PPAR Ligands for Cancer Chemoprevention

Peroxisome proliferator-activated receptors: Key regulators of tumor progression and growth

One of the main causes of death on the globe is cancer. Peroxisome-proliferator-activated receptors (PPARs) are nuclear hormone receptors, including PPARα, PPARδ and PPARγ, which are important in regulating cancer cell proliferation, survival, apoptosis, and tumor growth. Activation of PPARs by endogenous or synthetic compounds regulates tumor progression in various tissues. Although each PPAR isotype suppresses or promotes tumor development depending on the specific tissues or ligands, the mechanism is still unclear. PPARs are receiving interest as possible therapeutic targets for a number of disorders. Numerous clinical studies are being conducted on PPARs as possible therapeutic targets for cancer. Therefore, this review will focus on the existing and future uses of PPARs agonists and antagonists in treating malignancies. PubMed, Science Direct, and Scopus databases were searched regarding the effect of PPARs on various types of cancers until the end of May 2023. The results of the review articles showed the therapeutic influence of PPARs on a wide range of cancer on in vitro, in vivo and clinical studies. However, further experimental and clinical studies are needed to be conducted on the influence of PPARs on various cancers.

Potential Chemopreventive Effects of Dietary Combination of Phytochemicals against Cancer Development

Cancer remains a major cause of cancer-related death worldwide. Over 70% of epithelial malignancies are sporadic and are related to lifestyle. Epidemiological studies suggest an inverse correlation between cancer incidence and fruit and vegetable intake. Numerous preclinical studies using in vitro (cell lines) and in vivo animal models of oncogenesis have reported the chemopreventive effects of dietary phytochemical agents through alterations in different biomarkers and signaling pathways. However, there is contrasting evidence from preclinical studies and clinical trials. To date, the most studied compounds include curcumin, resveratrol, isoflavones, green tea extract (epigallocatechin gallate), black raspberry powder (anthocyanins and ellagitannins), bilberry extract (anthocyanins), ginger extract (gingerol derivatives), and pomegranate extract (ellagitannins and ellagic acid). Overall, the clinical evidence of the preventive effects of dietary phytochemicals against cancer development is still weak, and the amount of these phytochemicals needed to exert chemopreventive effects largely exceeds the common dietary doses. Therefore, we propose a combination treatment of natural compounds that are used clinically for another purpose in order to obtain excess inhibitory efficacy via low-dose administration and discuss the possible reasons behind the gap between preclinical research and clinical trials.

Potential Therapeutic Effects of PPAR Ligands in Glioblastoma

Glioblastoma (GB), also known as grade IV astrocytoma, represents the most aggressive form of brain tumor, characterized by extraordinary heterogeneity and high invasiveness and mortality. Thus, a great deal of interest is currently being directed to investigate a new therapeutic strategy and in recent years, the research has focused its attention on the evaluation of the anticancer effects of some drugs already in use for other diseases. This is the case of peroxisome proliferator-activated receptors (PPARs) ligands, which over the years have been revealed to possess anticancer properties. PPARs belong to the nuclear receptor superfamily and are divided into three main subtypes: PPAR-α, PPAR-β/δ, and PPAR-γ. These receptors, once activated by specific natural or synthetic ligands, translocate to the nucleus and dimerize with the retinoid X receptors (RXR), starting the signal transduction of numerous genes involved in many physiological processes. PPARs receptors are activated by specific ligands and participate principally in the preservation of homeostasis and in lipid and glucose metabolism. In fact, synthetic PPAR-α agonists, such as fibrates, are drugs currently in use for the clinical treatment of hypertriglyceridemia, while PPAR-γ agonists, including thiazolidinediones (TZDs), are known as insulin-sensitizing drugs. In this review, we will analyze the role of PPARs receptors in the progression of tumorigenesis and the action of PPARs agonists in promoting, or not, the induction of cell death in GB cells, highlighting the conflicting opinions present in the literature.

The PPARγ Agonist Rosiglitazone Suppresses Syngeneic Mouse SCC (Squamous Cell Carcinoma) Tumor Growth through an Immune-Mediated Mechanism

Recent evidence suggests that PPARγ agonists may promote anti-tumor immunity. We show that immunogenic PDV cutaneous squamous cell carcinoma (CSCC) tumors are rejected when injected intradermally at a low cell number (1 × 106) into immune competent syngeneic hosts, but not immune deficient mice. At higher cell numbers (5 × 106 PDV cells), progressively growing tumors were established in 14 of 15 vehicle treated mice while treatment of mice with the PPARγ agonist rosiglitazone resulted in increased tumor rejection (5 of 14 tumors), a significant decrease in PDV tumor size, and a significant decrease in tumor cell Ki67 labeling. Rosiglitazone treatment had no effect on tumor rejection, tumor volume or PDV tumor cell proliferation in immune deficient NOD.CB17-PrkdcSCID/J mice. Rosiglitazone treatment also promoted an increase in tumor infiltrating CD3+ T-cells at both early and late time points. In contrast, rosiglitazone treatment had no significant effect on myeloid cells expressing either CD11b or Gr-1 but suppressed a late accumulation of myeloid cells expressing both CD11b and Gr-1, suggesting a potential role for CD11b+Gr-1+ myeloid cells in the late anti-tumor immune response. Overall, our data provides evidence that the PPARγ agonist rosiglitazone promotes immune-mediated anti-neoplastic activity against tumors derived from this immunogenic CSCC cell line.

Standardized Punica Granatum Pericarp Extract, Suppresses Tumor Proliferation and Angiogenesis in a Mouse Model of Melanoma: Possible Involvement of PPARα and PPARγ Pathways

Melanoma is a challenging disease to treat. Punica granatum L. has a potential anticancer effect. This study determined the antiproliferative and antiangiogenic potential of the extract from pomegranate pericarp (PPE) in melanoma. Melanoma cells (1 × 10⁶) were injected to C57BL6 mice subcutaneously. On 8^th day, mice were randomly divided into 9 groups. Group 1 was considered as control and received distilled water. Groups 2 to 5 received 50, 100, 200 or 400 mg/kg of standardized PPE, orally. Group 6 received 400 mg/kg PPE and PPAR-γ antagonist (T0070907, 5 mg/kg/day). Group 7 received 400 mg/kg PPE and PPAR-α antagonist (GW6471, 10 mg/kg/day). Groups 8 and 9 received PPAR antagonists alone. On the 16th day, mice were euthanized and the tumor samples were analyzed by immunohistochemistry staining for Ki-67 and CD31. Vascular endothelial growth factor (VEGF) plasma level was determined by ELISA. PPE at the doses of 50, 100, 200, and 400 mg/kg decreased tumor weight to 1.28, 1.03, 0.82, and 0.58 g, respectively, in comparison with 1.46 g in control group. Tumor volume reduced to 2.1, 1.7, 1.35 and 0.95 cm3 at the mentioned doses, in comparison with 2.4 cm³ in control group (P < 0.05 for all groups). VEGF, Ki-67 and CD31 were decreased dose dependently in the treatment groups (P < 0.05). PPARα and PPARγ antagonists significantly reduced the extract effects (P < 0.05). It was concluded that PPE may have a potential implication in melanoma treatment through activation of PPARα and PPARγ receptors.

Peroxisome Proliferator-Activated Receptor Ligands and Their Role in Chronic Myeloid Leukemia: Therapeutic Strategies

Imatinib therapy remains the gold standard for treatment of chronic myeloid leukemia; however, the acquired resistance to this therapeutic agent in patients has urged the scientists to devise modalities for overcoming this chemoresistance. For this purpose, initially therapeutic agents with higher tyrosine kinase activity were introduced, which had the potential for inhibiting even mutant forms of Bcr-Abl. Furthermore, coupling imatinib with peroxisome proliferator-activated receptor ligands also showed beneficial effects in chronic myeloid leukemia cell proliferation. These combination protocols inhibited cell growth and induced apoptosis as well as differentiation in chronic myeloid leukemia cell lines. In addition, peroxisome proliferator-activated receptors ligands increased imatinib uptake by upregulating the expression of human organic cation transporter 1. Taken together, peroxisome proliferator-activated receptors ligands are currently being considered as novel promising therapeutic candidates for chronic myeloid leukemia treatment, because they can synergistically enhance the efficacy of imatinib. In this article, we reviewed the potential of peroxisome proliferator-activated receptors ligands for use in chronic myeloid leukemia treatment. The mechanism of action of these therapeutics modalities are also presented in detail.

Marine low molecular weight natural products as potential cancer preventive compounds

Due to taxonomic positions and special living environments, marine organisms produce secondary metabolites that possess unique structures and biological activities. This review is devoted to recently isolated and/or earlier described marine compounds with potential or established cancer preventive activities, their biological sources, molecular mechanisms of their action, and their associations with human health and nutrition. The review covers literature published in 2003–2013 years and focuses on findings of the last 2 years.

A comparison of peroxisome proliferator-activated receptor-α agonist and antagonist on human umbilical vein endothelial cells angiogenesis

Background:

There are controversial reports about the antiangiogenic effects of peroxisome proliferator-activated receptor α (PPARα). In the current study, we compared the effects of PPARα agonist and antagonist on human umbilical vein endothelial cells (HUVECs) angiogenesis with matrigel assay.

Materials and Methods:

HUVECs (1 × 10⁵ cells/well) treated with PPARα agonist (fenofibrate) and antagonist (GW6471) were cultured on matrigel for 24 h. Treated cells were stained with calcein and investigated by fluorescent microscopy. The obtained images were also analyzed by AngioQuant software. Finally, the data were analyzed using SPSS 15 software, Kruskal-Wallis and one way ANOVA.

Results:

Statistical analysis showed that fenofibrate significantly inhibit the tube formation (size, length, junction) (P < 0.05) but there was a trend to increased angiogenesis in GW6471 treated group (P > 0.05).

Conclusion:

These results showed that PPARα agonist is effective in suppression of angiogenesis. Further studies are needed to confirm these results in in vivo studies.

Preclinical cancer chemoprevention studies using animal model of inflammation-associated colorectal carcinogenesis

Inflammation is involved in all stages of carcinogenesis. Inflammatory bowel disease, such as ulcerative colitis and Crohn’s disease is a longstanding inflammatory disease of intestine with increased risk for colorectal cancer (CRC). Several molecular events involved in chronic inflammatory process are reported to contribute to multi-step carcinogenesis of CRC in the inflamed colon. They include over-production of free radicals, reactive oxygen and nitrogen species, up-regulation of inflammatory enzymes in arachidonic acid biosynthesis pathway, up-regulation of certain cytokines, and intestinal immune system dysfunction. In this article, firstly I briefly introduce our experimental animal models where colorectal neoplasms rapidly develop in the inflamed colorectum. Secondary, data on preclinical cancer chemoprevention studies of inflammation-associated colon carcinogenesis by morin, bezafibrate, and valproic acid, using this novel inflammation-related colorectal carcinogenesis model is described.

Cancer chemoprevention by carotenoids

Carotenoids are natural fat-soluble pigments that provide bright coloration to plants and animals. Dietary intake of carotenoids is inversely associated with the risk of a variety of cancers in different tissues. Preclinical studies have shown that some carotenoids have potent antitumor effects both in vitro and in vivo, suggesting potential preventive and/or therapeutic roles for the compounds. Since chemoprevention is one of the most important strategies in the control of cancer development, molecular mechanism-based cancer chemoprevention using carotenoids seems to be an attractive approach. Various carotenoids, such as β-carotene, a-carotene, lycopene, lutein, zeaxanthin, β-cryptoxanthin, fucoxanthin, canthaxanthin and astaxanthin, have been proven to have anti-carcinogenic activity in several tissues, although high doses of β-carotene failed to exhibit chemopreventive activity in clinical trials. In this review, cancer prevention using carotenoids are reviewed and the possible mechanisms of action are described.

Altered peroxisome-proliferator activated receptors expression in human endometrial cancer

Peroxisome proliferator-activated receptors (PPARs) belong to a family of nuclear hormone receptors acting as transcriptional factors, recently involved also in carcinogenesis. Present study was undertaken to evaluate the presence and subcellular localization of different PPAR isoforms (α, β, γ) in healthy endometrial tissue (n = 10) and endometrial carcinoma (FIGO I, endometrioides type, G1, n = 35). We sought to analyze PPARs mRNA content as well as protein immunohistochemical expression that was further quantified by Western Blot technique. For both PPARα and PPARβ, protein expression was significantly higher in endometrial cancers compared to normal endometrial mucosa. In opposite, PPARγ protein expression was lower in endometrial cancer cells. In each case, immunohistochemical reaction was confined to the perinuclear and/or nuclear region. At the transcriptional level, the content of mRNA of all PPAR subunits did not follow the protein pattern of changes. These results provide evidence for altered PPAR's protein expression and disregulation of posttranslational processes in endometrial cancers.

Cancer chemopreventive ability of conjugated linolenic acids

Conjugated fatty acids (CFA) have received increased interest because of their beneficial effects on human health, including preventing cancer development. Conjugated linoleic acids (CLA) are such CFA, and have been reviewed extensively for their multiple biological activities. In contrast to other types of CFAs including CLA that are found at low concentrations (less than 1%) in natural products, conjugated linolenic acids (CLN) are the only CFAs that occur in higher quantities in natural products. Some plant seeds contain a considerably high concentration of CLN (30 to 70 wt% lipid). Our research group has screened CLN from different plant seed oils to determine their cancer chemopreventive ability. This review describes the physiological functions of CLN isomers that occur in certain plant seeds. CLN are able to induce apoptosis through decrease of Bcl-2 protein in certain human cancer cell lines, increase expression of peroxisome proliferator-activated receptor (PPAR)-γ, and up-regulate gene expression of p53. Findings in our preclinical animal studies have indicated that feeding with CLN resulted in inhibition of colorectal tumorigenesis through modulation of apoptosis and expression of PPARγ and p53. In this review, we summarize chemopreventive efficacy of CLN against cancer development, especially colorectal cancer.

Protein expression analysis of inflammation-related colon carcinogenesis

Background:

Chronic inflammation is a risk factor for colorectal cancer (CRC) development. The aim of this study was to determine the differences in protein expression between CRC and the surrounding nontumorous colonic tissues in the mice that received azoxymethane (AOM) and dextran sodium sulfate (DSS) using a proteomic analysis.

Materials and Methods:

Male ICR mice were given a single intraperitoneal injection of AOM (10 mg/kg body weight), followed by 2% (w/v) DSS in their drinking water for seven days, starting one week after the AOM injection. Colonic adenocarcinoma developed after 20 weeks and a proteomics analysis based on two-dimensional gel electrophoresis and ultraflex TOF/TOF mass spectrometry was conducted in the cancerous and nontumorous tissue specimens.

Results:

The proteomic analysis revealed 21 differentially expressed proteins in the cancerous tissues in comparison to the nontumorous tissues. There were five markedly increased proteins (beta-tropomyosin, tropomyosin 1 alpha isoform b, S100 calcium binding protein A9, and an unknown protein) and 16 markedly decreased proteins (Car1 proteins, selenium-binding protein 1, HMG-CoA synthase, thioredoxin 1, 1 Cys peroxiredoxin protein 2, Fcgbp protein, Cytochrome c oxidase, subunit Va, ETHE1 protein, and 7 unknown proteins).

Conclusions:

There were 21 differentially expressed proteins in the cancerous tissues of the mice that received AOM and DSS. Their functions include metabolism, the antioxidant system, oxidative stress, mucin production, and inflammation. These findings may provide new insights into the mechanisms of inflammation-related colon carcinogenesis and the establishment of novel therapies and preventative strategies to treat carcinogenesis in the inflamed colon.

The use of Cox-2 and PPARγ signaling in anti-cancer therapies

Increased production of the pro-inflammatory enzyme cyclooxygenase-2 (Cox-2) and altered expression and activity of peroxisome proliferator-activated receptor γ (PPARγ) have been observed in many malignancies. Both the PPARγ ligands and the Cox-2 inhibitors possess anti-inflammatory and anti-neoplastic effects in vitro and have been assessed for their therapeutic potential in several pre-clinical and clinical studies. Recently, multiple interactions between PPARγ and Cox-2 signaling pathways have been revealed. Understanding of the cross-talk between PPARγ and Cox-2 might provide important novel strategies for the effective treatment and/or prevention of cancer. This article summarizes recent achievements involving the functional interactions between the PPARγ and Cox-2 signaling pathways and discusses the implications of such interplay for clinical use.

Nuclear receptors in head and neck cancer: current knowledge and perspectives

Disease management of head and neck cancer has improved significantly. However, a high rate of early recurrences and metastasis still counteract improvement of long-term survival. Hence, the quest for molecular mechanisms and key regulatory factors exploitable by targeted therapies is still ongoing. Such potential candidates may include also nuclear receptors, belonging to a superfamily of transcription factors implicated in a broad spectrum of physiological and pathophysiological processes. As dysfunction of nuclear receptor signaling contributes to a variety of proliferative diseases, they are major targets for drug discovery and hold promising potential for the development of improved anticancer treatment strategies. Several nuclear receptors have also been associated with head and neck cancer, and strategies targeting these molecules are currently tested in clinical trials. However, reports and molecular knowledge on the pathobiological relevance of nuclear receptors for cancers of the head and neck is currently rather fragmented. Hence, this review provides a general overview of nuclear receptors' molecular functions and summarizes their potential prognostic and therapeutic relevance for this tumor entity.

Prognostic and therapeutic potential of nuclear receptors in head and neck squamous cell carcinomas

Head and neck squamous cell carcinomas are among the most common neoplasms worldwide and characterized by local tumor aggressiveness, high rate of early recurrences, development of metastasis, and second primary cancers. Despite modern therapeutic strategies and sophisticated surgical management, overall survival-rates remained largely unchanged over the last decades. Thus, the need for novel treatment options for this tumor entity is undeniable. A key event in carcinogenesis is the uncontrolled modulation of genetic programs. Nuclear receptors belong to a large superfamily of transcription factors implicated in a broad spectrum of physiological and pathophysiological processes, including cancer. Several nuclear receptors have also been associated with head and neck cancer. This review will summarize their mode of action, prognostic/therapeutic relevance, as well as preclinical and clinical studies currently targeting nuclear receptors in this tumor entity.

PPARalpha Ligands as Antitumorigenic and Antiangiogenic Agents

Peroxisome proliferator-activated receptors (PPARs) belong to the nuclear receptor family of ligand-activated transcription factors. This subfamily is composed of three members-PPARalpha, PPARdelta, and PPARgamma-that differ in their cell and tissue distribution as well as in their target genes. PPARalpha is abundantly expressed in liver, brown adipose tissue, kidney, intestine, heart, and skeletal muscle; and its ligands have been used to treat diseases such as obesity and diabetes. The recent finding that members of the PPAR family, including the PPARalpha, are expressed by tumor and endothelial cells together with the observation that PPAR ligands regulate cell growth, survival, migration, and invasion, suggested that PPARs also play a role in cancer. In this review, we focus on the contribution of PPARalpha to tumor and endothelial cell functions and provide compelling evidence that PPARalpha can be viewed as a new class of ligand activated tumor "suppressor" gene with antiangiogenic and antitumorigenic activities. Given that PPAR ligands are currently used in medicine as hypolipidemic drugs with excellent tolerance and limited toxicity, PPARalpha activation might offer a novel and potentially low-toxic approach for the treatment of tumor-associated angiogenesis and cancer.