dose, aromatic amine-induced bladder carcinogenesis to develop a useful model for evaluating bladder cancer prevention approaches such as cyclooxygenase-2 inhibition. As demonstrated by these examples, mice with specific (and human-like) genetic susceptibilities for cancer provide powerful new tools for testing and characterizing interventions that may inhibit the process of carcinogenesis in humans.

Leptin and prostate cancer.

BACKGROUND: Higher prostate cancer mortality rates among US immigrants from countries with lower rates suggest environmental influences on prostate carcinogenesis (e.g., diet, body composition). METHODS: In a study identifying determinants of clinically relevant prostate cancer, we compared plasma concentrations of leptin, an adiposity-related hormone, in 48 men with tumors 0.5 cc in volume or with histologic evidence of extraprostatic extension but without metastases ("high-volume disease"), matched by age (+/- 5 years) and year at diagnosis (+/- 1 year). RESULTS: Men with high-volume disease exhibited higher leptin concentrations overall and after stratification by age, testosterone level, height, and body mass index (BMI). Analysis revealed that men with elevated leptin concentrations had an increased risk of diagnosis with high-volume disease (odds ratio (OR) = 2.35, 95% confidence interval (CI) = 1.01-5.44), as did men with high leptin and high testosterone (OR = 9.73, 95% CI = 2.05-46.24) and men >/=5'8" with high leptin (OR = 3.67, 95% CI = 1.40-9.63). CONCLUSIONS: Leptin may affect the risk of clinically relevant prostate cancer through testosterone and factors related to stature and obesity.

Cancer prevention studies in p53-deficient mice.

Future progress in mechanism-based cancer prevention research may be facilitated by animal models displaying specific genetic susceptibilities for cancer, such as mice deficient in 1 (+/-) or both (-/-) alleles of the p53 tumor suppressor gene. We observed in p53-/- mice that calorie restriction (CR) increased the latency of spontaneous tumor development (mostly lymphomas) by approximately 75%, decreased serum insulin-like growth factor-1 (IGF-1) and leptin levels, slowed thymocyte cell cycle traverse, and induced apoptosis in immature thymocytes. In p53+/- mice, CR and a 1 d/wk fast each delayed spontaneous tumor development (a mix of lymphomas, sarcomas, and epithelial tumors) and decreased serum IGF-1 and leptin levels, even when begun late in life. In p53+/-Wnt-1 transgenic mice, a mammary tumor model, the same interventions increased mammary tumor latency and reduced mean serum IGF-1 and leptin levels to <50% of those of control mice. We capitalized on the susceptibility of p53+/- mice to chronic, low-dose aromatic amine-induced bladder carcinogenesis to develop a useful model for evaluating bladder cancer prevention approaches. These examples clearly indicate that mice with specific (and humanlike) genetic susceptibilities for cancer are powerful models for testing interventions that may inhibit carcinogenesis in humans.

Low-dose genistein induces cyclin-dependent kinase inhibitors and G(1) cell-cycle arrest in human prostate cancer cells.

Genistein, a naturally occurring isoflavone found chiefly in soy products, reportedly has antiprostate cancer effects, but the mechanisms underlying these effects are unknown. We studied the antiproliferative and apoptosis-inducing effects of genistein in the androgen-sensitive human prostate cancer cell line LNCaP. Viable cell number was assessed by the 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide assay; cell-cycle progression and apoptosis were evaluated by flow cytometry; apoptosis was also assessed by a histone enzyme-linked immunosorbent assay; and the expression of several cell-cycle- and apoptosis-related genes and their gene products was determined by northern blot analysis, western blot analysis, and/or assays based on polymerase chain reaction. Physiologic concentrations of genistein (< or = 20 microM) decreased LNCaP viable cell number in a dose-dependent manner, induced a G(1) cell-cycle block, decreased prostate-specific antigen mRNA expression, and increased p27(KIP1) and p21(WAF1) (mRNA and protein) but had no effect on apoptosis or the mRNA expression of the apoptosis- and cell-cycle-related markers bcl-2, bax, Rb, and proliferating cell nuclear antigen. Higher concentrations of genistein (> 20 microM) did induce apoptosis. We conclude that genistein (at physiologic concentrations) exerts potent antiproliferative effects on LNCaP cells by inducing a G(1) cell-cycle block. The antiproliferative effects of genistein may be mediated by increased levels of p27(KIP1) and p21(WAF1), which are negative cell-cycle regulators that act as cyclin-dependent kinase inhibitors and that have been recently linked with prostate carcinogenesis. These findings may provide insights into the mechanisms underlying the apparent antiprostate cancer effects of soy consumption observed in epidemiologic studies.

Effects of calorie restriction on thymocyte growth, death and maturation.

We previously reported that calorie restriction (CR) significantly delays the spontaneous development of thymic lymphomas and other neoplasms in p53-deficient mice and their wild-type littermates. The purpose of the present study was to further characterize the anti-lymphoma effects of CR by assessing thymocyte growth, death and maturation in response to acute (6 day) and chronic (28 day) CR regimens. Male C57BL/6J mice fed a CR diet (restricted to 60% of control ad libitum intake) for 6 days displayed a severe reduction in thymic size and cellularity, as well as a decrease in splenic size and cellularity; these declines were sustained through 28 days of CR. Mice maintained on a CR diet for 28 days also displayed a significant depletion in the cell numbers of all four major thymocyte subsets defined by CD4 and CD8 expression. Analysis within the immature CD4(-)8(-) thymocyte subset further revealed an alteration in normal CD44 and CD25 subset distribution. In particular, CR for 28 days resulted in a significant decrease in the percentage of the proliferative CD44(-)25(-) subset. In addition, a significant increase in the percentage of the early, pro-T cell CD44(+)25(-) population was detected, indicative of a CR-induced delay in thymocyte maturation. Taken together, these findings suggest that CR suppresses (through several putative mechanisms) lymphomagenesis by reducing the pool of immature thymocytes that constitute the lymphoma-susceptible subpopulation.

Development of a database for assessing dietary phytoestrogen intake.

For the past two decades, epidemiologists have observed lower risks of lung, breast, prostate, colon, and other cancers in populations that frequently consume fruits and vegetables. Numerous phytoestrogens have been shown to be anticarcinogenic under experimental conditions and may account for at least part of the cancer-prevention effects of fruit and vegetable consumption. These plant constituents include isoflavonoids, coumestans, lignans, phytosterols, and flavonoids. DietSys, the nutrient analysis program associated with the National Cancer Institute Health Habits and History Questionnaire (HHHQ), and other nationally available nutrient analysis databases do not fully assess these constituents. Therefore, we modified DietSys to include these components in foods on the basis of published values. In addition, as part of an epidemiological study of prostate cancer, we modified the food-frequency component of the HHHQ to include the main foods contributing to phytoestrogen intake. Although there are limitations to the consistency and quality of many of the values because they were gathered from a variety of sources, our approach should provide a useful first tool for assessing the epidemiological association between phytoestrogen consumption and cancer risk. Furthermore, this work has already facilitated the identification of the major dietary contributors with phytoestrogen activity and prioritized future laboratory analyses of specific foods toward the development of a more complete and accurate database.

Phytoestrogen intake and prostate cancer: a case-control study using a new database.

In the last several years, attention has been focused on comparing the Western diet, which is rich in fat, protein, and refined carbohydrates, with the Asian diet, which is rich in phytoestrogens, as a possible explanation for the contrasting rates of clinically relevant prostate cancer. Phytoestrogens, plant-derived nutrients, include several isoflavones, flavonoids, lignans, phytosterols, and coumestans, some of which have been postulated as having anticarcinogenic properties. Using a new database, we examined the role of phytoestrogen intake and prostate cancer risk in 83 Caucasian cases and 107 controls. Controls reported consuming higher amounts of foods containing genistein, daidzein, and coumestrol and lower amounts of foods containing campesterol and stigmasterol. Multivariate analysis, after adjustment for age, family history of prostate cancer, alcohol consumption, and total calorie intake, showed an inverse association between coumestrol (p = 0.03) and daidzein (p = 0.07) and prostate cancer risk. Genistein, the most studied phytoestrogen, showed a slight protective effect (p = 0.26). However, a positive association was found between campesterol (p = 0.08) and stigmasterol (p = 0.03) and risk of prostate cancer. These results are suggestive of a possible relationship between phytoestrogen intake and prostate cancer risk. Larger comprehensive studies are needed to further refine the role of phytoestrogen intake in prostate cancer risk.

Mechanistic studies of the effects of the retinoid N-(4-hydroxyphenyl)retinamide on prostate cancer cell growth and apoptosis.

To explore the mechanisms underlying the chemopreventive effects of the synthetic retinoid N-(4-hydroxyphenyl)retinamide (4-HPR) in prostate cancer, we evaluated the anti-proliferative and apoptosis-inducing effects of 4-HPR in the androgen-sensitive human prostate cancer cell line LNCaP. 4-HPR decreased the number of viable LNCaP cells (as measured by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay) in a dose-dependent manner. Although 4-HPR exerted a modest G1 cell-cycle block (as determined by flow cytometry), its effect on reduced cell number appeared to result primarily from induction of apoptosis (as measured by an enzyme-linked immunosorbent assay and flow-cytometric assays). The mitogenic effects of R1881, a non-metabolizable androgen that potently induces LNCaP cell proliferation, was completely blocked by greater than 0.5 microM 4-HPR. Furthermore, increasing the R1881 concentration in the presence of 2.0 microM 4-HPR increased apoptotic cell death. 4-HPR decreased prostate-specific antigen (PSA) protein levels in conditioned medium and decreased PSA mRNA expression. 4-HPR also decreased the ratio of bcl-2 to bax mRNA expression in LNCaP cells by approximately 45%, indicating that the apoptotic effects of 4-HPR may be mediated, at least in part, by alterations in the bcl-2/bax-regulated apoptotic pathway. N-acetylcysteine (4 mM) completely blocked the anti-proliferative and apoptotic-inducing effects of 4-HPR, suggesting that an oxidative mechanism may be involved. We concluded that (i) 4-HPR exerts growth-suppressive and apoptotic effects on LNCaP cells, (ii) 4-HPR can interact with androgen to suppress proliferation and induce apoptosis, (iii) the apoptotic effects of 4-HPR may be mediated in part by the bcl-2/bax pathway, and (iv) a pro-oxidant mechanism may contribute to the anti-proliferative and apoptotic-inducing effects of 4-HPR.

Mechanism-based cancer prevention approaches: targets, examples, and the use of transgenic mice.

Humans are exposed to a wide variety of carcinogenic insults, including endogenous and man-made chemicals, radiation, physical agents, and viruses. The ultimate goal of carcinogenesis research is to elucidate the processes involved in the induction of human cancer so that interventions may be developed to prevent the disease, either in the general population or in susceptible subpopulations. Progress to date in the carcinogenesis field, particularly regarding the mechanisms of chemically induced cancer, has revealed several points along the carcinogenesis pathway that may be amenable to mechanism-based prevention strategies. The purpose of this review is to examine the basic mechanisms and stages of chemical carcinogenesis, with an emphasis on ways in which preventive interventions can modify those processes. Possible ways of interfering with tumor initiation events include the following: i) modifying carcinogen activation by inhibiting enzymes responsible for that activation or by direct scavenging of DNA-reactive electrophiles and free radicals; ii) enhancing carcinogen detoxification processes by altering the activity of the detoxifying enzymes; and iii) modulating certain DNA repair processes. Possible ways of blocking the processes involved in the promotion and progression stages of carcinogenesis include the following: i) scavenging of reactive oxygen species; ii) altering the expression of genes involved in cell signaling, particularly those regulating cell proliferation, apoptosis, and differentiation; and iii) decreasing inflammation. In addition, the utility for mechanism-based cancer prevention research of new animal models that are based on the overexpression or inactivation of specific cancer-related genes is examined.

Splenic marginal zone B-cell and thymic T-cell lymphomas in p53-deficient mice.

The molecular pathology and histogenesis of lymphomas in 56 retired breeder male and 14 12-week-old male homozygous p53-deficient (p53-/-) mice (C57BL/6TacfBR-[KO]p53 N4) were evaluated. Lymphomas were assessed by serial morphologic techniques, immunohistochemistry, flow cytometry, and analysis of T cell receptor (TCR) or immunoglobulin heavy chain (IgH) gene rearrangements. We found two common types of lymphomas. T-cell lymphomas arose in the thymus through a sequence of lymphocyte depletion, medullary hyperplasia, and unilateral lymphoma. Tumor cells were CD3+, CD90+, and usually TCRalpha/beta+ and possessed clonal TCRbeta gene rearrangements. Thymic lymphoblastic lymphomas (LL) were highly malignant and quickly metastasized to the splenic white pulp and liver, even when the thymus was only slightly increased in weight. In the spleen, a novel lymphoma was found. Marginal zone hyperplasia led to marginal zone lymphoma (MZL), a well-differentiated lymphoma that usually expressed CD45R (B220) and CD5 at low levels and that had clonal IgH gene rearrangements. IgH gene rearrangements were also seen in spleens with marginal zone hyperplasias only. Hyperplastic and neoplastic marginal zone B cells expressed IgM at low to normal levels, as seen by FACS analysis and immunohistochemistry. These tumors only metastasized to the liver at a later stage, as they became less differentiated. Several mice had both types of tumors present in the spleen. Two B-cell lymphoblastic lymphomas of uncertain origin were also found. In this article, we discuss the possible mechanisms responsible for development of the lymphomas seen in these p53-deficient mice.