Troglitazone induces p27Kip1-associated cell-cycle arrest through down-regulating Skp2 in human hepatoma cells

Leucine drives LAT1-related SNAIL upregulation in glucose-starved pancreatic cancer cells

Pancreatic cancer, a highly fibrotic and hypovascular tumor, is thought to have unique metabolic characteristics in surviving and proliferating in malnutritional microenvironments. In this study, we compared the differences in the ability of pancreatic cancer cells to adapt to glucose-free conditions with liver cancer cells, which are representative of hypervascular tumors. Three pancreatic cancer cells and two liver cancer cells were used to examine the transcriptional expression levels of molecules involved in intracellular amino acid uptake, epithelial-mesenchymal transition (EMT), and cancer stemness under glucose deprivation. The results showed that the proliferative activity of pancreatic cancer cells under glucose deprivation was significantly lower than that of liver cancer cells, but the expression levels of amino acid transporters were significantly higher. Among them, L-type amino acid transporter 1 (LAT1) upregulation was unique in concert with increased expression of the EMT regulator SNAIL and the cancer stemness marker doublecortin-like kinase 1. LAT1 knockdown canceled the upregulation of SNAIL in glucose-starved pancreatic cancer cells, suggesting a mechanistic link between the two molecules. When LAT1 was stimulated by its substrate leucine, the SNAIL expression was upregulated dose-dependently. Collectively, pancreatic cancer cells reprogrammed metabolism to adapt to energy crises involving leucine-induced SNAIL upregulation.

Exploiting the potential of the ubiquitin-proteasome system in overcoming tyrosine kinase inhibitor resistance in chronic myeloid leukemia

The advent of tyrosine kinase inhibitors (TKI) targeting BCR-ABL has drastically changed the treatment approach of chronic myeloid leukemia (CML), greatly prolonged the life of CML patients, and improved their prognosis. However, TKI resistance is still a major problem with CML patients, reducing the efficacy of treatment and their quality of life. TKI resistance is mainly divided into BCR-ABL-dependent and BCR-ABL-independent resistance. Now, the main clinical strategy addressing TKI resistance is to switch to newly developed TKIs. However, data have shown that these new drugs may cause serious adverse reactions and intolerance and cannot address all resistance mutations. Therefore, finding new therapeutic targets to overcome TKI resistance is crucial and the ubiquitin-proteasome system (UPS) has emerged as a focus. The UPS mediates the degradation of most proteins in organisms and controls a wide range of physiological processes. In recent years, the study of UPS in hematological malignant tumors has resulted in effective treatments, such as bortezomib in the treatment of multiple myeloma and mantle cell lymphoma. In CML, the components of UPS cooperate or antagonize the efficacy of TKI by directly or indirectly affecting the ubiquitination of BCR-ABL, interfering with CML-related signaling pathways, and negatively or positively affecting leukemia stem cells. Some of these molecules may help overcome TKI resistance and treat CML. In this review, the mechanism of TKI resistance is briefly described, the components of UPS are introduced, existing studies on UPS participating in TKI resistance are listed, and UPS as the therapeutic target and strategies are discussed.

Roles of the peroxisome proliferator-activated receptors (PPARs) in the pathogenesis of hepatocellular carcinoma (HCC)

Hepatocellular carcinoma (HCC) holds a prominent position among global cancer types. Classically, HCC manifests in individuals with a genetic predisposition when they encounter risk elements, particularly in the context of liver cirrhosis. Peroxisome proliferator-activated receptors (PPARs), which are transcription factors activated by fatty acids, belong to the nuclear hormone receptor superfamily and play a pivotal role in the regulation of energy homeostasis. At present, three distinct subtypes of PPARs have been recognized: PPARα, PPARγ, and PPARβ/δ. They regulate the transcription of genes responsible for cellular development, energy metabolism, inflammation, and differentiation. In recent years, with the rising incidence of HCC, there has been an increasing focus on the mechanisms and roles of PPARs in HCC. PPARα primarily mediates the occurrence and development of HCC by regulating glucose and lipid metabolism, inflammatory responses, and oxidative stress. PPARβ/δ is closely related to the self-renewal ability of liver cancer stem cells (LCSCs) and the formation of the tumor microenvironment. PPARγ not only influences tumor growth by regulating the glucose and lipid metabolism of HCC, but its agonists also have significant clinical significance for the treatment of HCC. Therefore, this review offers an exhaustive examination of the role of the three PPAR subtypes in HCC progression, focusing on their mediation of critical cellular processes such as glucose and lipid metabolism, inflammation, oxidative stress, and other pivotal signaling pathways. At the end of the review, we discuss the merits and drawbacks of existing PPAR-targeted therapeutic strategies and suggest a few alternative combinatorial therapeutic approaches that diverge from conventional methods.

Development of a prognostic model for anoikis and identifies hub genes in hepatocellular carcinoma

Considering the high fatality of hepatocellular carcinoma (HCC), current prognostic systems are insufficient to accurately forecast HCC patients' outcomes. In our study, nine anoikis‑related genes (PTRH2, ITGAV, ANXA5, BIRC5, BDNF, BSG, DAP3, SKP2, and EGF) were determined to establish a risk scoring model using LASSO regression, which could be validated in ICGC dataset. Kaplan-Meier curves and time-dependent receiver operating characteristic (ROC) curve analysis confirmed the risk score possessed an accurate predictive value for the prognosis of HCC patients. The high-risk group showed a higher infiltration of aDCs, macrophages, T-follicular helper cells, and Th2 cells. Besides, PD-L1 was significantly higher in the high-risk group compared to the low-risk group. Several anoikis‑related genes, such as ANX5, ITGAV, BDNF and SKP2, were associated with drug sensitivity in HCC. Finally, we identified BIRC5 and SKP2 as hub genes among the nine model genes using WGCNA analysis. BIRC5 and SKP2 were over-expressed in HCC tissues, and their over-expression was associated with poor prognosis, no matter in our cohort by immunohistochemical staining or in the TCGA cohort by mRNA-Seq. In our cohort, BIRC5 expression was highly associated with the T stage, pathologic stage, histologic grade and AFP of HCC patients. In general, our anoikis-related risk model can enhance the ability to predict the survival outcomes of HCC patients and provide a feasible therapeutic strategy for immunotherapy and drug resistance in HCC. BIRC5 and SKP2 are hub genes of anoikis‑related genes in HCC.

Peroxisome proliferator-activated receptor gamma as a therapeutic target for hepatocellular carcinoma: Experimental and clinical scenarios

Hepatocellular carcinoma (HCC) is the most common type of liver cancer worldwide. Viral hepatitis is a significant risk factor for HCC, although metabolic syndrome and diabetes are more frequently associated with the HCC. With increasing prevalence, there is expected to be > 1 million cases annually by 2025. Therefore, there is an urgent need to establish potential therapeutic targets to cure this disease. Peroxisome-proliferator-activated receptor gamma (PPARγ) is a ligand-activated transcription factor that plays a crucial role in the patho-physiology of HCC. Many synthetic agonists of PPARγ suppress HCC in experimental studies and clinical trials. These synthetic agonists have shown promising results by inducing cell cycle arrest and apoptosis in HCC cells and preventing the invasion and metastasis of HCC. However, some synthetic agonists also pose severe side effects in addition to their therapeutic efficacy. Thus natural PPARγ agonists can be an alternative to exploit this potential target for HCC treatment. In this review, the regulatory role of PPARγ in the pathogenesis of HCC is elucidated. Furthermore, the experimental and clinical scenario of both synthetic and natural PPARγ agonists against HCC is discussed. Most of the available literature advocates PPARγ as a potential therapeutic target for the treatment of HCC.

Potential Therapeutics Targeting Upstream Regulators and Interactors of EHMT1/2

Euchromatin histone lysine methyltransferases (EHMTs) are epigenetic regulators responsible for silencing gene transcription by catalyzing H3K9 dimethylation. Dysregulation of EHMT1/2 has been reported in multiple cancers and is associated with poor clinical outcomes. Although substantial insights have been gleaned into the downstream targets and pathways regulated by EHMT1/2, few studies have uncovered mechanisms responsible for their dysregulated expression. Moreover, EHMT1/2 interacting partners, which can influence their function and, therefore, the expression of target genes, have not been extensively explored. As none of the currently available EHMT inhibitors have made it past clinical trials, understanding upstream regulators and EHMT protein complexes may provide unique insights into novel therapeutic avenues in EHMT-overexpressing cancers. Here, we review our current understanding of the regulators and interacting partners of EHMTs. We also discuss available therapeutic drugs that target the upstream regulators and binding partners of EHMTs and could potentially modulate EHMT function in cancer progression.

Emerging Roles of SKP2 in Cancer Drug Resistance

More than half of all cancer patients receive chemotherapy, however, some of them easily acquire drug resistance. Resistance to chemotherapy has become a massive obstacle to achieve high rates of pathological complete response during cancer therapy. S-phase kinase-associated protein 2 (Skp2), as an E3 ligase, was found to be highly correlated with drug resistance and poor prognosis. In this review, we summarize the mechanisms that Skp2 confers to drug resistance, including the Akt-Skp2 feedback loop, Skp2-p27 pathway, cell cycle and mitosis regulation, EMT (epithelial-mesenchymal transition) property, enhanced DNA damage response and repair, etc. We also addressed novel molecules that either inhibit Skp2 expression or target Skp2-centered interactions, which might have vast potential for application in clinics and benefit cancer patients in the future.

Emerging roles of F-box proteins in cancer drug resistance

Chemotherapy continues to be a major treatment strategy for various human malignancies. However, the frequent emergence of chemoresistance compromises chemotherapy efficacy leading to poor prognosis. Thus, overcoming drug resistance is pivotal to achieve enhanced therapy efficacy in various cancers. Although increased evidence has revealed that reduced drug uptake, increased drug efflux, drug target protein alterations, drug sequestration in organelles, enhanced drug metabolism, impaired DNA repair systems, and anti-apoptotic mechanisms, are critically involved in drug resistance, the detailed resistance mechanisms have not been fully elucidated in distinct cancers. Recently, F-box protein (FBPs), key subunits in Skp1-Cullin1-F-box protein (SCF) E3 ligase complexes, have been found to play critical roles in carcinogenesis, tumor progression, and drug resistance through degradation of their downstream substrates. Therefore, in this review, we describe the functions of FBPs that are involved in drug resistance and discuss how FBPs contribute to the development of cancer drug resistance. Furthermore, we propose that targeting FBPs might be a promising strategy to overcome drug resistance and achieve better treatment outcome in cancer patients. Lastly, we state the limitations and challenges of using FBPs to overcome chemotherapeutic drug resistance in various cancers.

Therapeutic potential of PPARγ natural agonists in liver diseases

Peroxisome proliferator‐activated receptor gamma (PPARγ) is a vital subtype of the PPAR family. The biological functions are complex and diverse. PPARγ plays a significant role in protecting the liver from inflammation, oxidation, fibrosis, fatty liver and tumours. Natural products are a promising pool for drug discovery, and enormous research effort has been invested in exploring the PPARγ‐activating potential of natural products. In this manuscript, we will review the research progress of PPARγ agonists from natural products in recent years and probe into the application potential and prospects of PPARγ natural agonists in the therapy of various liver diseases, including inflammation, hepatic fibrosis, non‐alcoholic fatty liver and liver cancer.

The Skp2 Pathway: A Critical Target for Cancer Therapy

Strictly regulated protein degradation by ubiquitin-proteasome system (UPS) is essential for various cellular processes whose dysregulation is linked to serious diseases including cancer. Skp2, a well characterized component of Skp2-SCF E3 ligase complex, is able to conjugate both K48-linked ubiquitin chains and K63-linked ubiquitin chains on its diverse substrates, inducing proteasome mediated proteolysis or modulating the function of tagged substrates respectively. Overexpression of Skp2 is observed in various human cancers associated with poor survival and adverse therapeutic outcomes, which in turn suggests that Skp2 engages in tumorigenic activity. To that end, the oncogenic properties of Skp2 are demonstrated by various genetic mouse models, highlighting the potential of Skp2 as a target for tackling cancer. In this article, we will describe the downstream substrates of Skp2 as well as upstream regulators for Skp2-SCF complex activity. We will further summarize the comprehensive oncogenic functions of Skp2 while describing diverse strategies and therapeutic platforms currently available for developing Skp2 inhibitors.

Activation of peroxisome proliferator-activated receptor-gamma inhibits tumor growth by negatively regulating nuclear factor-κB activation in patients with hepatocellular carcinoma

BACKGROUND

The prognosis of advanced hepatocellular carcinoma (HCC) is poor because of its rapid progression. Peroxisome proliferator-activated receptor-gamma (PPARγ) is known to inhibit tumor growth in vitro; however, the behavior of PPARγ in clinical cases of HCC remains uncertain.

METHODS

Surgical specimens were collected from 104 HCC patients. The anti-neoplastic effects of PPARγ were evaluated.

RESULTS

PPARγ and its ligand expression were increased in some cases of HCC. When HCC patients were divided into two groups, tumor size was larger in patients with low PPARγ expression. Moreover, low PPARγ expression in HCC was an independent predictor of poorer prognosis. PPARγ expression was positively correlated with PPARγ activation and negatively correlated with NF-κB activation in HCC. PPARγ activation inhibited cell proliferation by inducing cell cycle arrest, through increased expression of p27(kip1) and decreased expression of cyclin D1 and interleukin-8. When HCC cells were treated with PPARγ ligands, PPARγ activation was increased and cell proliferation was inhibited in a dose-dependent manner. In contrast, PPARγ ligands negatively regulated NF-κB activation.

CONCLUSIONS

Activation of PPARγ induces cell cycle arrest and inhibits tumor progression by negatively regulating NF-κB activation in HCC. Therefore, PPARγ is an important endogenous regulator of HCC progression, and is a potential therapeutic target for HCC.

Small molecule therapeutics targeting F-box proteins in cancer

The ubiquitin proteasome system (UPS) plays vital roles in maintaining protein equilibrium mainly through proteolytic degradation of targeted substrates. The archetypical SCF ubiquitin E3 ligase complex contains a substrate recognition subunit F-box protein that recruits substrates to the catalytic ligase core for its polyubiquitylation and subsequent proteasomal degradation. Several well-characterized F-box proteins have been demonstrated that are tightly linked to neoplasia. There is mounting information characterizing F-box protein-substrate interactions with the rationale to develop unique therapeutics for cancer treatment. Here we review that how F-box proteins function in cancer and summarize potential small molecule inhibitors for cancer therapy.

Anticancer effect of rosiglitazone in rats treated with N-nitrosodiethylamine via inhibition of DNA synthesis: an implication for hepatocellular carcinoma

Rosiglitazone, peroxisome proliferator-activated receptor-γ (PPARγ) ligand, is a clinically tested drug used in the treatment of diabetes.

Ezetimibe prevents the development of non‑alcoholic fatty liver disease induced by high‑fat diet in C57BL/6J mice

There is currently no established treatment for non-alcoholic fatty liver disease (NAFLD), including its most extreme form, non-alcoholic steatohepatitis (NASH). Ezetimibe, an inhibitor of Niemann-Pick C1 Like 1-dependent cholesterol absorption, improves diet-induced hyperlipidemia and attenuates liver steatosis and insulin resistance. The aim of the present study was to determine whether ezetimibe treatment is able to inhibit the development of NAFLD, and to elucidate the underlying mechanism, using C57BL/6J (B6) mice maintained on a high-fat diet. Male B6 mice (20 weeks of age) were divided into the following two groups (n=7 in each group): Mice fed a high-fat diet for four weeks and mice fed a high-fat diet with 0.0064% (wt/wt) ezetimibe (5 mg/kg/day) for four weeks. Administration of ezetimibe significantly reduced liver steatosis and fibrosis. Ezetimibe reduced serum cholesterol, hepatic fat accumulation and insulin resistance in the liver of mice fed the high-fat diet. Furthermore, ezetimibe significantly reduced hepatic mRNA expression of Acc1 and Scd1, which are involved in hepatic fatty acid synthesis. Ezetimibe significantly reduced hepatic Cd36 gene expression, upregulation of which is significantly associated with insulin resistance, hyperinsulinemia and increased steatosis. The protein expression of SKP2, a viable therapeutic target in human cancer, was also reduced by ezetimibe. These findings suggest that ezetimibe may be an effective therapy for high fat-induced NAFLD, including NASH.

Roles of F-box proteins in human digestive system tumors (Review)

F-box proteins (FBPs), the substrate-recognition subunit of E3 ubiquitin (Ub) ligase, are the important components of Ub proteasome system (UPS). FBPs are involved in multiple cellular processes through ubiquitylation and subsequent degradation of their target proteins. Many studies have described the roles of FBPs in human cancers. Digestive system tumors account for a large proportion of all the tumors, and their mortality is very high. This review summarizes for the first time the roles of FBPs in digestive system tumorige-nesis and tumor progression, aiming at finding new routes for the rational design of targeted anticancer therapies in digestive system tumors.

HBV core promoter mutations promote cellular proliferation through E2F1-mediated upregulation of S-phase kinase-associated protein 2 transcription

BACKGROUND & AIMS

Hepatitis B virus (HBV) core promoter (CP) mutations have been associated with an increased risk of hepatocellular carcinoma (HCC) in clinical studies. We previously reported that a combination of CP mutations seen in HCC patients, expressed in HBx gene, increased SKP2 (S-phase kinase-associated protein 2) expression, thereby promoting cellular proliferation. Here, we investigate the possible mechanisms by which CP mutations upregulate SKP2.

METHODS

We used immunoblotting and ATPlite assay to validate the effect of CP mutations in full-length HBV genome on cell cycle regulator levels and cell proliferation. Activation of SKP2 mRNA was assessed by quantitative real-time PCR in primary human hepatocytes (PHH) and HCC cell lines. Effect of CP mutations on SKP2 promoter activity was determined by luciferase assay. Target regulation of E2F1 on SKP2 was analyzed by siRNAs.

RESULTS

CP mutations in full-length HBV genome upregulated SKP2 expression, thereby downregulating cell cycle inhibitors and accelerating cellular proliferation. CP mutations enhanced SKP2 promoter activity but had no effect on SKP2 protein stability. Mapping of the SKP2 promoter identified a region necessary for activation by CP mutations that contains an E2F1 response element. Knocking down E2F1 reduced the effects of CP mutations on SKP2 and cellular proliferation. The effect of CP mutations on E2F1 might be mediated through hyperphosphorylation of RB.

CONCLUSIONS

HBV CP mutations enhance SKP2 transcription by activating the E2F1 transcription factor and in turn downregulate cell cycle inhibitors, thus providing a potential mechanism for an association between CP mutations and HCC.

CITED2 is a novel direct effector of peroxisome proliferator-activated receptor γ in suppressing hepatocellular carcinoma cell growth

BACKGROUND

Previous reports from these authors found that activation of peroxisome proliferator-activated receptor gamma (PPARγ) suppressed hepatocellular carcinoma (HCC). This study sought to identify the molecular target of PPARγ and characterize its antitumor effect in HCC.

METHODS

Optimal PPARγ binding activity was obtained using the PPARγ agonist rosiglitazone (100 μM) as determined by enzyme-linked immunosorbent assay. Under PPARγ activation, 114 PPARγ downstream targets associated with cancer development were identified by oligonucleotide microarray and Gene Ontology analysis. Among them, Cbp/p300-interacting transactivator, with Glu/Asp-rich carboxy-terminal domain, 2 (CITED2) was the most prominent PPARγ-bound target, as determined by chromatin immunoprecipitation-polymerase chain reaction.

RESULTS

CITED2 messenger RNA and protein was significantly down-regulated in primary HCCs compared with their adjacent nontumor tissues. PPARγ induced expression of CITED2 in HCC cell lines after adenovirus-PPARγ transduction. The biological function of CITED2 was evaluated by loss- and gain-of-function assays. CITED2 knockdown in the hepatocyte cell line LO2 and HCC cell line Hep3B significantly increased cell viability and clonogenicity, and promoted G1 -S phase transition in both cell lines. In contrast, ectopic expression of CITED2 in HepG2 and BEL7404 HCC cell lines significantly suppressed cell growth. The tumor suppressive effect of CITED2 was associated with up-regulation of cyclin-dependent kinase inhibitors p15(INK4B) , p21(Wat1/Cip1) , p27(Kip1) , antiproliferative regulator interferon alpha 1, proapoptotic mediators including tumor necrosis factor receptor superfamily member 1A (TNFRSF1A), TNFRSF25, caspase-8, granzyme A, and the tumor suppressor gene maspin. CITED2 was also associated with the down-regulation of cell cycle regulator cyclin D1, oncogene telomerase reverse transcriptase, and proinvasion/metastasis gene matrix metallopeptidase 2.

CONCLUSIONS

CITED2 is a direct effector of PPARγ for tumor suppression. Cancer 2013. © 2012 American Cancer Society.

p53, SKP2, and DKK3 as MYCN Target Genes and Their Potential Therapeutic Significance

Neuroblastoma is the most common extra-cranial solid tumor of childhood. Despite significant advances, it currently still remains one of the most difficult childhood cancers to cure, with less than 40% of patients with high-risk disease being long-term survivors. MYCN is a proto-oncogene implicated to be directly involved in neuroblastoma development. Amplification of MYCN is associated with rapid tumor progression and poor prognosis. Novel therapeutic strategies which can improve the survival rates whilst reducing the toxicity in these patients are therefore required. Here we discuss genes regulated by MYCN in neuroblastoma, with particular reference to p53, SKP2, and DKK3 and strategies that may be employed to target them.

Functional role of peroxisome-proliferator-activated receptor γ in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the fifth most common cancer worldwide. Major risk factors of HCC include infection with hepatitis B or C viruses, alcohol and non-alcoholic fatty liver disease. HCC is difficult to diagnose at early stage, and has a very poor survival rate when diagnosed at a late stage. The majority of HCC-related deaths result from local invasion (to cause liver failure) or distant metastases. There is an urgent need to identify effective molecular targets for the treatment of the disease. As the target of an established class of therapeutic agent thiazolidinediones (TZDs), peroxisome-proliferator-activated receptor γ (PPARγ) has been widely studied for its role in the development of HCC. A substantial body of evidence based on in vitro and in vivo models indicates that the activation of PPARγ is able to inhibit HCC cell proliferation and tumor growth through inducing cell cycle arrest and apoptosis via the regulation of a panel of downstream effector molecules. PPARγ activation also induces an inhibitory effect on HCC metastasis. Meanwhile, there is new evidence suggesting that PPARγ inhibition could also be anti-tumorigenic. In the present review, we summarize the available information on the role of PPARγ in HCC development and spread, and discuss whether PPARγ activation by TZDs could play a role in the treatment of HCC, summarizing both in vitro and in vivo. Considering the available data, PPARγ seems to exert beneficial effects against HCC and may therefore represent as a therapeutic target.

Rosiglitazone reverses mitomycin C resistance in human gastric cancer cells

INTRODUCTION

To explore the mechanisms of rosiglitazone (ROS), a selective peroxisome proliferator-activated receptor gamma ligand, in reversing mitomycin C (MMC) resistance in a human drug-resistant gastric cancer cell line.

METHODS

The vincristine-resistant human gastric cancer cell line SGC7901/VCR and its parental cell line SGC7901 were treated with ROS, MMC (negative control), cyclosporine A+MMC (positive control) or ROS+MMC. A tetrazolium blue (methyl thiazolyl tetrazolium) assay was used to evaluate the sensitivity to these treatments. Flow cytometry analysis and acridine orange-ethidium bromide (AO-EB) fluorescent staining were used to determine the effects of ROS on MMC-induced apoptosis. Reverse transcription polymerase chain reaction and western blotting were used to measure the expression of multidrug resistant 1 (MDR1), Livin and P-glycoprotein (P-gp).

RESULTS

ROS administration dose dependently increased the reversal index in MMC-treated SCG7901/VCR cells. ROS increased apoptosis in SGC7901/VCR cells compared with the blank group and MMC group. ROS+MMC also increased apoptosis in SGC7901/VCR cells compared with other groups (P < 0.05 or P < 0.01). The mRNA expression of MDR1 and Livin and the protein expression of P-gp in SGC7901/VCR cells were significantly higher than those in SGC7901 cells (P < 0.01). However, ROS or ROS+MMC treatment markedly upregulated the mRNA expression of MDR1 and Livin and the protein expression of P-gp in SGC7901/VCR cells (P < 0.01).

CONCLUSIONS

ROS reverses MMC resistance in human gastric cancer SGC7901/VCR cells by reducing expression of MDR1, Livin and P-gp and increasing apoptosis.

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.

The APC/C Ubiquitin Ligase: From Cell Biology to Tumorigenesis

The ubiquitin proteasome system (UPS) is required for normal cell proliferation, vertebrate development, and cancer cell transformation. The UPS consists of multiple proteins that work in concert to target a protein for degradation via the 26S proteasome. Chains of an 8.5-kDa protein called ubiquitin are attached to substrates, thus allowing recognition by the 26S proteasome. Enzymes called ubiquitin ligases or E3s mediate specific attachment to substrates. Although there are over 600 different ubiquitin ligases, the Skp1-Cullin-F-box (SCF) complexes and the anaphase promoting complex/cyclosome (APC/C) are the most studied. SCF involvement in cancer has been known for some time while APC/C's cancer role has recently emerged. In this review we will discuss the importance of APC/C to normal cell proliferation and development, underscoring its possible contribution to transformation. We will also examine the hypothesis that modulating a specific interaction of the APC/C may be therapeutically attractive in specific cancer subtypes. Finally, given that the APC/C pathway is relatively new as a cancer target, therapeutic interventions affecting APC/C activity may be beneficial in cancers that are resistant to classical chemotherapy.

Human recombinant Cripto-1 increases doubling time and reduces proliferation of HeLa cells independent of pro-proliferation pathways

Human oncofetal protein Cripto-1 (CR-1) is overexpressed in many types of cancers. CR-1 binds to cell surface Glypican-1 to activate Erk1/2 MAPK and Akt pathways leading to cell proliferation. However, we show that treatment with recombinant CR-1 reduces proliferation of HeLa cells by increasing the doubling time without triggering cell death or cell cycle arrest. Using a comparative study with U-87 MG cells, we show that the pro-proliferative pathway of CR-1 is not effective in HeLa cells due to lower expression of Glypican-1. Further we show that treatment with recombinant CR-1 increases PTEN in HeLa cells leading to downregulation of PI3K/Akt pathway. The anti-proliferative effect gets potentiated when the pro-proliferative pathway is blocked.

The Role of PPARgamma in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is the third leading cause of cancer-related death worldwide. This cancer develops mainly in cirrhotic patients. The cirrhotic liver is considered to be a preneoplastic organ, suggesting the rationale for cancer prevention. PPARγ is a nuclear transcription factor whose activation leads to interaction in the metabolism of lipids, insulin sensitization of peripheral cells, anti-inflammatory action. It can also induce differentiation and inhibits proliferation of cancer cells. Until now, data using PPARγ ligands in HCC have demonstrated mainly in in vitro models that its activation could be due to an antiproliferative effect. PPARγ ligand administration has also been associated with a diminution of liver fibrosis in animal models, and potentially also on tumoral cell death. Soma data show that the favorable effect of natural and synthetized PPARγ agonists could also be independent of PPARγ activation. Furthermore, in some situations, PPARγ antagonists have also an anticancer effect. Therefore, we can conclude that the link between activation of the PPARγ pathway and an anticancer activity is suggested but until now not firmly established in HCC.

Mechanisms by which thiazolidinediones induce anti-cancer effects in cancers in digestive organs

Increasing evidence suggests that thiazolidinediones (TZDs) could have a therapeutic potential for patients with cancers. Here, the evidence on the mechanisms by which TZDs could contribute to different steps of cancer biology in the digestive system is summarized. According to studies, TZDs induce anti-cancer actions through 3 main pathways: (1) cell growth arrest, (2) induction of apoptosis, and (3) inhibition of cell invasion. Cell growth arrest is induced by an increased level of p27(Kip1). p27(Kip1) accumulation results from the inhibition of the ubiquitin-proteasome system and/or inhibition of MEK-ERK signaling. TZDs induce apoptosis through increased levels of apoptotic molecules, such as p53 and PTEN and/or decreased level of anti-apoptotic molecules, such as Bcl-2 and survivin. Inhibition of MEK-ERK signaling-mediated up-regulation of E-cadherin and claudin-4, and/or decreased expression of matrix metalloproteinases (MMPs) such as MMP-2 and MMP-9, play a role in the TZD-induced inhibition of cancer cell invasion. Thus, TZDs are capable of inducing anti-tumor action in a variety of ways in gastrointestinal cancers.

Gene Expression Changes Induced by PPAR Gamma Agonists in Animal and Human Liver

Thiazolidinediones are a class of Peroxisome Proliferator Activated Receptor γ (PPARγ) agonists that reduce insulin resistance in type 2 diabetic patients. Although no detectable hepatic toxicity has been evidenced in animal studies during preclinical trials, these molecules have nevertheless induced hepatic adverse effects in some treated patients. The mechanism(s) of hepatotoxicity remains equivocal. Several studies have been conducted using PCR analysis and microarray technology to identify possible target genes and here we review the data obtained from various in vivo and in vitro experimental models. Although PPARγ is expressed at a much lower level in liver than in adipose tissue, PPARγ agonists exert various PPARγ-dependent effects in liver in addition to PPARγ-independent effects. Differences in effects are dependent on the choice of agonist and experimental conditions in rodent animal studies and in rodent and human liver cell cultures. These effects are much more pronounced in obese and diabetic liver. Moreover, our own recent studies have shown major interindividual variability in the response of primary human hepatocyte populations to troglitazone treatment, supporting the occurrence of hepatotoxicity in only some individuals.

In vitro and in vivo therapeutic efficacy of the PPAR-γ agonist troglitazone in combination with cisplatin against malignant pleural mesothelioma cell growth

Malignant pleural mesothelioma (MPM), an aggressive and refractory tumor type, is increasing in frequency throughout the world. Peroxisome proliferator activated receptor-γ (PPAR-γ) agonists have anticancer activity against several cancer cell lines in vitro and in vivo. However, there have been no reports that PPAR-γ agonists induce growth inhibition of MPM cell lines. In this study, we investigated the inhibitory effect of a PPAR-γ agonist in combination with an anticancer agent on MPM cell growth in vitro and in vivo. We examined the therapeutic efficacy of the PPAR-γ agonist troglitazone (TGZ) in combination with cisplatin against a human MPM cell line, both in vitro and orthotopically inoculated into severe combined immunodeficient (SCID) mice. Troglitazone (TGZ) alone inhibited MPM cell growth in vitro in a dose-dependent manner via induction of G1 cell cycle arrest and apoptosis. The combination of TGZ and cisplatin showed an additive inhibitory effect on MPM cell growth compared to treatment with either individual drug. Treatment with 500 mg/kg or 1000 mg/kg TGZ effectively inhibited the production of thoracic tumors and pleural effusion in EHMES-10 cell-bearing SCID mice. Moreover, treatment with 500 mg/kg TGZ in combination with 3 mg/kg cisplatin more effectively prolonged survival compared to treatment with either individual drug. These results suggest that TGZ in combination with cisplatin may become a novel therapy for MPM.

Overexpression of PPARγ can down-regulate Skp2 expression in MDA-MB-231 breast tumor cells

Skp2 is frequent amplified and overexpressed in breast cancer, making it a potential molecular target for cancer therapy. The objective of this study was to examine the effect of PPARγ overexpression on Skp2 expression in breast cancer cell lines. First, we investigated the role of PPARγ and Skp2 in human breast cancer progression. Immunohistochemical analysis of 70 specimens on formalin-fixed paraffin sections was performed. Furthermore in vitro, Western blot analysis was used to study the relationship between PPARγ and Skp2. We found that the expression of PPARγ and Skp2 expression was inverse correlation whether in vivo or in vitro. In addition, PPARγ overexpression can down-regulate the expression of Skp2 mRNA and protein in breast cancer cells. PPARγ overexpression decreased breast cancer cell proliferation and induced spontaneous apoptosis even in the absence of exogenous ligand. These PPARγ-overexpressing cells were dramatically more sensitive to PPARγ ligand-induced apoptosis compared with parental or Myc-control transfected cells. Overexpressing of Skp2 partially reversed PPARγ's pro-apoptotic and anti-proliferative abilities. These results suggested that PPARγ's pro-apoptotic and anti-proliferative abilities appear to be triggered at least in part by the modulation of Skp2.

Negative regulation of the oncogenic transcription factor FoxM1 by thiazolidinediones and mithramycin

The Forkhead Box transcription factor FoxM1 regulates expression of genes that promote cell cycle progression, and it plays essential roles in the development of liver, lung, prostate and colorectal tumors. Thiazolidinediones (TZDs) activate the peroxisome proliferator-activated receptor gamma (PPARγ), a ligand-activated nuclear receptor transcription factor. We found that treatment of the human hepatoma cell lines HepG2 and PLC/PRF/5 cells with TZDs leads to inhibition of FoxM1 gene expression. No PPARγ/retinoid X receptor (RXR) consensus DNA binding sites were detected in the FoxM1 promoter extending to -10 kb upstream, and knockdown of PPARγ had no impact on TZD mediated downregulation of FoxM1 expression. Previously, others showed that PPARγ agonists inhibit the expression and DNA-binding activity of the Sp1 transcription factor. Here we show that Sp1 binds to the FoxM1 promoter region and positively regulates FoxM1 transcription, while mithramycin, a chemotherapy drug that specifically binds GC rich sequences in the DNA and inhibits activities of Sp1, inhibits expression of FoxM1. Our data suggest that TZD mediated suppression of Sp1 is responsible for downregulation of FoxM1 gene expression. Inhibition of FoxM1 expression by TZDs provides a new mechanism for TZD mediated negative regulation of cancer cell growth. FoxM1 expression and activity in cancer cells can be targeted using PPARγ agonists or the anti-neoplastic antibiotic mithramycin.

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.

[The effect of rosiglitazone on the cell proliferation and the expressions of p27 and skp2 in helicobacter pylori infected human gastric epithelial cells]

BACKGROUND/AIMS

Ligands for peroxisome proliferator-activated receptorgamma (PPARgamma), a member of the ligand-activated nuclear receptor superfamily, exhibit anti-tumoral effects and are associated with de novo synthesis of proteins involved in regulating the cell cycle and cell survival/death. Helicobacter pylori (H. pylori) is an etiologic agent for gastric adenocarcinoma, and raises the cell turnover of gastric epithelium. The aim of this study was to investigate the effect of PPARgamma ligand rosiglitazone on the cell proliferation and the expressions of p27 and Skp2 protein in H. pylori infected gastric epithelial cells.

METHODS

We examined the expression of PPARgamma by Western blot in H. pylori infected AGS human gastric epithelial cells. The effect of rosiglitazone on the survival of H. pylori infected AGS cells was assessed by cell viability assay. After the treatment of rosiglitazone in H. pylori infected AGS cells, the expressions of p27 and Skp2 were assessed by Western blot.

RESULTS

The expression of PPARgamma protein was increased in H. pylori infected AGS cells. Cell growth was inhibited and decreased in dose- and time- dependent manner in H. pylori infected AGS cells treated with rosiglitazone. A decrease in Skp2 expression and a reciprocal increase in p27 expression were found in dose- and time-dependent manner in H. pylori infected AGS cells treated with rosiglitazone.

CONCLUSIONS

Rosiglitazone inhibited the growth of H. pylori infected AGS cells. Rosiglitazone attenuated Skp2 expression, thereby promoting p27 accumulation in H. pylori infected human gastric epithelial cells. Further studies will be needed to find the effects of accumulation on cell turnover in H. pylori infection and the role in the H. pylori-associated gastric carcinogenesis.

Adaptation and failure of pancreatic beta cells in murine models with different degrees of metabolic syndrome

The events that contribute to the expansion of beta-cell mass and enhanced beta-cell function in insulin-resistant states have not been elucidated fully. Recently, we showed that beta-cell adaptation failed dramatically in adult, insulin-resistant POKO mice, which contrasts with the appropriate expansion of beta cells in their ob/ob littermates. Thus, we hypothesised that characterisation of the islets in these mouse models at an early age should provide a unique opportunity to: (1) identify mechanisms involved in sensing insulin resistance at the level of the beta cells, (2) identify molecular effectors that contribute to increasing beta-cell mass and function, and (3) distinguish primary events from secondary events that are more likely to be present at more advanced stages of diabetes. Our results define the POKO mouse as a model of early lipotoxicity. At 4 weeks of age, it manifests with inappropriate beta-cell function and defects in proliferation markers. Other well-recognised pathogenic effectors that were observed previously in 16-week-old mice, such as increased reactive oxygen species (ROS), macrophage infiltration and endoplasmic reticulum (ER) stress, are also present in both young POKO and young ob/ob mice, indicating the lack of predictive power with regards to the severity of beta-cell failure. Of interest, the relatively preserved lipidomic profile in islets from young POKO mice contrasted with the large changes in lipid composition and the differences in the chain length of triacylglycerols in the serum, liver, muscle and adipose tissue in adult POKO mice. Later lipotoxic insults in adult beta cells contribute to the failure of the POKO beta cell. Our results indicate that the rapid development of insulin resistance and beta-cell failure in POKO mice makes this model a useful tool to study early molecular events leading to insulin resistance and beta-cell failure. Furthermore, comparisons with ob/ob mice might reveal important adaptive mechanisms in beta cells with either therapeutic or diagnostic potential.

PPARgamma-independent antitumor effects of thiazolidinediones

The thiazolidinedione (TZD) family of PPARgamma agonists, especially troglitazone and ciglitazone, induce cell cycle arrest, differentiation, and apoptosis in cancer cells. Mounting evidence indicates that TZDs interfere with multiple signaling mechanisms independently of PPARgamma activation, which affect many aspects of cellular functions governing cell cycle progression and survival of cancer cells. Here, we review the "off-target" mechanisms that underlie the antitumor effects of TZDs with emphasis on three key pathways, namely, inhibition of Bcl-2/Bcl-xL function, proteasomal degradation of cell cycle- and apoptosis-regulatory proteins, and transcriptional repression of androgen receptor (AR) through Sp1 degradation. Relative to tumor cells, nonmalignant cells are resistant to these PPARgamma-independent antitumor effects, which underscores the translational potential of these agents. Furthermore, dissociation of these antitumor effects from their PPARgamma agonist activity provides a rationale for using TZDs as scaffolds for lead optimization to develop a novel class of antitumor agents with a unique mode of mechanism.

Switching in discoid domain receptor expressions in SLUG-induced epithelial-mesenchymal transition

BACKGROUND

Acquired features of cells under epithelial-mesenchymal transition (EMT) have not yet been fully identified. The current study was conducted to assess alterations in both the proliferative potential and the responsiveness to extracellular matrices (ECMs) in EMT.

METHODS

MDCK cells and SLUG-transfected MDCK clones (SLUG-MDCK) were used in this study. The cell cycle was analyzed by using flow cytometry and Western blotting. ECM-stimulated cell proliferation was examined by using the following ECMs, type I collagen, type IV collagen, fibronectin, and laminin. Protein phosphorylation was detected by immunoprecipitation-Western by using the 4G10 antibody.

RESULTS

Both G1 and G2/M arrest were found in the SLUG-MDCK cells, and the responsible molecules for the cell-cycle arrests were, at least in part, p21WAF1/Cip1 and Wee1. Once in contact with type I collagen, the SLUG-MDCK cells, showing the Wee1 degradation, dramatically started to proliferate up to 6-fold in cell number at Day 5, in contrast to only a 2-fold increase in the control. The analysis of the collagen receptors in the SLUG-MDCK cells disclosed a striking increase in the discoid domain receptor (DDR) 2 expression and a clear decrease in the DDR1 expression. The immunoprecipitated DDR2 protein extracted from SLUG-MDCK cells, which were cultured on collagen for 30 minutes, was tyrosine-phosphorylated, indicating valid functionality of the up-regulated receptor. The altered expression from DDR1 to DDR2 was also found in the naturally dedifferentiated sister cell lines of human liver cancer.

CONCLUSIONS

Collectively, SLUG-induced EMT may alter the expression profile of receptor tyrosine kinases, including DDRs.

Overexpression of Jab1 in hepatocellular carcinoma and its inhibition by peroxisome proliferator-activated receptor{gamma} ligands in vitro and in vivo

PURPOSE

Jun activation domain-binding protein 1 (Jab1) is the fifth subunit of the COP9 signalosome and exhibits oncogenic activity. We investigated Jab1 expression in hepatocellular carcinoma (HCC) tissues and cell lines and tested the effect of peroxisome proliferator-activated receptor gamma (PPARgamma) ligands on Jab1 expression.

EXPERIMENTAL DESIGN

Jab1 expression in HCC tissues and cell lines was studied by real-time reverse transcription-PCR, immunohistochemical staining, and Western blotting. Promoter activity and chromatin immunoprecipitation assays were done to address the inhibition of Jab1 promoter by PPARgamma ligands. RNA interference was used to clarify PPARgamma ligand-induced inhibition of Jab1. Anticancer and Jab1-suppressing activity of PPARgamma ligands was tested in nude mice.

RESULTS

Jab1 was detected in the nucleus and cytoplasm of HCC tissues and 37% (37 of 99) of tissues exhibited Jab1 overexpression. Jab1 expression correlated with sex and hepatitis C virus infection, whereas it was negatively associated with hepatitis B virus infection. Additionally, Jab1 was overexpressed in HCC cell lines. PPARgamma ligands troglitazone and rosiglitazone down-regulated Jab1 expression in HCC cells, and troglitazone directly suppressed Jab1 promoter activity by inhibiting Sp1- and Tcf4-mediated transcription. This suppression was mediated via both PPARgamma-dependent and PPARgamma-independent mechanisms. Ectopic expression of Jab1 counteracted troglitazone-induced growth inhibition. Animal studies verified that intratumor or i.p. injection of troglitazone attenuated HCC growth and reduced Jab1 expression in tumor tissues.

CONCLUSIONS

Our results indicate that Jab1 is overexpressed in HCC and PPARgamma ligands may suppress Jab1 to inhibit the proliferation of HCC cells.

The aorta-gonad-mesonephros-derived stroma cell line DAS104-4 induces differentiation of leukemic cells

While critical steps in the regulation of leukemia cell development have been intensively studied in recent years, less is known about the interactions of leukemic cells with their stroma. Previously, we have shown that human acute myeloid leukemia (AML) cells differentiate upon injection into murine blastocysts. We here describe that human AML Kasumi-1 cells, cocultured with murine aorta-gonad-mesonephros (AGM) region-derived DAS104-4 stromal cells, decrease proliferation and colony formation efficiency; and up-regulate myelo-monocytic cell surface markers. Gene expression analysis showed decreased transcription of the AML1-ETO fusion gene and increased transcription of p16 (INK4A), p21 (WAF1) and C/EBPalpha genes. Coculture can induce myeloid differentiation also in patient-derived AML cells. Our findings strengthen the notion that the embryonic milieu can regulate the proliferation and differentiation of leukemic cells.

Thiazolidinediones modulate the expression of beta-catenin and other cell-cycle regulatory proteins by targeting the F-box proteins of Skp1-Cul1-F-box protein E3 ubiquitin ligase independently of peroxisome proliferator-activated receptor gamma

Considering the role of aberrant beta-catenin signaling in tumorigenesis, we investigated the mechanism by which the peroxisome proliferator-activated receptor gamma (PPARgamma) agonist troglitazone facilitated beta-catenin down-regulation. We demonstrate that troglitazone and its more potent PPARgamma-inactive analogs Delta2TG and STG28 mediated the proteasomal degradation of beta-catenin in prostate cancer cells by up-regulating the expression of beta-transducin repeat-containing protein (beta-TrCP), an F-box component of the Skp1-Cul1-F-box protein E3 ubiquitin ligase. Evidence indicates that although small interfering RNA-mediated beta-TrCP knockdown protected cells against STG28-facilitated beta-catenin ablation, ectopic beta-TrCP expression enhanced the degradation. The involvement of beta-TrCP in beta-catenin degradation was also corroborated by the pull-down analysis and the concurrent down-regulation of known beta-TrCP substrates examined, including Wee1, Ikappabetaalpha, cdc25A, and nuclear factor-kappaB/p105. Furthermore, glycogen synthase kinase-3beta represented a key regulator in the effect of these thiazolidinedione derivatives on beta-catenin proteolysis even though these agents increased its phosphorylation level. It is noteworthy that this drug-induced beta-TrCP up-regulation was accompanied by the concomitant down-regulation of Skp2 and Fbw7, thereby affecting many of the target proteins of these two F-box proteins (such as p27 and cyclin E). As a consequence, the ability of troglitazone to target these F-box proteins provides a molecular basis to account for its reported effect on modulating the expression of aforementioned cell-cycle regulatory proteins. Despite this complicated mode of pharmacological actions, normal prostate epithelial cells, relative to LNCaP cells, were less susceptible to the effects of STG28 on modulating the expression of beta-catenin and beta-TrCP, suggesting the translation potential of using STG28 as a scaffold to develop more potent chemopreventive agents.

The PPARgamma agonist pioglitazone inhibits early neoplastic occurrence in the rat liver

Hepatocellular carcinoma (HCC) is increasing worldwide and is the fifth main cause of cancer-related death. HCC develops on a preneoplastic organ, the cirrhotic liver. Therefore, chemoprevention could play a role in the therapy of HCC. We evaluated the preventive effects of pioglitazone, a peroxisome proliferator-activated receptor gamma agonist, on the induction of early carcinogenic events. We monitored pre-neoplastic foci induced by a two-stage initiation/promotion model of hepatocarcinogenesis in rats, using diethylnitrosamine and acetylaminofluorene. Pioglitazone treatment was initiated the day after the first diethylnitrosamine injection. By quantitative morphometry and Western blot, we showed that pioglitazone significantly decreases the size of pre-neoplastic foci. Analysis of proliferation and apoptosis, assessed by immunohistochemistry, demonstrated decreased proliferation but no effect on cell death in rats treated with pioglitazone. These events were associated with an increased expression of the cyclin-dependent kinase inhibitor p27(kip1), compared to the non treated group. In conclusion, pioglitazone inhibits early carcinogenic transformation in a two-step rat model. As pioglitazone has a low toxicity profile, we believe it would be interesting to evaluate its effect in chemoprevention of HCC in humans in a clinical setting.

Luteolin promotes degradation in signal transducer and activator of transcription 3 in human hepatoma cells: an implication for the antitumor potential of flavonoids

In this study, we have investigated the underlying molecular mechanism for the potent proapoptotic effect of luteolin on human hepatoma cells both in vitro and in vivo, focusing on the signal transducer and activator of transcription 3 (STAT3)/Fas signaling. A clear apoptosis was found in the luteolin-treated HLF hepatoma cells in a time- and dosage-dependent manner. In concert with the caspase-8 activation by luteolin, an enhanced expression in functional Fas/CD95 was identified. Consistent with the increased Fas/CD95 expression, a drastic decrease in the Tyr(705) phosphorylation of STAT3, a known negative regulator of Fas/CD95 transcription, was found within 20 minutes in the luteolin-treated cells, leading to down-regulation in the target gene products of STAT3, such as cyclin D1, survivin, Bcl-xL, and vascular endothelial growth factor. Of interest, the rapid down-regulation in STAT3 was consistent with an accelerated ubiquitin-dependent degradation in the Tyr(705)-phosphorylated STAT3, but not the Ser(727)-phosphorylated one, another regulator of STAT3 activity. The expression level of Ser(727)-phosphorylated STAT3 was gradually decreased by the luteolin treatment, followed by a fast and clear down-regulation in the active forms of CDK5, which can phosphorylate STAT3 at Ser(727). An overexpression in STAT3 led to resistance to luteolin, suggesting that STAT3 was a critical target of luteolin. In nude mice with xenografted tumors using HAK-1B hepatoma cells, luteolin significantly inhibited the growth of the tumors in a dosage-dependent manner. These data suggested that luteolin targeted STAT3 through dual pathways-the ubiquitin-dependent degradation in Tyr(705)-phosphorylated STAT3 and the gradual down-regulation in Ser(727)-phosphorylated STAT3 through inactivation of CDK5, thereby triggering apoptosis via up-regulation in Fas/CD95.

F-box protein Skp2: a novel transcriptional target of E2F

The F-box-containing protein Skp2 plays a critical role in coordinating the G1/S transition and progression through the S phase of the mammalian cell cycle. Skp2 is overexpressed in a broad spectrum of human cancers and the expression level correlates with tumor malignancy. However, the Skp2 gene is neither amplified nor rearranged in most human cancers and the underlying mechanism of Skp2 overexpression remains poorly understood. We show here that the Skp2 gene contains a functional E2F response element (hSRE2). Ectopic expression of E2F1 induces expression of the endogenous Skp2 gene in human fibroblast cells, whereas antisense-mediated knockdown of E2F1 in human tumor cell lines reduces expression of endogenous Skp2 gene. The hSRE2 element not only participates in activation of Skp2 promoter function during normal cell cycle progression into S phase, it is also required for the high-level Skp2 gene expression in many human tumor cell lines. These results reveal Skp2 as a novel target for E2F regulation that is disrupted in several human tumor cell lines.

Pioglitazone, a synthetic ligand for PPARγ, induces apoptosis in RB-deficient human colorectal cancer cells

No published data are available about the expression of peroxisome proliferator-activated receptor gamma (PPARgamma) and the role of PPARgamma in retinoblastoma protein (RB)-deficient human colorectal cancer (CRC) cells (SNU-C4 and SNU-C2A). Our aim was to investigate whether PPARgamma is expressed in SNU-C4 and SNU-C2A cells and to elucidate possible molecular mechanisms underlying the effect of pioglitazone, a synthetic ligand for PPARgamma, on cell growth in these cell lines. RT-PCR and Western blot analysis showed that both human CRC cell lines expressed PPARgamma mRNA and protein. Pioglitazone inhibited the cell growth of both cell lines through G2/M phase block and apoptosis. In addition, pioglitazone caused a down-regulation of the X chromosome-linked inhibitor of apoptosis (XIAP), Bcl-2, and cyclooxygenase-2 (COX-2) under conditions leading to PPARgamma down-regulation. These results suggest that pioglitazone may have therapeutic relevance or significance in the treatment of human CRC, and the down-regulation of XIAP, Bcl-2, and COX-2 may contribute to pioglitazone-induced apoptosis in these and other RB-deficient cell lines and tumors.

RNA silencing of Cks1 induced G2/M arrest and apoptosis in human lung cancer cells

Cdc kinase subunit 1 (Cks1) has been shown to involve in the regulation of cell cycle progression and p27Kip1 degradation. To define the role of Cks1 in lung tumorigenesis, we examined the expression of Cks1 in human lung cancer cell lines and tested the effect of Cks1-specific small interfering RNA (siRNA) on these cells. Reverse transcriptase-polymerase chain reaction (RT-PCR) and western blot analysis showed that Cks1 was highly expressed in human lung cancer cells. Transfection of Cks1 siRNA down-regulated Cdc2 kinase activity and induced G2/M arrest in Cks1- overexpressing H358 lung cancer cells. Long-term treatment of Cks1 siRNA induced caspase activation and apoptosis in H358 cells. On the contrary, Cks1 siRNA did not affect viability of normal human lung fibroblasts under the same experimental condition. Collectively, our results suggest that Cks1 participates in the steps of lung tumorigenesis and this gene may be a target for the treatment of lung cancer.

Cyclooxygenase-2 in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the fifth most common cancer and the third leading cause of cancer related mortality worldwide. The incidence of HCC is rising worldwide, especially in the United States. The overall survival of patients with HCC is grim and currently no efficient secondary prevention or systemic treatments are available. Recent evidence suggests that COX-2 signaling is implicated in hepatocarcinogenesis and COX-2 inhibitors prevent HCC cell growth in vitro and in animal models. However, given the recently reported side effect associated with some of the COX-2 inhibitors, it is imperative to develop chemotherapeutic strategy that simultaneously targets COX-2 and other related key molecules in hepatocarcinogenesis or to utilize agents inhibiting COX-2 signaling in conjunction with other standard chemotherapy or radiation therapy. Such combinational therapeutic approaches are expected to provide synergistic anti-tumor effect with lesser side effect. In this regard, the recently delineated interplay between COX-2-derived PG signaling and other growth-regulatory pathways such as EGFR, Met, iNOS, VEGF and n-3 polyunsaturated fatty acids is expected to provide important therapeutic implications. This review summarizes the recent advances in understanding the mechanisms for COX-2-derived PG signaling in hepatocarcinogenesis and focuses on the newly unveiled interactions between PG cascade and other key signaling pathways that coordinately regulate HCC growth. Understanding these mechanisms and interplays will facilitate the development of more effective chemopreventive and therapeutic strategies.

Ciglitazone-induced p27 gene transcriptional activity is mediated through Sp1 and is negatively regulated by the MAPK signaling pathway

We have previously demonstrated that the PPARgamma ligand, ciglitazone, increases p27kip1 protein levels in HT-29 colon cancer cells through both inhibition of proteasome associated degradation and activation of transcriptional activity. [F. Chen, L.E. Harrison, Cell Signal. 17 (2005) 809] The purpose of this investigation was to further elucidate the mechanism of ciglitazone-induced activation of p27 gene transcription. We observed that the region -774/-462 of the p27 promoter plays a key role in ciglitazone-induced gene transcriptional activity and this region contains two Sp1 binding sites. When the p27PF-luc reporter was co-transfected with Sp1 expression plasmids, ciglitazone-induced p27PF-luc activity significantly increased, while mithramycin A, a Sp1 inhibitor, was able to abrogate its effects. Ciglitazone exposure increased both Sp1 protein expression and Sp1-DNA binding, which was also associated with a decrease of Erk1/2 phosphorylation. A similar increase of Sp1-DNA binding was observed when phosphorylation of Erk1/2 was inhibited by pretreatment with the MAP kinase inhibitor, U0126. In addition, a significant increase of p27PF-luc reporter luciferase activity was noted after MAP kinase inhibition, which could be abolished with co-treatment with mithramycin A. Based on these data, we postulate that ciglitazone induces p27 gene transcription through increased Sp1 binding to its promoter region, which in turn is mediated through increased Sp1 protein levels and decreased inhibitory regulation by the MAP kinase pathway.

Pivotal role of peroxisome proliferator-activated receptor gamma (PPARgamma) in regulation of erythroid progenitor cell proliferation and differentiation

OBJECTIVE

The aim of this study was to reveal the role of peroxisome proliferator-activated receptor gamma (PPARgamma) in erythropoiesis.

METHODS

The effects of PPARgamma ligands on cellular proliferation and differentiation were investigated in erythroid colony-forming cells (ECFCs) purified from human peripheral blood.

RESULTS

RT-PCR analysis revealed that PPARgamma mRNA is expressed in ECFCs. Synthetic PPARgamma ligands, troglitazone or pioglitazone, suppressed cellular proliferation without inducing apoptosis and delayed maturation of ECFCs, as determined by flow cytometry. The delay in erythroid maturation by troglitazone was confirmed by the down-regulation of gamma-globin, beta-globin and GATA-1 mRNA, and the maintenance of GATA-2 mRNA.

CONCLUSIONS

Our results suggest that PPARgamma modulates the differentiation process of erythroid progenitor cells, and plays a crucial role in regulating the balance of hematopoiesis.