Intestinal-specific PPARgamma deficiency enhances tumorigenesis in ApcMin/+ mice

Dual function of activated PPARγ by ligands on tumor growth and immunotherapy

As one of the peroxisome-proliferator-activated receptors (PPARs) members, PPARγ is a ligand binding and activated nuclear hormone receptor, which is an important regulator in metabolism, proliferation, tumor progression, and immune response. Increased evidence suggests that activation of PPARγ in response to ligands inhibits multiple types of cancer proliferation, metastasis, and tumor growth and induces cell apoptosis including breast cancer, colon cancer, lung cancer, and bladder cancer. Conversely, some reports suggest that activation of PPARγ is associated with tumor growth. In addition to regulating tumor progression, PPARγ could promote or inhibit tumor immunotherapy by affecting macrophage differentiation or T cell activity. These controversial findings may be derived from cancer cell types, conditions, and ligands, since some ligands are independent of PPARγ activity. Therefore, this review discussed the dual role of PPARγ on tumor progression and immunotherapy.

Exosome and lipid metabolism-related genes in pancreatic adenocarcinoma: a prognosis analysis

OBJECTIVE

The purpose of the study was to investigate the role of exosome and lipid metabolism-related genes (EALMRGs) mRNA levels in the diagnosis and prognosis of Pancreatic Adenocarcinoma (PAAD).

METHODS

The mRNA expression pattern of PAAD and pan-cancers with prognostic data were obtained from The Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GEO) database. EALMRGs were acquired from GeneCards and MSigDB database after merging and deduplication. Prognostic EALMRGs were screened through univariate COX regression analysis, and a prognostic model was constructed based on these genes by least absolute shrinkage and selection operator (LASSO) regression. The prognostic value of EALMRGs was then validated in pan-cancer data. The time characteristics ROC curve analysis was performed to evaluate the effectiveness of the prognostic genes.

RESULTS

We identified 5 hub genes (ABCB1, CAP1, EGFR, PPARG, SNCA) according to high and low-risk groups of prognoses. The risk formula was verified in three other cohort of pancreatic cancer patients and was explored in pan-cancer data. Additionally, T cell and dendritic cell infiltration was significantly increased in low-risk group. The expression of the 5 hub genes was also identified in single-cell sequencing data of pancreatic cancer with pivotal pathways. Additionally, functional enrichment analysis based on pancreatic cancer data in pancreatic cancer showed that protein serine/threonine kinase activity, focal adhesion, actin binding, cell-substrate junction, organic acid transport, and regulation of transporter activity were significant related to the expression of genes in EALMRGs.

CONCLUSIONS

Our risk formula shows potential prognostic value in multiple cancers and manifest pivotal alterations in immune infiltration and biological pathway in pancreatic cancer.

PPARγ agonist inhibits c-Myc-mediated colorectal cancer tumor immune escape

As a master transcription factor, c-Myc plays an important role in promoting tumor immune escape. In addition, PPARγ (peroxisome proliferator-activated receptor γ) regulates cell metabolism, inflammation, and tumor progression, while the effect of PPARγ on c-Myc-mediated tumor immune escape is still unclear. Here we found that cells treated with PPARγ agonist pioglitazone (PIOG) reduced c-Myc protein expression in a PPARγ-dependent manner. qPCR analysis showed that PIOG had no significant effect on c-Myc gene levels. Further analysis showed that PIOG decreased c-Myc protein half-life. Moreover, PIOG increased the binding of c-Myc to PPARγ, and induced c-Myc ubiquitination and degradation. Importantly, c-Myc increased PD-L1 and CD47 immune checkpoint protein expression and promoted tumor immune escape, while PIOG inhibited this event. These findings suggest that PPARγ agonist inhibited c-Myc-mediated tumor immune escape by inducing its ubiquitination and degradation.

PPARγ agonist pioglitazone enhances colorectal cancer immunotherapy by inducing PD-L1 autophagic degradation

Blockade of PD-1/PD-L1 immune checkpoint could be an effective antitumor strategy for multiple types of cancer, but it is low response rate for colorectal cancer patients with unclear mechanism. Here we found that PPARγ agonist pioglitazone could reduce PD-L1 protein levels without effect on its gene expression. Further analysis showed that pioglitazone induced PD-L1 autophagic degradation in a PPARγ-dependent manner. Pioglitazone promoted PD-L1 translocation to lysosome by immunofluorescence analysis, which was associated with the increased binding of PPARγ to PD-L1. Moreover the combined pioglitazone with PD-1 antibody enhanced colorectal tumor immunotherapy, which was involved in reduced PD-L1 levels and increased CD8⁺ T cells. These findings suggest that PPARγ agonist could induce PD-L1 autophagic degradation resulting in increased colorectal tumor immunotherapy.

Sex differences feed into nuclear receptor signaling along the digestive tract

Sex differences in physiology are noted in clinical and animal studies. However, mechanisms underlying these observed differences between males and females remain elusive. Nuclear receptors control a wide range of physiological pathways and are expressed in the gastrointestinal tract, including the mouth, stomach, liver and intestine. We investigated the literature pertaining to ER, AR, FXR, and PPAR regulation and highlight the sex differences in nutrient metabolism along the digestive system. We chose these nuclear receptors based on their metabolic functions, and hormonal actions. Intriguingly, we noted an overlap in target genes of ER and FXR that modulate mucosal integrity and GLP-1 secretion, whereas overlap in target genes of PPARα with ER and AR modulate lipid metabolism. Sex differences were seen not only in the basal expression of nuclear receptors, but also in activation as their endogenous ligand concentrations fluctuate depending on nutrient availability. Finally, in this review, we speculate that interactions between the nuclear receptors may influence overall metabolic decisions in the gastrointestinal tract in a sex-specific manner.

Inflammatory Bowel Disease: New Insights into the Interplay between Environmental Factors and PPARγ

The pathophysiological processes of inflammatory bowel diseases (IBDs), i.e., Crohn's disease (CD) and ulcerative colitis (UC), are still not completely understood. The exact etiology remains unknown, but it is well established that the pathogenesis of the inflammatory lesions is due to a dysregulation of the gut immune system resulting in over-production of pro-inflammatory cytokines. Increasing evidence underlines the involvement of both environmental and genetic factors. Regarding the environment, the microbiota seems to play a crucial role. Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors that exert pleiotropic effects on glucose homeostasis, lipid metabolism, inflammatory/immune processes, cell proliferation, and fibrosis. Furthermore, PPARs modulate interactions with several environmental factors, including microbiota. A significantly impaired PPARγ expression was observed in UC patients' colonic epithelial cells, suggesting that the disruption of PPARγ signaling may represent a critical step of the IBD pathogenesis. This paper will focus on the role of PPARγ in the interaction between environmental factors and IBD, and it will analyze the most suitable in vitro and in vivo models available to better study these relationships.

PPARG expression in colorectal cancer and its association with staging and clinical evolution

Purpose

To evaluate the gene expression of peroxisome proliferator activated receptors gamma (PPARG) in colorectal tumors and to correlate this data with clinical variables of the patients.

Methods

We analyzed the gene expression of PPARG in 50 samples of colorectal tumors using real-time reverse transcription polymerase chain reaction, and 20 adjacent normal tissue samples as control. The results of these quantifications were correlated with the respective patients’ medical records’ clinical information.

Results

PPARG expression was not different in the tumor tissue compared to the control tissue. Patients older than 60 years, histological type with mucinous differentiation, more advanced staging at the time of diagnosis, and patients who evolved with recurrence of the disease or death did not present higher PPARG expression.

Conclusion

Expression of PPARGD was not associated with worse prognosis.

Ecdysone steroid hormone remote controls intestinal stem cell fate decisions via the PPARγ-homolog Eip75B in Drosophila

Developmental studies revealed fundamental principles on how organ size and function is achieved, but less is known about organ adaptation to new physiological demands. In fruit flies, juvenile hormone (JH) induces intestinal stem cell (ISC) driven absorptive epithelial expansion balancing energy uptake with increased energy demands of pregnancy. Here, we show 20-Hydroxy-Ecdysone (20HE)-signaling controlling organ homeostasis with physiological and pathological implications. Upon mating, 20HE titer in ovaries and hemolymph are increased and act on nearby midgut progenitors inducing Ecdysone-induced-protein-75B (Eip75B). Strikingly, the PPARγ-homologue Eip75B drives ISC daughter cells towards absorptive enterocyte lineage ensuring epithelial growth. To our knowledge, this is the first time a systemic hormone is shown to direct local stem cell fate decisions. Given the protective, but mechanistically unclear role of steroid hormones in female colorectal cancer patients, our findings suggest a tumor-suppressive role for steroidal signaling by promoting postmitotic fate when local signaling is deteriorated.

A semi-automated technique for adenoma quantification in the ApcMin mouse using FeatureCounter

Colorectal cancer is a major contributor to death and disease worldwide. The Apc^Min mouse is a widely used model of intestinal neoplasia, as it carries a mutation also found in human colorectal cancers. However, the method most commonly used to quantify tumour burden in these mice is manual adenoma counting, which is time consuming and poorly suited to standardization across different laboratories. We describe a method to produce suitable photographs of the small intestine of Apc^Min mice, process them with an ImageJ macro, FeatureCounter, which automatically locates image features potentially corresponding to adenomas, and a machine learning pipeline to identify and quantify them. Compared to a manual method, the specificity (or True Negative Rate, TNR) and sensitivity (or True Positive Rate, TPR) of this method in detecting adenomas are similarly high at about 80% and 87%, respectively. Importantly, total adenoma area measures derived from the automatically-called tumours were just as capable of distinguishing high-burden from low-burden mice as those established manually. Overall, our strategy is quicker, helps control experimenter bias, and yields a greater wealth of information about each tumour, thus providing a convenient route to getting consistent and reliable results from a study.

Dual gene deficient models of ApcMin/+ mouse in assessing molecular mechanisms of intestinal carcinogenesis

The Apc^Min/+ mouse, carrying an inactivated allele of the adenomatous polyposis coli (Apc) gene, is a widely used animal model of human colorectal tumorigenesis. While crossed with other gene knockout or knock-in mice, these mice possess advantages in investigation of human intestinal tumorigenesis. Intestinal tumor pathogenesis involves multiple gene alterations; thus, various double gene deficiency models could provide novel insights into molecular mechanisms of tumor biology, as well as gene-gene interactions involved in intestinal tumor development and assessment of novel strategies for preventing and treating intestinal cancer. This review discusses approximately 100 double gene deficient mice and their associated intestinal tumor development and progression phenotypes. The dual gene knockouts based on the Apc mutation background consist of inflammation and immune-related, cell cycle-related, Wnt/β-catenin signaling-related, tumor growth factor (TGF)-signaling-related, drug metabolism-related, and transcription factor genes, as well as some oncogenes and tumor suppressors. Future studies should focus on conditional or inducible dual or multiple mouse gene knockout models to investigate the molecular mechanisms underlying intestinal tumor development, as well as potential drug targets.

Thyroid-Specific PPARγ Deletion Is Benign in the Mouse

Peroxisome proliferator-activated receptor γ (PPARγ) is widely expressed at low levels and regulates many physiological processes. In mice and humans, there is evidence that PPARγ can function as a tumor suppressor. A PAX8-PPARγ fusion protein (PPFP) is oncogenic in a subset of thyroid cancers, suggesting that inhibition of endogenous PPARγ function by the fusion protein could contribute to thyroid oncogenesis. However, the function of PPARγ within thyrocytes has never been directly tested. Therefore, we have created a thyroid-specific genetic knockout of murine Pparg and have studied thyroid biology in these mice. Thyroid size and histology, the expression of thyroid-specific genes, and serum T4 levels all are unaffected by loss of thyroidal PPARγ expression. PPFP thyroid cancers have increased activation of AKT, and mice with thyroid-specific expression of PPFP combined with thyroid-specific loss of PTEN (a negative regulator of AKT) develop thyroid cancer. Therefore we created mice with combined thyroid-specific deletions of Pparg and Pten to test if there is oncogenic synergy between these deletions. Pten deletion alone results in benign thyroid hyperplasia, and this is unchanged when combined with deletion of Pparg. We conclude that, at least in the contexts studied, thyrocyte PPARγ does not play a significant role in the development or function of the thyroid and does not function as a tumor suppressor.

Preventive Effects of Heat-Killed Enterococcus faecalis Strain EC-12 on Mouse Intestinal Tumor Development

Establishing effective methods for preventing colorectal cancer by so-called “functional foods” is important because the global burden of colorectal cancer is increasing. Enterococcus faecalis strain EC-12 (EC-12), which belongs to the family of lactic acid bacteria, has been shown to exert pleiotropic effects, such as anti-allergy and anti-infectious effects, on mammalian cells. In the present study, we aimed to evaluate the preventive effects of heat-killed EC-12 on intestinal carcinogenesis. We fed 5-week-old male and female Apc mutant Min mice diets containing 50 or 100 ppm heat-killed EC-12 for 8 weeks. In the 50 ppm treated group, there was 4.3% decrease in the number of polyps in males vs. 30.9% in females, and significant reduction was only achieved in the proximal small intestine of female mice. A similar reduction was observed in the 100 ppm treated group. Moreover, heat-killed EC-12 tended to reduce the levels of c-Myc and cyclin D1 mRNA expression in intestinal polyps. Next, we confirmed that heat-killed EC-12 suppressed the transcriptional activity of the T-cell factor/lymphoid enhancer factor, a transcriptional factor involved in cyclin D1 mRNA expression in intestinal polyps. Our results suggest that heat-killed EC-12 very weakly suppresses intestinal polyp development in Min mice, in part by attenuating β-catenin signaling, and this implies that heat-killed EC-12 could be used as a “functional food”.

Thermodynamics in cancers: opposing interactions between PPAR gamma and the canonical WNT/beta-catenin pathway

Cancer cells are the site of numerous metabolic and thermodynamic abnormalities. We focus this review on the interactions between the canonical WNT/beta-catenin pathway and peroxisome proliferator-activated receptor gamma (PPAR gamma) in cancers and their implications from an energetic and metabolic point of view. In numerous tissues, PPAR gamma activation induces inhibition of beta-catenin pathway, while the activation of the canonical WNT/beta-catenin pathway inactivates PPAR gamma. In most cancers but not all, PPAR gamma is downregulated while the WNT/beta-catenin pathway is upregulated. In cancer cells, upregulation of the WNT/beta-catenin signaling induces dramatic changes in key metabolic enzymes that modify their thermodynamic behavior. This leads to activation of pyruvate dehydrogenase kinase1 (PDK-1) and monocarboxylate lactate transporter. Consequently, phosphorylation of PDK-1 inhibits the pyruvate dehydrogenase complex (PDH). Thus, a large part of pyruvate cannot be converted into acetyl-coenzyme A (acetyl-CoA) in mitochondria and only a part of acetyl-CoA can enter the tricarboxylic acid cycle. This leads to aerobic glycolysis in spite of the availability of oxygen. This phenomenon is referred to as the Warburg effect. Cytoplasmic pyruvate is converted into lactate. The WNT/beta-catenin pathway induces the transcription of genes involved in cell proliferation, i.e., MYC and CYCLIN D1. This ultimately promotes the nucleotide, protein and lipid synthesis necessary for cell growth and multiplication. In cancer, activation of the PI3K-AKT pathway induces an increase of the aerobic glycolysis. Moreover, prostaglandin E2 by activating the canonical WNT pathway plays also a role in cancer. In addition in many cancer cells, PPAR gamma is downregulated. Moreover, PPAR gamma contributes to regulate some key circadian genes. In cancers, abnormalities in the regulation of circadian rhythms (CRs) are observed. CRs are dissipative structures which play a key-role in far-from-equilibrium thermodynamics. In cancers, metabolism, thermodynamics and CRs are intimately interrelated.

Peroxisome Proliferator-Activated Receptor Modulation during Metabolic Diseases and Cancers: Master and Minions

The prevalence of obesity and metabolic diseases (such as type 2 diabetes mellitus, dyslipidaemia, and cardiovascular diseases) has increased in the last decade, in both industrialized and developing countries. This also coincided with our observation of a similar increase in the prevalence of cancers. The aetiology of these diseases is very complex and involves genetic, nutritional, and environmental factors. Much evidence indicates the central role undertaken by peroxisome proliferator-activated receptors (PPARs) in the development of these disorders. Due to the fact that their ligands could become crucial in future target-therapies, PPARs have therefore become the focal point of much research. Based on this evidence, this narrative review was written with the purpose of outlining the effects of PPARs, their actions, and their prospective uses in metabolic diseases and cancers.

Commonalities in the Association between PPARG and Vitamin D Related with Obesity and Carcinogenesis

The PPAR nuclear receptor family has acquired great relevance in the last decade, which is formed by three different isoforms (PPARα, PPARβ/δ, and PPAR ϒ). Those nuclear receptors are members of the steroid receptor superfamily which take part in essential metabolic and life-sustaining actions. Specifically, PPARG has been implicated in the regulation of processes concerning metabolism, inflammation, atherosclerosis, cell differentiation, and proliferation. Thus, a considerable amount of literature has emerged in the last ten years linking PPARG signalling with metabolic conditions such as obesity and diabetes, cardiovascular disease, and, more recently, cancer. This review paper, at crossroads of basic sciences, preclinical, and clinical data, intends to analyse the last research concerning PPARG signalling in obesity and cancer. Afterwards, possible links between four interrelated actors will be established: PPARG, the vitamin D/VDR system, obesity, and cancer, opening up the door to further investigation and new hypothesis in this fascinating area of research.

Naringin, a natural dietary compound, prevents intestinal tumorigenesis in Apc (Min/+) mouse model

PURPOSE

Naringin is a natural dietary flavonoid compound. We aimed to evaluate the effects of naringin on intestinal tumorigenesis in the adenomatous polyposis coli multiple intestinal neoplasia (Apc (Min/+)) mouse model.

METHODS

Apc (Min/+) mice were given either naringin (150 mg/kg) or vehicle by p.o. gavage daily for 12 consecutive weeks. Mice were killed with ether, and blood samples were collected to assess the concentrations of IL-6 and PGE2. Total intestines were removed, and the number of polyps was examined. Tissue samples of intestinal polyps were subjected to the assays of histopathology, immunohistochemical analysis and Western blotting analysis.

RESULTS

Apc (Min/+) mice fed with naringin developed less and smaller polyps in total intestines. Naringin prevented intestinal tumorigenesis without adverse effects. Histopathologic analysis revealed the reduction of dysplastic cells and dysplasia in the adenomatous polyps. The treatments' effects might arise from its anti-proliferation, induction of apoptosis and modulation of GSK-3β and APC/β-catenin signaling pathways. Naringin also exerted its effects on tumorigenesis through anti-chronic inflammation.

CONCLUSION

Naringin prevented intestinal tumorigenesis likely through a collection of activities including anti-proliferation, induction of apoptosis, modulation of GSK-3β and APC/β-catenin pathways and anti-inflammation. Naringin is a potential chemopreventive agent for reducing the risk of colonic cancers.

Altered Interactions between the Gut Microbiome and Colonic Mucosa Precede Polyposis in APCMin/+ Mice

Mutation of the adenomatous polyposis coli (APC gene), an early event in the adenoma-carcinoma sequence, is present in 70-80% of sporadic human colorectal adenomas and carcinomas. To test the hypothesis that mutation of the APC gene alters microbial interactions with host intestinal mucosa prior to the development of polyposis, culture-independent methods (targeted qPCR assays and Illumina sequencing of the 16S rRNA gene V1V2 hypervariable region) were used to compare the intestinal microbial composition of 30 six-week old C57BL/6 APCMin/+ and 30 congenic wild type (WT) mice. The results demonstrate that similar to 12-14 week old APCMin/+ mice with intestinal neoplasia, 6 week old APCMin/+ mice with no detectable neoplasia, exhibit an increased relative abundance of Bacteroidetes spp in the colon. Parallel mouse RNA sequence analysis, conducted on a subset of proximal colonic RNA samples (6 APCMin/+, 6 WT) revealed 130 differentially expressed genes (DEGs, fold change ≥ 2, FDR <0.05). Hierarchical clustering of the DEGs was carried out by using 1-r dissimilarity measurement, where r stands for the Pearson correlation, and Ward minimum variance linkage, in order to reduce the number of input variables. When the cluster centroids (medians) were included along with APC genotype as input variables in a negative binomial (NB) regression model, four of seven mouse gene clusters, in addition to APC genotype, were significantly associated with the increased relative abundance of Bacteroidetes spp. Three of the four clusters include several downregulated genes encoding immunoglobulin variable regions and non-protein coding RNAs. These results support the concept that mutation of the APC gene alters colonic-microbial interactions prior to polyposis. It remains to be determined whether interventions directed at ameliorating dysbiosis in APCMin/+mice, such as through probiotics, prebiotics or antibiotics, could reduce tumor formation.

CARD9 Promotes Sex-Biased Colon Tumors in the APCmin Mouse Model

Caspase recuitment domain-containing protein 9 (CARD9) functions in different inflammation pathways to elicit responses to microbial signals and is known to affect intestinal inflammation. Examining the APC(min) mouse model of intestinal tumorigenesis and using stringently controlled, sex- and age-matched pairs of CARD9-competent and CARD9-deficient mice, we have found that CARD9 has a restricted but strong effect on tumorigenesis in the large intestine. We have found that CARD9 reduces viability specifically in males and promotes tumorigenesis specifically in the large intestines of these male mice. To our knowledge, this is the first gene ablation in APC(min) mice that solely affects colon tumors in male subjects and, as such, may have significant clinical implications. Additional data suggest correlative disruption of plasma cytokine expression and immune infiltration of the tumors. We speculate that known sex-specific differences in human colorectal cancer may involve inflammation, particularly CARD9-dependent inflammation.

Protective Effects of Turbinaria ornata and Padina pavonia against Azoxymethane-Induced Colon Carcinogenesis through Modulation of PPAR Gamma, NF-κB and Oxidative Stress

The aim of this study was to investigate the antiproliferative and protective effects of the brown seaweeds, Turbinaria ornata and Padina pavonia, against azoxymethane (AOM)-induced colon carcinogenesis in mice. Both algal extracts showed anti-proliferative effects on the human carcinoma cell line HCT-116 in vitro, with T. ornata demonstrating a more potent effect. Male albino Swiss mice received intraperitoneal injections of AOM (10 mg/kg) once a week for two consecutive weeks and 100 mg/kg of either T. ornata or P. pavonia extracts. AOM-induced mice exhibited alterations in the histological structure of the colon, elevated lipid peroxidation and nitric oxide, declined glutathione content and reduced activity of superoxide dismutase and glutathione peroxidase. In addition, AOM induced downregulation of peroxisome proliferator activated receptor gamma (PPARγ) and p53 mRNA expression, with concomitant upregulation of nuclear factor-kappa B (NF-κB) in colon tissue. Administration of either algal extract markedly alleviated the recorded alterations. In conclusion, the current study suggests that T. ornata and P. pavonia, through their antioxidant and anti-inflammatory effects, are able to attenuate colon inflammation by downregulating NF-κB expression. Furthermore, the protective effects of both algae against AOM-initiated carcinogenesis were attributed, at least in part, to their ability to upregulate colonic PPARγ and p53 expression.

Chemotherapy and chemoprevention by thiazolidinediones

Thiazolidinediones (TZDs) are synthetic ligands of Peroxisome-Proliferator-Activated Receptor gamma (PPARγ). Troglitazone, rosiglitazone, and pioglitazone have been approved for treatment of diabetes mellitus type II. All three compounds, together with the first TZD ciglitazone, also showed an antitumor effect in preclinical studies and a beneficial effect in some clinical trials. This review summarizes hypotheses on the role of PPARγ in tumors, on cellular targets of TZDs, antitumor effects of monotherapy and of TZDs in combination with other compounds, with a focus on their role in the treatment of differentiated thyroid carcinoma. The results of chemopreventive effects of TZDs are also considered. Existing data suggest that the action of TZDs is highly complex and that actions do not correlate with cellular PPARγ expression status. Effects are cell-, species-, and compound-specific and concentration-dependent. Data from human trials suggest the efficacy of TZDs as monotherapy in prostate cancer and glioma and as chemopreventive agent in colon, lung, and breast cancer. TZDs in combination with other therapies might increase antitumor effects in thyroid cancer, soft tissue sarcoma, and melanoma.

Development of Small Molecules Targeting the Wnt Signaling Pathway in Cancer Stem Cells for the Treatment of Colorectal Cancer

Colorectal cancer (CRC) was ranked third in morbidity and mortality in the United States in 2013. Although substantial progress has been made in surgical techniques and postoperative chemotherapy in recent years, the prognosis for colon cancer is still not satisfactory, mainly because of cancer recurrence and metastasis. The latest studies have shown that cancer stem cells (CSCs) play important roles in cancer recurrence and metastasis. Drugs that target CSCs might therefore have great therapeutic potential in prevention of cancer recurrence and metastasis. The wingless-int (Wnt) signaling pathway in CSCs has been suggested to play crucial roles in colorectal carcinogenesis, and has become a popular target for anti-CRC therapy. Dysregulation of the Wnt signaling pathway, mostly by inactivating mutations of the adenomatous polyposis coli tumor suppressor or oncogenic mutations of β-catenin, has been implicated as a key factor in colorectal tumorigenesis. Abnormal increases of β-catenin levels represents a common pathway in Wnt signaling activation and is also observed in other human malignancies. These findings highlight the importance of developing small-molecule drugs that target the Wnt pathway. Herein we provide an overview on the current development of small molecules that target the Wnt pathway in colorectal CSCs and discuss future research directions.

Chemopreventive drugs: Mechanisms via inhibition of cancer stem cells in colorectal cancer

Recent epidemiological studies, basic research and clinical trials on colorectal cancer (CRC) prevention have helped identify candidates for effective chemopreventive drugs. However, because of the conflicting results of clinical trials or side effects, the effective use of chemopreventive drugs has not been generalized, except for patients with a high-risk for developing hereditary CRC. Advances in genetic and molecular technologies have highlighted the greater complexity of carcinogenesis, especially the heterogeneity of tumors. We need to target cells and processes that are critical to carcinogenesis for chemoprevention and treatment of advanced cancer. Recent research has shown that intestinal stem cells may serve an important role in tumor initiation and formation of cancer stem cells. Moreover, studies have shown that the tumor microenvironment may play additional roles in dedifferentiation, to enable tumor cells to take on stem cell features and promote the formation of tumorigenic stem cells. Therefore, early tumorigenic changes of stem cells and signals for dedifferentiation may be good targets for chemoprevention. In this review, I focus on cancer stem cells in colorectal carcinogenesis and the effect of major chemopreventive drugs on stem cell-related pathways.

Intestinal tumor suppression in ApcMin/+ mice by prostaglandin D2 receptor PTGDR

Our earlier work showed that knockout of hematopoietic prostaglandin D synthase (HPGDS, an enzyme that produces prostaglandin D₂) caused more adenomas in Apc^Min/+ mice. Conversely, highly expressed transgenic HPGDS allowed fewer tumors. Prostaglandin D₂ (PGD₂) binds to the prostaglandin D₂ receptor known as PTGDR (or DP1). PGD₂ metabolites bind to peroxisome proliferator-activated receptor γ (PPARG). We hypothesized that Ptgdr or Pparg knockouts may raise numbers of tumors, if these receptors take part in tumor suppression by PGD₂. To assess, we produced Apc^Min/+ mice with and without Ptgdr knockouts (147 mice). In separate experiments, we produced Apc^Min/+ mice expressing transgenic lipocalin-type prostaglandin D synthase (PTGDS), with and without heterozygous Pparg knockouts (104 mice). Homozygous Ptgdr knockouts raised total numbers of tumors by 30–40% at 6 and 14 weeks. Colon tumors were not affected. Heterozygous Pparg knockouts alone did not affect tumor numbers in Apc^Min/+ mice. As mentioned above, our Pparg knockout assessment also included mice with highly expressed PTGDS transgenes. Apc^Min/+ mice with transgenic PTGDS had fewer large adenomas (63% of control) and lower levels of v-myc avian myelocytomatosis viral oncogene homolog (MYC) mRNA in the colon. Heterozygous Pparg knockouts appeared to blunt the tumor-suppressing effect of transgenic PTGDS. However, tumor suppression by PGD₂ was more clearly mediated by receptor PTGDR in our experiments. The suppression mechanism did not appear to involve changes in microvessel density or slower proliferation of tumor cells. The data support a role for PGD₂ signals acting through PTGDR in suppression of intestinal tumors.

Targeted Cox2 gene deletion in intestinal epithelial cells decreases tumorigenesis in female, but not male, ApcMin/+ mice

Mice heterozygous for mutations in the adenomatous polyposis coli gene (Apc(+/-) mice) develop intestinal neoplasia. Apc(+/-) tumor formation is thought to be dependent on cyclooxygenase 2 (COX2) expression; both pharmacologic COX2 inhibition and global Cox2 gene deletion reduce the number of intestinal tumors in Apc(+/-) mice. COX2 expression is reported in epithelial cells, fibroblasts, macrophages and endothelial cells of Apc(+/-) mouse polyps. However, the cell type(s) in which COX2 expression is required for Apc(+/-) tumor induction is not known. To address this question, we developed Apc(Min/+) mice in which the Cox2 gene is specifically deleted either in intestinal epithelial cells or in myeloid cells. There is no significant difference in intestinal polyp number between Apc(Min/+) mice with a targeted Cox2 gene deletion in myeloid cells and their control littermate Apc(Min/+) mice. In contrast, Apc(Min/+) mice with a targeted Cox2 deletion in intestinal epithelial cells have reduced intestinal tumorigenesis when compared to their littermate control Apc(Min/+) mice. However, two gender-specific effects are notable. First, female Apc(Min/+) mice developed more intestinal tumors than male Apc(Min/+) mice. Second, targeted intestinal epithelial cell Cox2 deletion decreased tumorigenesis in female, but not in male, Apc(Min/+) mice. Considered in the light of pharmacologic studies and studies with global Cox2 gene knockout mice, our data suggest that (i) intrinsic COX2 expression in intestinal epithelial cells plays a gender-specific role in tumor development in Apc(Min/+) mice, and (ii) COX2 expression in cell type(s) other than intestinal epithelial cells also modulates intestinal tumorigenesis in Apc(Min/+) mice, by a paracrine process.

Chemoprevention of colon and small intestinal tumorigenesis in APC(min/+) mice by SHetA2 (NSC721689) without toxicity

The occurrence of intestinal polyps in people at high risk for developing colorectal cancer provides an opportunity to test the efficacy of chemoprevention agents. In this situation of treating otherwise healthy people, the potential for toxicity must be minimal. The small-molecule flexible heteroarotinoid (Flex-Het), called SHetA2, has chemoprevention activity in organotypic cultures in vitro and lack of toxicity at doses capable of inhibiting xenograft tumor growth in vivo. The objective of this study was to evaluate SHetA2 chemoprevention activity and toxicity in the APC(min/+) murine model. Oral administration of SHetA2 at 30 and 60 mg/kg five days per week for 12 weeks significantly reduced development of intestinal polyps by 40% to 60% depending on the dose and sex of the treatment group. Immunohistochemical and Western blot analysis of polyps showed reduced levels of cyclin D1 and proliferating cell nuclear antigen in both SHetA2 treatment groups. Western blot analysis also showed SHetA2 induction of E-cadherin, Bax, and caspase-3 cleavage along with reduction in Bcl-2, COX-2, and VEGF, consistent with SHetA2 regulation of apoptosis, inflammation, and angiogenesis. Neither dose caused weight loss nor gross toxicity in APC(min/+) or wild-type littermates. Magnetic resonance imaging (MRI) of cardiac function showed no evidence of SHetA2 toxicity. SHetA2 did not alter left ventricular wall thickness. In summary, SHetA2 exerts chemoprevention activity without overt or cardiac toxicity in the APC(min/+) model. SHetA2 modulation of biomarkers in colon polyps identifies potential pharmacodynamic endpoints for SHetA2 clinical trials.

Deficiency of caveolin-1 in Apc(min/+) mice promotes colorectal tumorigenesis

Caveolin-1 (Cav1), a scaffold protein of membrane caveolae and coactivator of peroxisome proliferator-activated receptor gamma (PPARg), inhibits oncogenic signaling through Ras and wingless. However, the in vivo role of Cav1 in colorectal cancer (CRC) remained unknown. To test whether loss of Cav1 accelerates tumorigenesis, we generated a novel mouse model of CRC by crossing C57BL/6 Apc(min/+) with B6129 Cav1 knockout (Cav1-/-) mice. Apc(min/+) Cav1-/- mice developed large, microinvasive and vascularized intraepithelial adenocarcinomas in the distal colon and rectum with higher incidence than Apc(min/+) Cav1+/- and Apc(min/+) Cav1+/+ littermates. Intratumoral gene signatures related to Ras and wingless signaling were elevated, nuclear localization of PPARg protein and expression of PPARg-target genes were reduced independently of Cav1. The PPARg-agonist rosiglitazone prevented tumor formation in mice irrespectively of the Cav1 status and upregulated expression of the Ras-inhibitory protein docking protein-1. Thus, codeficiency of Cav1 and adenomatous polyposis coli facilitated formation of CRC, and activation of PPARg may offer novel strategies for treatment of CRC.

PPARG Epigenetic Deregulation and Its Role in Colorectal Tumorigenesis

Peroxisome proliferator-activated receptor gamma (PPARγ) plays critical roles in lipid storage, glucose metabolism, energy homeostasis, adipocyte differentiation, inflammation, and cancer. Its function in colon carcinogenesis has largely been debated; accumulating evidence, however, supports a role as tumor suppressor through modulation of crucial pathways in cell differentiation, apoptosis, and metastatic dissemination. Epigenetics adds a further layer of complexity to gene regulation in several biological processes. In cancer, the relationship with epigenetic modifications has provided important insights into the underlying molecular mechanisms. These studies have highlighted how epigenetic modifications influence PPARG gene expression in colorectal tumorigenesis. In this paper, we take a comprehensive look at the current understanding of the relationship between PPARγ and cancer development. The role that epigenetic mechanisms play is also addressed disclosing novel crosstalks between PPARG signaling and the epigenetic machinery and suggesting how this dysregulation may contribute to colon cancer development.

Rosiglitazone and AS601245 decrease cell adhesion and migration through modulation of specific gene expression in human colon cancer cells

PPARs are nuclear receptors activated by ligands. Activation of PPARγ leads to a reduction of adhesion and motility in some cancer models. PPARγ transcriptional activity can be negatively regulated by JNK-mediated phosphorylation. We postulated that the use of agents able to inhibit JNK activity could increase the effectiveness of PPARγ ligands. We analysed the effects of rosiglitazone (PPARγ ligand) and AS601245 (a selective JNK inhibitor) alone or in association on adhesion and migration of CaCo-2, HT29, and SW480 human colon cancer cells and investigated, through microarray analysis, the genes involved in these processes. Cell adhesion and migration was strongly inhibited by rosiglitazone and AS601245. Combined treatment with the two compounds resulted in a greater reduction of the adhesion and migration capacity. Affymetrix analysis in CaCo-2 cells revealed that some genes which were highly modulated by the combined treatment could be involved in these biological responses. Rosiglitazone, AS601245 and combined treatment down-regulated the expression of fibrinogen chains in all three cell lines. Moreover, rosiglitazone, alone or in association with AS601245, caused a decrease in the fibrinogen release. ARHGEF7/β-PIX gene was highly down-regulated by combined treatment, and western blot analysis revealed that β-PIX protein is down-modulated in CaCo-2, HT29 and SW480 cells, also. Transfection of cells with β-PIX gene completely abrogated the inhibitory effect on cell migration, determined by rosiglitazone, AS601245 and combined treatment. Results demonstrated that β-PIX protein is involved in the inhibition of cell migration and sustaining the positive interaction between PPARγ ligands and anti-inflammatory agents in humans.

The role of peroxisome proliferator-activated receptors in carcinogenesis and chemoprevention

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that are involved in regulating glucose and lipid homeostasis, inflammation, proliferation and differentiation. Although all of these functions might contribute to the influence of PPARs in carcinogenesis, there is a distinct need for a review of the literature and additional experimentation to determine the potential for targeting PPARs for cancer therapy and cancer chemoprevention. As PPAR agonists include drugs that are used for the treatment of metabolic diseases, a more complete understanding of the roles of PPARs in cancer will aid in determining any increased cancer risk for patients undergoing therapy with PPAR agonists.

Runx1 is a tumor suppressor gene in the mouse gastrointestinal tract

The Runx1 transcription factor plays an important role in tissue homeostasis through its effects on stem/progenitor cell populations and differentiation. The effect of Runx1 on epithelial differentiation of the secretory cell lineage of the colon was recently demonstrated. This study aimed to examine the role of Runx1 in tumor development in epithelial cells of the gastrointestinal tract. Conditional knockout mice that lacked Runx1 expression in epithelial cells of the GI tract were generated. These mice were crossed onto the Apc(Min) background, killed and their intestinal tumor phenotypes were compared with Apc(Min) Runx1 wild-type control mice. Apc-wild-type Runx1-mutant mice were also examined for tumor development. Colons from Runx1 knockout and wild-type mice were used for genome-wide mRNA expression analyses followed by gene-specific quantitative RT-PCR of whole colon and colon epithelium to identify Runx1 target genes. Runx1 deficiency in intestinal epithelial cells significantly enhanced tumorigenesis in Apc(Min) mice. Notably, epithelial Runx1 deficiency in Apc-wild-type mice was sufficient to cause tumor development. Absence of Runx1 was associated with global changes in the expression of genes involved in inflammation and intestinal metabolism, and with gene sets indicative of a metastatic phenotype and poor prognosis. Gene-specific analysis of Runx1-deficient colon epithelium revealed increased expression of genes linked to an expansion of the stem/progenitor cell population. These results identify Runx1 as a novel tumor suppressor gene for gastrointestinal tumors and support a role for Runx1 in maintaining the balance between the intestinal stem/progenitor cell population and epithelial differentiation of the GI tract.

PPAR gamma, bioactive lipids, and cancer progression

In this article we review the evolution of cancer research involving PPARgamma, including mechanisms, target genes, and clinical applications. For the last thirteen years, the effects of PPARgamma activity on tumor biology have been studied intensely. Most of this research has focused upon the potential for employing agonists of this nuclear receptor in cancer treatment. As a monotherapy such agonists have shown little success in clinical trials, while they have shown promise as components of combination treatments both in culture and in animal models. Other investigations have explored a possible role for PPARgamma as a tumor suppressor, and as an inducer of differentiation of cancer stem cells. Whereas early studies have yielded variable conclusions regarding the prevalence of PPARgamma mutations in cancer, the protein level of this receptor has been more recently identified as a significant prognostic marker. We predict that indicators of PPARgamma activity may also serve as predictive markers for tailoring treatments.

PPARγ Promotes Growth and Invasion of Thyroid Cancer Cells

Undifferentiated (anaplastic) thyroid cancer (ATC) is one of the most aggressive human malignancies and no effective therapy is currently available. We show here that PPARγ levels are elevated in cells derived from ATC. Depletion of PPARγ in HTh74 ATC cells resulted in decreased cell growth, cell cycle arrest and a reduction in pRb and cyclin A and B1 levels. We further showed that both flank and orthotopic thyroid tumors derived from PPARγ-depleted cells grew more slowly than PPARγ-expressing cells. When PPARγ was overexpressed in more differentiated thyroid cancer BCPAP cells which lack PPARγ, there was increased growth and raised pRb and cyclin A and B1 levels. Finally, PPARγ depletion in ATC cells decreased their invasive capacity whereas overexpression in PTC cells increased invasiveness. These data suggest that PPARγ may play a detrimental role in thyroid cancer and that targeting it therapeutically may lead to improved treatment of advanced thyroid cancer.

Insertional mutagenesis identifies multiple networks of cooperating genes driving intestinal tumorigenesis

The evolution of colorectal cancer suggests the involvement of many genes. To identify new drivers of intestinal cancer, we performed insertional mutagenesis using the Sleeping Beauty transposon system in mice carrying germline or somatic Apc mutations. By analyzing common insertion sites (CISs) isolated from 446 tumors, we identified many hundreds of candidate cancer drivers. Comparison to human data sets suggested that 234 CIS-targeted genes are also dysregulated in human colorectal cancers. In addition, we found 183 CIS-containing genes that are candidate Wnt targets and showed that 20 CISs-containing genes are newly discovered modifiers of canonical Wnt signaling. We also identified mutations associated with a subset of tumors containing an expanded number of Paneth cells, a hallmark of deregulated Wnt signaling, and genes associated with more severe dysplasia included those encoding members of the FGF signaling cascade. Some 70 genes had co-occurrence of CIS pairs, clustering into 38 sub-networks that may regulate tumor development.

Pathophysiological Roles of PPARgamma in Gastrointestinal Epithelial Cells

Although the highest levels of PPARγ expression in the body have been reported in the gastrointestinal epithelium, little is known about the physiological functions of that receptor in the gut. Moreover, there is considerable controversy concerning the effects of thiazolidinedione PPARγ agonists on the two major diseases of the gastrointestinal track: colorectal cancer and inflammatory bowel disease. We will undertake to review both historical and recently published data with a view toward summarizing what is presently known about the roles of PPARγ in both physiological and pathological processes in the gastrointestinal epithelium.

PPARgamma: The Portrait of a Target Ally to Cancer Chemopreventive Agents

Peroxisome proliferator-activated receptor-gamma (PPARγ), one of three ligand-activated transcription factors named PPAR, has been identified as a molecular target for cancer chemopreventive agents. PPARγ was initially understood as a regulator of adipocyte differentiation and glucose homeostasis while later on, it became evident that it is also involved in cell differentiation, apoptosis, and angiogenesis, biological processes which are deregulated in cancer. It is now established that PPARγ ligands can induce cell differentiation and yield early antineoplastic effects in several tumor types. Moreover, several bioactive natural products with cancer protecting potential are shown to operate through activation of PPARγ. Overall, PPARγ appears to be a prevalent target ally to cancer chemopreventive agents and therefore pursuing research in this area is of great relevance.

Peroxisome proliferator-activated receptors and progression of colorectal cancer

The peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily. These receptors are also ligand-dependent transcription factors responsible for the regulation of cellular events that range from glucose and lipid homeostases to cell differentiation and apoptosis. The importance of these receptors in lipid homeostasis and energy balance is well established. In addition to these metabolic and anti-inflammatory properties, emerging evidence indicates that PPARs can function as either tumor suppressors or accelerators, suggesting that these receptors are potential candidates as drug targets for cancer prevention and treatment. However, conflicting results have emerged regarding the role of PPARs on colon carcinogenesis. Therefore, further investigation is warranted prior to considering modulation of PPARs as an efficacious therapy for colorectal cancer chemoprevention and treatment.

Peroxisome proliferator-activated receptor γ and colorectal cancer

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily and ligand-activated transcription factors. PPARγ plays an important role in adipocyte differentiation, lipid storage and energy dissipation in adipose tissue, and is involved in the control of inflammatory reactions as well as in glucose metabolism through the improvement of insulin sensitivity. Growing evidence has demonstrated that activation of PPARγ has an antineoplastic effect in tumors, including colorectal cancer. High expression of PPARγ is detected in human colon cancer cell lines and adenocarcinoma. This review describes the molecular mechanisms by which PPARγ regulates tumorigenesis in colorectal cancer, and examines current clinical trials evaluating PPARγ agonists as therapeutic agents for colorectal cancer.

The Ras inhibitors caveolin-1 and docking protein 1 activate peroxisome proliferator-activated receptor γ through spatial relocalization at helix 7 of its ligand-binding domain

Peroxisome proliferator-activated receptor γ (PPARγ) is a transcription factor that promotes differentiation and cell survival in the stomach. PPARγ upregulates and interacts with caveolin-1 (Cav1), a scaffold protein of Ras/mitogen-activated protein kinases (MAPKs). The cytoplasmic-to-nuclear localization of PPARγ is altered in gastric cancer (GC) patients, suggesting a so-far-unknown role for Cav1 in spatial regulation of PPARγ signaling. We show here that loss of Cav1 accelerated proliferation of normal stomach and GC cells in vitro and in vivo. Downregulation of Cav1 increased Ras/MAPK-dependent phosphorylation of serine 84 in PPARγ and enhanced nuclear translocation and ligand-independent transcription of PPARγ target genes. In contrast, Cav1 overexpression sequestered PPARγ in the cytosol through interaction of the Cav1 scaffolding domain (CSD) with a conserved hydrophobic motif in helix 7 of PPARγ's ligand-binding domain. Cav1 cooperated with the endogenous Ras/MAPK inhibitor docking protein 1 (Dok1) to promote the ligand-dependent transcriptional activity of PPARγ and to inhibit cell proliferation. Ligand-activated PPARγ also reduced tumor growth and upregulated the Ras/MAPK inhibitors Cav1 and Dok1 in a murine model of GC. These results suggest a novel mechanism of PPARγ regulation by which Ras/MAPK inhibitors act as scaffold proteins that sequester and sensitize PPARγ to ligands, limiting proliferation of gastric epithelial cells.

Identification of Mom12 and Mom13, two novel modifier loci of Apc (Min) -mediated intestinal tumorigenesis

Colorectal cancer is a heterogeneous disease resulting from a combination of genetic and environmental factors. The C57BL/6J (B6) Apc (Min/+) mouse develops polyps throughout the gastrointestinal tract and has been a valuable model for understanding the genetic basis of intestinal tumorigenesis. Apc (Min/+) mice have been used to study known oncogenes and tumor suppressor genes on a controlled genetic background. These studies often utilize congenic knockout alleles, which can carry an unknown amount of residual donor DNA. The Apc (Min) model has also been used to identify modifer loci, known as Modifier of Min (Mom) loci, which alter Apc (Min) -mediated intestinal tumorigenesis. B6 mice carrying a knockout allele generated in WW6 embryonic stem cells were crossed to B6 Apc (Min/+) mice to determine the effect on polyp multiplicity. The newly generated colony developed significantly more intestinal polyps than Apc (Min/+) controls. Polyp multiplicity did not correlate with inheritance of the knockout allele, suggesting the presence of one or more modifier loci segregating in the colony. Genotyping of simple sequence length polymorphism (SSLP) markers revealed residual 129X1/SvJ genomic DNA within the congenic region of the parental knockout line. An analysis of polyp multiplicity data and SSLP genotyping indicated the presence of two Mom loci in the colony: 1) Mom12, a dominant modifier linked to the congenic region on chromosome 6, and 2) Mom13, which is unlinked to the congenic region and whose effect is masked by Mom12. The identification of Mom12 and Mom13 demonstrates the potential problems resulting from residual heterozygosity present in congenic lines.

A Sleeping Beauty transposon-mediated screen identifies murine susceptibility genes for adenomatous polyposis coli (Apc)-dependent intestinal tumorigenesis

It is proposed that a progressive series of mutations and epigenetic events leads to human colorectal cancer (CRC) and metastasis. Furthermore, data from resequencing of the coding regions of human CRC suggests that a relatively large number of mutations occur in individual human CRC, most at low frequency. The functional role of these low-frequency mutations in CRC, and specifically how they may cooperate with high-frequency mutations, is not well understood. One of the most common rate-limiting mutations in human CRC occurs in the adenomatous polyposis coli (APC) gene. To identify mutations that cooperate with mutant APC, we performed a forward genetic screen in mice carrying a mutant allele of Apc (Apc(Min)) using Sleeping Beauty (SB) transposon-mediated mutagenesis. Apc(Min) SB-mutagenized mice developed three times as many polyps as mice with the Apc(Min) allele alone. Analysis of transposon common insertion sites (CIS) identified the Apc locus as a major target of SB-induced mutagenesis, suggesting that SB insertions provide an efficient route to biallelic Apc inactivation. We also identified an additional 32 CIS genes/loci that may represent modifiers of the Apc(Min) phenotype. Five CIS genes tested for their role in proliferation caused a significant change in cell viability when message levels were reduced in human CRC cells. These findings demonstrate the utility of using transposon mutagenesis to identify low-frequency and cooperating cancer genes; this approach will aid in the development of combinatorial therapies targeting this deadly disease.

Therapeutic potential of peroxisome proliferator-activated receptors in chronic inflammation and colorectal cancer

Peroxisome proliferatoreactivated receptors (PPARs) are members of the nuclear hormone receptor superfamily and have been implicated in a variety of physiologic and pathologic processes, such as nutrient metabolism, energy homeostasis, inflammation, and cancer. This article highlights breakthroughs in our understanding of the potential roles of PPARs in inflammatory bowel disease and colorectal cancer. PPARs might hold the key to some of the questions that are pertinent to the pathophysiology of inflammatory diseases and colorectal cancer and could possibly serve as drug targets for new antiinflammatory therapeutic and anticancer agents.

PPARgamma Ligand as a Promising Candidate for Colorectal Cancer Chemoprevention: A Pilot Study

Activating synthetic ligands for peroxisome proliferator-activated receptor gamma (PPARγ), such as pioglitazone, are commonly used to treat persons with diabetes mellitus with improvement of insulin resistance. Several reports have clearly demonstrated that PPARγ ligands could inhibit colorectal cancer cell growth and induce apoptosis. Meanwhile, aberrant crypt foci (ACF) have come to be established as a biomarker of the risk of CRC in azoxymethane-treated mice and rats. In humans, ACF can be detected using magnifying colonoscopy. Previously, CRC and adenoma were used as a target for chemopreventive agents, but it needs a long time to evaluate, however, ACF can be a surrogate marker of CRC even for a brief period. In this clinical study, we investigated the chemopreventive effect of pioglitazone on the development of human ACF as a surrogate marker of CRC. Twenty-nine patients were divided into two groups, 20 were in the endoscopically normal control group and 9 were in the pioglitazone (15 mg/day) group, and ACF and adenoma were examined before and after 1-month treatment. The number of ACF was significantly decreased (5.8 ± 1.1 to 3.3 ± 2.3) after 1 month of pioglitazone treatment, however, there was no significant change in the number of crypts/ACF or in the number and size of adenomas. Pioglitazone may have a clinical application as a cancer-preventive drug. This investigation is just a pilot study, therefore, further clinical studies are needed to show that the PPARγ ligand may be a promising candidate as a chemopreventive agent for colorectal carcinogenesis.

[Expression of nuclear hormone receptors PPAR, LXR and RXR in the liver and lipid and glucose levels in blood in susceptible and resistant to hepatocarcinogenesis mice strains]

Earlier it was shown that male mice of the DD/He strain were highly susceptible to ortho-aminoasotoluene (OAT) induced hepatocarcinogenesis, and resistant to spontaneous liver tumor development as compared to the CC57BR/Mv strain. In the present work we have made a comparative investigation of peroxisome proliferator-activated receptor (PPAR), liver X-receptor (LXR) and retinoic X-receptor (RXR) mRNA levels in liver as well as concentrations of corticosterone, glucose, lipids and insulin in blood of male DD/He and CC57BR/Mv mice. Using the multiplex RT-PCR method it was found that PPAR-alpha, PPAR-gamma, RXR-alpha and RXR-beta mRNA content was essentially decreased in the liver of DD mice as compared to mice of the CC57BR strain. No significant interstrain differences of LXR-alpha and LXR-beta mRNA content were found. In DD micetere was more then the 3-fold decrease of blood content of corticosterone, which is involved in PPAR and RXR regulation. DD mice demonstrated a significant decrease in blood serum glucose and insulin concentrations as well as higher reactivity to insulin as compared with CC57BR mice. Elevated blood total cholesterol and cholesterol HDL level were found in DD mice whereas triglyceride content was basically the same in both mouse strains. It is known that glucocorticoids, PPAR and RXR play crucial role in transcription regulation of inflammation response. Therefore our data allow to suggest that decreased corticosterone level in blood, PPAR and RXR mRNA content in liver of the DD strain may lead to induction of inflammation by OAT exposure, resulting in a high incidence of tumorigenesis in this strain.

Inhibitory role of peroxisome proliferator-activated receptor gamma in hepatocarcinogenesis in mice and in vitro

Although peroxisome proliferator-activated receptor gamma (PPARgamma) agonist have been shown to inhibit hepatocellular carcinoma (HCC) development, the role of PPARgamma in hepatocarcinogenesis remains unclear. We investigated the therapeutic efficacy of PPARgamma against HCC. PPARgamma-deficient (PPARgamma(+/-)) and wild-type (PPARgamma(+/+)) littermates were used in a diethylnitrosamine (DEN)-induced HCC model and treated with PPARgamma agonist (rosiglitazone) or the vehicle alone for 8 months. The effects of PPARgamma on HCC cell growth and apoptosis were examined using PPARgamma-expressing adenovirus (Ad-PPARgamma). PPARgamma(+/-) mice were more susceptible to DEN-induced HCC than PPARgamma(+/+) mice (94% versus 62%, P < 0.05), and rosiglitazone significantly reduced the incidence of HCC in PPARgamma(+/+) mice (vehicle 62% versus treatment 24%, P < 0.01), but not in PPARgamma(+/-) mice, indicating that PPARgamma suppresses hepatocellular carcinogenesis. A pronounced expression of PPARgamma was observed in a HCC cell line (Hep3B) infected with Ad-PPARgamma. Such induction markedly suppressed HCC cell viability (P < 0.01). Further, Hep3B infection with Ad-PPARgamma revealed a decreased proportion of cells in S-phase (12.92% versus 11.58%, P < 0.05), with arrest at G(2)/M phase (38.2% versus 55.68%, P < 0.001), and there was concomitant phosphorylation of the key G(2)/M phase inhibitors cdc25C and cdc2. PPARgamma overexpression increased cell apoptosis (21.47% versus 35.02%, P < 0.01), mediated by both extrinsic (Fas and tumor necrosis factor-alpha) and intrinsic (caspase-9, caspase-3, caspase-7, and poly[ADP-ribose] polymerase) pathways. Moreover, PPARgamma directly induced a putative tumor suppressor gene, growth differentiation factor-15.

CONCLUSION

Loss of one PPARgamma allele is sufficient to enhance susceptibility to HCC. PPARgamma suppresses tumor cell growth through reducing cell proliferation and inducing G(2)/M phase arrest, apoptosis, and up-regulating growth differentiation factor-15. Thus, PPARgamma acts as a tumor-suppressor gene in the liver.

Peroxisome proliferator-activated receptor γ and colorectal cancer

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily and ligand-activated transcription factors. PPARγ plays an important role in adipocyte differentiation, lipid storage and energy dissipation in adipose tissue, and is involved in the control of inflammatory reactions as well as in glucose metabolism through the improvement of insulin sensitivity. Growing evidence has demonstrated that activation of PPARγ has an antineoplastic effect in tumors, including colorectal cancer. High expression of PPARγ is detected in human colon cancer cell lines and adenocarcinoma. This review describes the molecular mechanisms by which PPARγ regulates tumorigenesis in colorectal cancer, and examines current clinical trials evaluating PPARγ agonists as therapeutic agents for colorectal cancer.

PPARs in Irradiation-Induced Gastrointestinal Toxicity

The use of radiation therapy to treat cancer inevitably involves exposure of normal tissues. Although the benefits of this treatment are well established, many patients experience distressing complications due to injury to normal tissue. These side effects are related to inflammatory processes, and they decrease therapeutic benefit by increasing the overall treatment time. Emerging evidence indicates that PPARs and their ligands are important in the modulation of immune and inflammatory reactions. This paper discusses the effects of abdominal irradiation on PPARs, their role and functions in irradiation toxicity, and the possibility of using their ligands for radioprotection.

Current understanding of the role of PPARγ in gastrointestinal cancers

Numerous studies have indicated that PPARγ plays multiple roles such as in inflammation, cell cycle control, cell proliferation, apoptosis, and carcinogenesis, thus PPARγ contributes to the homeostasis. Many in vitro studies have showed that ligand-induced activation of PPARγ possess antitumor effect in many cancers including CRC. However, the role of PPARγ in gastrointestinal cancers, especially in colorectal cancer, is rather controversial. Nevertheless, some recent studies with the positive results on the possible application of PPARγ ligands, such as Bezafibrate or Rosiglitazone in gastrointestinal cancers, have suggested a potential usefulness of PPARγ agonists in cancer prevention and therapy. In this review, the authors discuss the recent developments in the role of PPARγ in gastrointestinal cancers.

Growth factor-antagonized rexinoid apoptosis involves permissive PPARgamma/RXR heterodimers to activate the intrinsic death pathway by NO

Growth factor (GF) deprivation and/or blocking of cognate signaling can induce apoptosis and is the basis of several cancer treatment paradigms. We observed that RXR agonists (rexinoids) induce apoptosis of tumor cells when GF support is abrogated. This "rexinoid apoptosis" involves activation of both iNOS and eNOS by RXR-PPARgamma and results in production of apoptogenic NO. IGF/EGF-induced IGF receptor 1-mediated MAP kinase blocks rexinoid apoptosis by RXR phosphorylation. Combining rexinoids with the MAPK inhibitor U0126 induced apoptosis in human cancer cells in vitro and ex vivo and blocked xenograft growth in vivo. Our results suggest a regulatory mechanism in which GF signaling antagonizes RXR-PPARgamma-mediated default apoptosis to sustain cell life.