Effects of docosahexaenoic acid (DHA) on intestinal polyp development in Apc{Delta}716 knockout mice

Epidemiological studies and animal experiments show an associationof dietary intake of fish oils and low incidence of severaltypes of cancers. The active ingredients of fish oils appearto be polyunsaturated fatty acids of     

No effects of Smad2 (madh2) null mutation on malignant progression of intestinal polyps in Apc(delta716) knockout mice

The loss of heterozygosity (LOH) in human chromosome 18q21 is found at high frequencies in advanced pancreatic and colorectal cancers. Several candidate tumor suppressor genes, such as SMAD2, SMAD4, and DCC, are located in this region. The homologues of these genes in the mouse are also clustered on chromosome 18. Mutations in the adenomatous polyposis coli (APC) gene are responsible for familial adenomatous polyposis, and we earlier constructed a mouse model for familial adenomatous polyposis, Apc(delta716). Although human APC is located on chromosome 5q, mouse Apc is on chromosome 18, 30 cM proximal to the Dcc-Smad4-Smad2 locus. Taking advantage of this fact, we constructed previously a cis-compound Apc(delta716) Smad4 mutant, the intestinal polyps of which progress to very invasive adenocarcinomas. To determine whether Smad2 mutations play similar roles in malignant progression, here we constructed compound mutant mice carrying Apc and Smad2 knockouts in the cis configuration. In contrast to the cis-compound Apc(delta716) Smad4 heterozygotes, the polyps in the cis-compound Apc(delta716) Smad2 heterozygotes showed no difference in the number, size, or histopathology from the polyps in the simple Apc(delta716) heterozygotes. These results suggest that, on human chromosome 18q21, the SMAD4 LOH plays a more significant role, and SMAD2 LOH is insufficient to cause malignant progression of colonic polyps.     

Suppression of intestinal and mammary neoplasia by lifetime administration of aspirin in Apc(Min/+) and Apc(Min/+), Msh2(-/-) mice

Numerous studies have indicated that exposure to nonsteroidal anti-inflammatory drugs is associated with a lowered risk of colorectal cancer. However, analyses of the effect of aspirin upon tumorigenesis in Apc(Min/+) mice have yielded contrasting results. We show that adult dietary exposure to aspirin does not suppress intestinal tumorigenesis in Apc(Min/+) mice, but that continual exposure from the point of conception does. To test whether this regime could suppress the phenotype of murine models of hereditary nonpolyposis colorectal cancer, Msh2-deficient mice were exposed to aspirin. This did not modify the mutator phenotype of Msh2(-/-) mice, but weakly extended survival. Finally, we analyzed (Apc(Min/+), Msh2(-/-)) mice and found that lifetime aspirin exposure significantly delayed the onset of both intestinal and mammary neoplasia. Thus embryonic and perinatal exposure to aspirin suppresses neoplasia specifically associated with the loss of Apc function, opening a potential window of opportunity for nonsteroidal anti-inflammatory drug intervention.     

Calorie restriction and diet composition modulate spontaneous intestinal tumorigenesis in Apc(Min) mice through different mechanisms

We evaluated the effects of diet on intestinal tumorigenesis in male Apc(Min) mice by comparing AIN-76A diet fed ad libitum (CON); calorie intake restricted by 40% of the CON (CR); diet high in olive oil and supplemented with freeze-dried fruit and vegetable extracts (OFV); and diet high in total fat (HF). Compared with CON, the frequency of intestinal polyps was reduced by 57% by CR (P < 0.001) and by 33% OFV diet (P = 0.04). Both effective interventions reduced total body weight, lean mass, and fat mass and increased daily urinary corticosterone output, but only CR reduced serum insulin-like growth factor I and leptin. We conclude that dietary interventions can partially offset genetic susceptibility to intestinal carcinogenesis.     

Cyclooxygenase 2- and prostaglandin E(2) receptor EP(2)-dependent angiogenesis in Apc(Delta716) mouse intestinal polyps.

To investigate angiogenesis during intestinal polyp development, we determined the microvessel density (MVD) in polyps of Apc knockout (Apc(Delta716)) mice, a model for human familial adenomatous polyposis. We scored MVD also in several compound mutants carrying Apc(Delta716), namely, mice with an additional mutation in Smad4, in which the polyps progress into invasive adenocarcinomas; mice with a cyclooxygenase (COX)-2 gene (Ptgs2) mutation, in which adenoma growth is suppressed; and mice with prostaglandin E(2) EP receptor gene mutations. In both simple Apc(Delta716) and compound Apc(Delta716) Smad4 mutants, MVD increased in a polyp size-dependent manner only in the polyps expanded beyond a threshold of about 1 mm in diameter. These results indicate that tumor angiogenesis is stimulated only after tumors grow to a certain size, and this angiogenic switch is common to both benign adenomas and malignant adenocarcinomas. In Apc(Delta716) polyposis attenuated by the COX-2 gene mutation, in contrast, MVD did not increase even in polyps larger than 1 mm. The same phenomenon was observed in the compound mutant mice with Apc(Delta716) and the EP(2) receptor gene mutations, but not in other EP compound mutants. We also immunohistochemically studied COX-2 and angiogenic factors, vascular endothelial growth factor and basic fibroblast growth factor. Interestingly, expression of these proteins was also increased in polyps larger than 1 mm. These results suggest that, in both benign and malignant mouse intestinal tumors, stromal expression of COX-2 results in elevated prostaglandin E(2) levels that stimulate cell surface receptor EP(2), followed by induction of vascular endothelial growth factor that causes tumor angiogenesis.     

The role of P-glycoprotein in intestinal tumorigenesis: disruption of mdr1a suppresses polyp formation in Apc(Min/+) mice

P-glycoprotein (P-gp) mediates the active transport of various substrates including xenobiotics, and it thus has a protective function in various cell types and tissues/organs including the intestinal epithelium. However, whether or not P-gp plays a positive role in the intestinal tumorigenesis is unclear. We have introduced disrupted alleles of the murine P-gp gene, mdr1a, into Apc(Min/+) mice to evaluate whether P-gp plays any role in intestinal carcinogenesis. Spontaneously occurring DNA damage was significantly increased in both the small and large intestine of mdr1a(-/-), Apc(Min/+) mice compared with mdr1a(+/+), Apc(Min/+) mice. Furthermore, we observed active proliferation and rapid migration/disappearance of enterocytes in the intestine of the compound mice deficient in mdr1a. Finally, we found that the number of polyps and cancers was markedly decreased in mdr1a(-/-), Apc(Min/+) mice (P=0.0016). P-gp thus appears to play a positive role during intestinal tumorigenesis.     

Polyethylene glycol inhibits intestinal neoplasia and induces epithelial apoptosis in Apc(min) mice

Efficacy of a safe and clinically utilized polyethylene glycol formulation (PEG-3350) to suppress intestinal tumors was investigated in the Apc(min) mouse-model of experimental carcinogenesis. Furthermore, based on our previous finding on the induction of apoptosis in HT-29 cells by PEG, we evaluated its ability to stimulate epithelial cell apoptosis in both Apc(min) mouse as well as AOM-treated rat as a potential molecular mechanism of chemoprevention. Twenty-two Apc(min) mice were randomized equally to PEG or vehicle (control) supplementation. Tumors were scored and uninvolved intestinal mucosal apoptosis was assayed using a modified terminal deoxynucleotidyl transferase-mediated nick end-labeling (TUNEL) assay and by immunohistochemical detection of cleaved caspase-3. Supplementation of Apc(min) mice with 10% PEG 3350 (in drinking water) resulted in a 48% (P<0.05) reduction in intestinal tumor burden and induced 2-3 fold increase in mucosal apoptosis. Dietary supplementation of polyethylene glycol (5%) also stimulated colonic mucosal apoptosis 4-5 fold in AOM-treated rats, the regimen that we previously reported to reduce tumor burden by 76% (P<0.05). In summary, we demonstrate, for the first time, that PEG does protect against Apc(min) mouse tumorigenesis. The correlation between pro-apoptotic actions and chemopreventive efficacy of PEG in these models strongly implicates induction of apoptosis as one of the impending mechanisms of chemoprevention.     

Growth stimulation of intestinal tumours in Apc(Min/+) mice by dietary L-methionine supplementation

We studied the effects of extra dietary methionine on the formation and growth of intestinal adenomas in the Min (Apc+/-) mouse, which is a murine model of the human familial adenomatous syndrome. The AIN-76A diet was supplemented with 0.7% L-methionine from week 4 after birth and the animals were killed at week 8. The number of tumours in Min mice was apparently not affected by the addition of extra methionine. However, the dietary methionine supplementation increased the surface area of small intestinal tumours by 41% (p=0.009). In the colon, extra methionine did not affect tumour size. In conclusion, extra dietary methionine promotes the growth of adenomas in the small intestine of Min mice.     

Fibersol-2 induces apoptosis of Apc-deficient colorectal Cancer (SW480) cells and decreases polyp formation in Apc MIN mice

The consumption of dietary fibers has been implicated with a lowered risk of human colorectal cancer. Proposed mechanisms involve alterations in the stool consistency, transit time, and formation of short-chain fatty acid by dietary fiber fermentation, and the reorganization of gut microbiota. Here we show that Fibersol-2, a digest-resistant maltodextrin, not only inhibits proliferation of colorectal SW480 cancer cell lines by increasing reactive oxygen species (ROS), but decreases the numbers of the adenoma count in Multiple Intestinal Neoplasia (MIN) mice carrying a mutation in the Adenomatous Polyposis Coli gene by 84 d of age. These observations provide direct evidence that Fibersol-2 intrinsically contains anti-cancer activity, independent of the intestinal metabolism and any potential interactions with the microbiota.     

The effects of a low-fat/high-fiber diet on sex hormone levels and menstrual cycling in premenopausal women: a 12-month randomized trial (the diet and hormone study)

BACKGROUND: Reduction of cumulative exposure to endogenous ovarian steroid hormones is a postulated method for reducing the risk of carcinoma of the breast and other malignancies. Although there are data from trials evaluating the effect of low-fat and high-fiber diets on sex hormone levels in premenopausal women, to the authors' knowledge none of these trials has combined a relatively large number of participants, follow-up of > 2-3 months, parallel controls receiving a usual diet, and careful timing of blood sampling within the menstrual cycle. METHODS: A total of 213 healthy women, ages 20-40 years, were randomly assigned to follow their usual diet or to adopt an isocaloric diet with goals of 20% calories as fat, total fiber of 25 g/day, and at least 8 fruit or vegetable servings per day. Serum levels of total estradiol (E2), sex hormone-binding globulin (SHBG), non-SHBG-bound estradiol (NSBE2), SHBG, and progesterone were evaluated during a menstrual cycle at baseline, and at 4 cycles (C4) and 12 cycles (C12) after the start of the intervention. Serum was collected during each test cycle 7-9 days after the detection of an luteinizing hormone peak in the urine. One hundred eighty-nine women provided serum at C4 and 176 women at C12. RESULTS: Serum E2 decreased by an average of 7.5% or 7.8 pg/mL (95% confidence interval [95% CI], -16.0-0.04) at C12 in the intervention group, versus a decrease of 0.9% or 0.9 pg/mL (95% CI, -9.5-7.7) in the control group (the P value for the difference between the treatment groups was 0.25). Results for NSBE2 were very similar to those for total estradiol. There were no material effects found to result from intervention with regard to SHBG or progesterone. The results did not differ by baseline age, body mass index, or baseline hormone level above or below the median, and were not likely to be affected by weight change, which amounted to a mean loss of only 0.23 kg in the diet group versus a gain of 0.17 kg in the control group. The decrease in serum E2 associated with intervention was not greater when subjects were stratified by self-reported adherence to the dietary goals. CONCLUSIONS: The results of the current study suggest that the effects of this isocaloric low-fat, high-fiber diet pattern on circulating ovarian steroids were modest or nonexistent. However, the observed 7.5% reduction in estradiol could have biologic significance if it persisted over many years. Moreover, underestimation of the true dietary effect could have occurred because of incomplete adherence to assigned diets. Weight loss and weight control through midlife could be a more effective and feasible approach to dietary intervention in reducing the risk of breast carcinoma.     

The polyp prevention trial continued follow-up study: no effect of a low-fat, high-fiber, high-fruit, and -vegetable diet on adenoma recurrence eight years after randomization.

The Polyp Prevention Trial (PPT) was a multicenter randomized clinical trial to evaluate the effects of a high-fiber (18 g/1,000 kcal), high-fruit and -vegetable (3.5 servings/1,000 kcal), and low-fat (20% of total energy) diet on the recurrence of adenomatous polyps in the large bowel over a period of 4 years. Although intervention participants reported a significantly reduced intake of dietary fat, and increased fiber, fruit, and vegetable intakes, their risk of recurrent adenomas was not significantly different from that of the controls. Since the PPT intervention lasted only 4 years, it is possible that participants need to be followed for a longer period of time before treatment differences in adenoma recurrence emerge, particularly if diet affects early events in the neoplastic process. The PPT-Continued Follow-up Study (PPT-CFS) was a post-intervention observation of PPT participants for an additional 4 years from the completion of the trial. Of the 1,905 PPT participants, 1,192 consented to participate in the PPT-CFS and confirmed colonoscopy reports were obtained on 801 participants. The mean time between the main trial end point colonoscopy and the first colonoscopy in the PPT-CFS was 3.94 years (intervention group) and 3.87 years (control group). The baseline characteristics of 405 intervention participants and 396 control participants in the PPT-CFS were quite similar. Even though the intervention group participants increased their fat intake and decreased their intakes of fiber, fruits, and vegetables during the PPT-CFS, they did not go back to their prerandomization baseline diet (P < 0.001 from paired t tests) and intake for each of the three dietary goals was still significantly different from that in the controls during the PPT-CFS (P < 0.001 from t tests). As the CFS participants are a subset of the people in the PPT study, the nonparticipants might not be missing completely at random. Therefore, a multiple imputation method was used to adjust for potential selection bias. The relative risk (95% confidence intervals) of recurrent adenoma in the intervention group compared with the control group was 0.98 (0.88-1.09). There were no significant intervention-control group differences in the relative risk for recurrence of an advanced adenoma (1.06; 0.81-1.39) or multiple adenomas (0.92; 0.77-1.10). We also used a multiple imputation method to examine the cumulative recurrence of adenomas through the end of the PPT-CFS: the intervention-control relative risk (95% confidence intervals) for any adenoma recurrence was 1.04 (0.98-1.09). This study failed to show any effect of a low-fat, high-fiber, high-fruit and -vegetable eating pattern on adenoma recurrence even with 8 years of follow-up. (Cancer Epidemiol Biomarkers Prev 2007;16(9):1745-52).     

Dietary methyl donor depletion protects against intestinal tumorigenesis in Apc(Min/+) mice

Despite recent population data, the influence of dietary folate supplementation on colon cancer risk remains controversial. This study examines the effects of folate deficiency, in combination with choline, methionine, and vitamin B12 depletion, on intestinal tumorigenesis in Apc(Min/+) mice. Methyl donor sufficient (MDS) and deficient (MDD) diets were started at five or 10 weeks of age and tumors evaluated at 16 weeks. MDD suppressed intestinal tumor formation in Apc(Min/+) mice (~80%) when started at five weeks of age. The protective effect was lost when MDD was initiated at 10 weeks of age, indicating an important time dependency on cancer suppression. Concomitant with cancer protection, MDD restricted body weight gain. Therefore, a second study was conducted in which MDS was given ad libitum or pair-fed with MDD. Although small intestinal tumors were reduced 54% in pair-fed MDS mice, MDD caused a further reduction (96%). In colon, although MDD did not affect tumor numbers, tumor size was reduced. Gene expression profiling of normal-appearing colonic mucosa after 11 weeks on MDD identified a total of 493 significantly downregulated genes relative to the MDS group. Pathway analysis placed many of these genes within general categories of inflammatory signaling and cell-cycle regulation, consistent with recently published human data obtained during folate depletion. Further studies are warranted to investigate the complex interplay of methyl donor status and cancer protection in high-risk populations.     

Ephrin-A1 promotes the malignant progression of intestinal tumors in Apc(min/+) mice

The ephrin-A1 and EphA receptors are frequently highly expressed in different human cancers, suggesting that they may promote tumor development and progression. We generated transgenic mice carrying Fabpl(4xat-132) ephrin-A1, which express ephrin-A1 in the intestinal epithelial cells. Those mice were then mated with Apc(min/+) mice to produce the compound mice, which overexpress ephrin-A1 in the intestinal tumors of Apc(min/+) mice. We compared the number, size and histopathological features of the intestinal tumors in the Fabpl(4xat-132) ephrin-A1/Apc(min/+) compound mice with those of the Apc(min/+) mice. The compound mice showed an increased number of intestinal tumors, significantly in the large intestine, and developed more invasive tumors. Among the 20 mice of each type examined, 5 Apc(min/+) mice developed 5 invasive tumors, 1 invasive tumor in each mouse, in the proximal or middle portions of the small intestine. On the other hand, 14 out of 20 compound mice developed 29 invasive tumors and 16 of them were in the distal small intestine and the large intestine, where transgenic ephrin-A1 was highly expressed. These results suggested that the increased expression of ephrin-A1 accelerated the malignant progression of the intestinal adenoma to invasive tumors.     

Deletion of the AU-rich RNA binding protein Apobec-1 reduces intestinal tumor burden in Apc(min) mice

The RNA-specific cytidine deaminase apobec-1 is an AU-rich RNA binding protein that binds the 3' untranslated region (UTR) of cyclooxygenase-2 (Cox-2) mRNA and stabilizes its turnover in vitro. Cox-2 overexpression accompanies intestinal adenoma formation in both humans and mice. Evidence from both genetic deletion studies as well as from pharmacologic inhibition has implicated Cox-2 in the development of intestinal adenomas in experimental animals and in adenomas and colorectal cancer in humans. Here, we show that small intestinal adenoma formation is dramatically reduced in compound Apc(min/+) apobec-1(-/-) mice when compared with the parental Apc(min/+) strain. This reduced tumor burden was found in association with increased small intestinal apoptosis and reduced proliferation in small intestinal crypt-villus units from compound Apc(min/+) apobec-1(-/-) mice. Intestinal adenomas from compound Apc(min/+) apobec-1(-/-) mice showed a <2-fold increase in Cox-2 mRNA abundance and reduced prostaglandin E(2) content compared with adenomas from the parental Apc(min/+) strain. In addition, there was reduced expression in adenomas from compound Apc(min/+) apobec-1(-/-) mice of other mRNAs (including epidermal growth factor receptor, peroxisome proliferator-activated receptor delta, prostaglandin receptor EP4, and c-myc), each containing the apobec-1 consensus binding site within their 3'-UTR. Adenovirus-mediated apobec-1 introduction into HCA-7 (colorectal cancer) cells showed a dose-dependent increase in Cox-2 protein and stabilization of endogenous Cox-2 mRNA. These findings suggest that deletion of apobec-1, by modulating expression of AU-rich RNA targets, provides an important mechanism for attenuating a dominant genetic restriction point in intestinal adenoma formation.     

Antitumor activity of the MEK inhibitor trametinib on intestinal polyp formation in ApcΔ716 mice involves stromal COX‐2

Extracellular signal‐regulated kinase is an MAPK that is most closely associated with cell proliferation, and the MEK/ERK signaling pathway is implicated in various human cancers. Although epidermal growth factor receptor, KRAS, and BRAF are considered major targets for colon cancer treatment, the precise roles of the MEK/ERK pathway, one of their major downstream effectors, during colon cancer development remain to be determined. Using Apc^Δ716 mice, a mouse model of familial adenomatous polyposis and early‐stage sporadic colon cancer formation, we show that MEK/ERK signaling is activated not only in adenoma epithelial cells, but also in tumor stromal cells including fibroblasts and vascular endothelial cells. Eight‐week treatment of Apc^Δ716 mice with trametinib, a small‐molecule MEK inhibitor, significantly reduced the number of polyps in the large size class, accompanied by reduced angiogenesis and tumor cell proliferation. Trametinib treatment reduced the COX‐2 level in Apc^Δ716 tumors in vivo and in primary culture of intestinal fibroblasts in vitro. Antibody array analysis revealed that trametinib and the COX‐2 inhibitor rofecoxib both reduced the level of CCL2, a chemokine known to be essential for the growth of Apc mutant polyps, in intestinal fibroblasts in vitro. Consistently, trametinib treatment reduced the Ccl2 mRNA level in Apc^Δ716 tumors in vivo. These results suggest that MEK/ERK signaling plays key roles in intestinal adenoma formation in Apc^Δ716 mice, at least in part, through COX‐2 induction in tumor stromal cells.     

Increase of oxidant-related triglycerides and phosphatidylcholines in serum and small intestinal mucosa during development of intestinal polyp formation in Min mice

Recent epidemiological studies have shown a positive association of a high-fat diet with the risk of colon cancer. Indeed, increments in the serum levels of triglycerides (TG) and cholesterols are positively related with colon carcinogenesis. We previously reported that an age-dependent hyperlipidemic state is characteristic of Min mice, an animal model for human familial adenomatous polyposis (FAP). However, qualitative and quantitative changes of lipid metabolism are poorly understood in this state. Here, we provide detailed analysis of serum lipids in Min mice using reverse-phased liquid chromatography/electrospray ionization mass spectrometry (RPLC/ESI-MS). We also demonstrate local analysis of lipid droplets in the villi of the small intestine using laser capture microdissection and a sensitive chip-based nanoESI-MS system. As a result, oxidized phosphatidylcholines (PC) such as aldehyde and carboxylic acid types were increased, even at an early stage of intestinal polyp formation in serum. In addition, hydroperoxidizable TG precursors containing linoleic acid (18:2n-6) were deposited at the tip of the villi with aging, and these hydroperoxidized TG were also increased in serum. Meanwhile, increments of the oxidizable TG precursors in serum and small intestinal mucosa were suppressed by treatment with pitavastatin, a novel third generation lipophilic statin. These results suggest that quantitative and qualitative lipid changes such as hydroperoxidizable TG precursors are important in the course of intestinal polyp formation and oxidative stress might lead to the development of intestinal polyp formation in Min mice.     

Prostaglandin E(2) protects intestinal tumors from nonsteroidal anti-inflammatory drug-induced regression in Apc(Min/+) mice.

Nonsteroidal anti-inflammatory drugs (NSAIDs) are antitumorigenic in humans as well as in animal models of intestinal neoplasia, such as the adenomatous polyposis coli (Min/+) (Apc(Min/+)) mouse. NSAIDs inhibit cyclooxygenase (COX) isozymes, which are responsible for the committed step in prostaglandin biosynthesis, and this has been considered the primary mechanism by which NSAIDs exert their antitumorigenic effects. However, mounting evidence suggests the existence of COX-independent mechanisms. In the present study, we attempted to clarify this issue by treating Apc(Min/+) mice bearing established tumors with NSAIDs (piroxicam and sulindac, 0.5 and 0.6 mg/mouse/day, respectively) for 6 days and concomitantly bypassing COX inhibition by treatment with the E prostaglandin (EP) receptor agonists 16,16-dimethyl-prostaglandin E(2) (PGE(2)) and 17-phenyl-trinor-PGE(2) (10 microg each, three times daily) administered via gavage and/or i.p. routes. Treatment with piroxicam and sulindac resulted in 95% and 52% fewer tumors, respectively, and a higher ratio of apoptosis:mitosis in tumors from sulindac-treated mice as compared with controls. These effects were attenuated by concomitant EP receptor agonist treatment, suggesting PGE(2) is important in the maintenance of tumor integrity. Immunological sequestration of PGE(2) with an anti-PGE(2) monoclonal antibody likewise resulted in 33% fewer tumors in Apc(Min/+) mice relative to untreated controls, additionally substantiating a role for PGE(2) in tumorigenesis. The EP receptor subtype EP1 mediates the effects of PGE(2) by increasing intracellular calcium levels ([Ca(2+)](i)), whereas antagonism of EP1 has been shown to attenuate tumorigenesis in Apc(Min/+) mice. We demonstrate that [Ca(2+)](i) is significantly elevated in tumors of Apc(Min/+) mice relative to the adjacent normal-appearing mucosa. Furthermore, treatment with piroxicam results in significantly lower [Ca(2+)](i) in tumors, and this effect is attenuated by concomitant treatment with the EP1/EP3 receptor agonist 17-phenyl-trinor-PGE(2). Overall, our results suggest that NSAIDs exert their antitumorigenic effects, in part, via interference with PGE(2) biosynthesis, and these effects may be mediated through changes in intracellular calcium levels.     

Suppression of UV carcinogenesis by difluoromethylornithine in nucleotide excision repair-deficient Xpa knockout mice

Xeroderma pigmentosum (XP) patients are deficient in nucleotide excision repair (NER) because of mutations in one of the genes coding for NER enzymes. This results predominantly in high frequency of UV-induced skin tumors at an early age; the most severe phenotype is found in patients of complementation group A (XPA). However, in a subset of these XPA patients no skin tumors appear, even at advanced age. Fibroblasts of this subset of patients are not capable of raising UV-induced enhanced reactivation (ER) of viruses and up-regulation of ornithine decarboxylase (ODC). We hypothesized that prevention of ODC induction would protect NER-deficient patients from cancer. To simulate the situation in XPA patients, we used a hairless Xpa knockout mouse model and down-regulated the ODC activity by difluoromethylornithine (DFMO) administered in the drinking water. The DFMO treatment significantly suppressed UV-induced carcinogenesis. In a crossover study, we additionally found that discontinuation of the DFMO treatment resulted in a rapid appearance of skin tumors, up to levels found in mice not treated with DFMO. Late-stage DFMO treatment significantly reduced the number of carcinomas by a factor of 2-3, and it appeared to select for carcinomas with high ODC activity. These results indicate that DFMO suppresses the outgrowth but not the initiation of UV-induced tumors. The DFMO treatment reduced the tumor load but did not offer the Xpa knockout mice full protection against UV carcinogenesis.