Comparison of the mechanism of induction of apoptosis in ovarian carcinoma cells by the conformationally restricted synthetic retinoids CD437 and 4-HPR

Vitamins in Gynecologic Malignancies

The combination of vitamin A and D derivatives with classical chemotherapeutic treatments results in more satisfactory outcomes. The use of drug combinations, such as 9cUAB130 with carboplatin and cisplatin with TAC-101, shows enhanced cytotoxic effects and reductions in ovarian tumor volume compared to single-drug treatments. Combining cisplatin with calcitriol and progesterone increases VDR expression, potentially enhancing the effectiveness of anticancer therapy in ovarian cancer. The effectiveness of vitamin derivatives in anticancer treatment may vary depending on the characteristics of the tumor and the cell line from which it originated. An increase in thiamine intake of one unit is associated with an 18% decrease in HPV infection. Higher intake of vitamin C by 50 mg/day is linked to a lower risk of cervical neoplasia. Beta-carotene, vitamin C, and vitamin E are associated with risk reductions of 12%, 15%, and 9% in endometrial cancer, respectively. A balanced daily intake of vitamins is important, as both deficiency and excess can influence cancer development. It has been observed that there is a U-shaped relationship between group B vitamins and metabolic markers and clinical outcomes.

Supramolecular Self-Assembly Mediated by Multiple Hydrogen Bonds and the Importance of C-S···N Chalcogen Bonds in N'-(Adamantan-2-ylidene)hydrazide Derivatives

The present article comprehensively examines six N′-(adamantan-2-ylidene)hydrazide derivatives using the Hirshfeld surface analysis, PIXEL energy for molecular dimers, lattice energies for crystal packing, and topological analysis for intramolecular and intermolecular interactions. The crystal structure of one of the N′-(adamantan-2-ylidene)hydrazide derivatives, namely, N′-(adamantan-2-ylidene)-5-bromothiophene-2-carbohydrazide 1, C₁₅H₁₇N₂OSBr, has been determined and analyzed in detail along with five closely related structures. The molecular conformation of 1 is locked by an intramolecular C–S···N chalcogen bond as found in one of its closely related structure, namely, N′-(adamantan-2-ylidene)thiophene-2-carbohydrazide. Furthermore, a detailed potential energy surface scan analysis has been performed to highlight the importance of a chalcogen bond. Two of these compounds possess syn-orientation for amide units, whereas the corresponding moiety exhibits anti-conformations in the remaining four structures. The Hirshfeld surface and its decomposed fingerprint plots provide a qualitative picture of acyl substituent effects on the intermolecular interactions toward crystal packing of these six structures. Intermolecular interaction energies for dimers observed in these structures calculated by density functional theory (B97D3/def2-TZVP) and PIXEL (MP2/6-31G**) methods are comparable. This study also identifies that multiple hydrogen bonds, including N/C–H···O/N and C–H···π interactions, are collectively responsible for a self-assembled synthon. The nature and strength of these interactions have been studied using atoms in molecule topological analysis. The in vitro antiproliferative activity of compound 1 was assessed against five human tumor cell lines and showed marked antiproliferative activity.

Bactericidal and Anti-biofilm Activity of the Retinoid Compound CD437 Against Enterococcus faecalis

Enterococcus faecalis (E. faecalis), a biofilm-forming pathogen, causes nosocomial infections. In recent years, drug resistance by enterococci has become increasingly severe due to widespread antibiotic abuse. Therefore, novel antibacterial agents are urgently needed. In this study, the synthetic retinoid compound CD437 was found to have potent bactericidal effect on E. faecalis. In addition, CD437 exhibited synergistic effects when administered in combination with gentamicin and additive effects when combined with ceftriaxone sodium. CD437 also inhibited biofilm formation by E. faecalis and exerted bactericidal effect on mature biofilm. Moreover, CD437 exhibited antibacterial and anti-biofilm effects against Staphylococcus. No bactericidal action of CD437 was observed against the gram-negative bacillus, but Pseudomonas aeruginosa biofilm extracellular polymeric substances (EPS) matrix formation was reduced. Overall, these findings indicate that CD437 has the potential to be developed as a novel antibacterial drug.

Fenretinide inhibits macrophage inflammatory mediators and controls hypertension in spontaneously hypertensive rats via the peroxisome proliferator-activated receptor gamma pathway

Fenretinide is a novel anticancer agent reported to exhibit anti-invasive and antimetastatic activities. It has also been shown to improve obesity and diabetes, although the effects of fenretinide on hypertension are still unknown, and the detailed mechanisms remain unclear. In this study, we have shown that treatment with lipopolysaccharide (LPS) decreased the expression of peroxisome proliferator-activated receptor γ (PPARγ) in RAW264.7 macrophages, and pretreatment with fenretinide reversed the effect of LPS on PPARγ expression. In addition, LPS-induced pro-inflammatory cytokine production, including tumor necrosis factor-α, interleukin 6, and monocyte chemoattractant protein 1 were dose-dependently reversed by fenretinide, and the effects of fenretinide on LPS-induced pro-inflammatory cytokine production were blocked by treatment with PPARγ antagonist. Moreover, fenretinide decreased LPS-induced inducible nitric oxide synthase expression and nitrogen oxide production. These effects were blocked by the pretreatment with PPARγ antagonist in a dose-dependent manner, indicating fenretinide activated PPARγ to exert anti-inflammation activity. In view of the role of inflammation in hypertension and the anti-inflammatory action of fenretinide, we found that administration of fenretinide in spontaneously hypertensive rats significantly decreased blood pressure. Taken together, these results indicate that fenretinide might be a potent antihypertensive agent that works by suppressing inflammation via activating PPARγ.

The antitumor toxin CD437 is a direct inhibitor of DNA polymerase α

CD437 is a retinoid-like small molecule that selectively induces apoptosis in cancer cells, but not in normal cells, through an unknown mechanism. We used a forward-genetic strategy to discover mutations in POLA1 that coincide with CD437 resistance (POLA1(R)). Introduction of one of these mutations into cancer cells by CRISPR-Cas9 genome editing conferred CD437 resistance, demonstrating causality. POLA1 encodes DNA polymerase α, the enzyme responsible for initiating DNA synthesis during the S phase of the cell cycle. CD437 inhibits DNA replication in cells and recombinant POLA1 activity in vitro. Both effects are abrogated by the identified POLA1 mutations, supporting POLA1 as the direct antitumor target of CD437. In addition, we detected an increase in the total fluorescence intensity and anisotropy of CD437 in the presence of increasing concentrations of POLA1 that is consistent with a direct binding interaction. The discovery of POLA1 as the direct anticancer target for CD437 has the potential to catalyze the development of CD437 into an anticancer therapeutic.

Targeting truncated RXRα for cancer therapy

Retinoid X receptor-alpha (RXRα), a unique member of the nuclear receptor superfamily, is a well-established drug target, representing one of the most important targets for pharmacologic interventions and therapeutic applications for cancer. However, how RXRα regulates cancer cell growth and how RXRα modulators suppress tumorigenesis are poorly understood. Altered expression and aberrant function of RXRα are implicated in the development of cancer. Previously, several studies had demonstrated the presence of N-terminally truncated RXRα (tRXRα) proteins resulted from limited proteolysis of RXRα in tumor cells. Recently, we discovered that overexpression of tRXRα can promote tumor growth by interacting with tumor necrosis factor-alpha-induced phosphoinositide 3-kinase and NF-κB signal transduction pathways. We also identified nonsteroidal anti-inflammatory drug Sulindac and analogs as effective inhibitors of tRXRα activities via a unique binding mechanism. This review discusses the emerging roles of tRXRα and modulators in the regulation of cancer cell survival and death as well as inflammation and our recent understanding of tRXRα regulation by targeting the alternate binding sites on its surface.

Non-genomic effects of the NR4A1/Nur77/TR3/NGFIB orphan nuclear receptor

The orphan nuclear receptor NR4A1/Nur77/TR3/NGFIB acts primarily as a transcription factor to regulate the expression of multiple genes. However, increasing research attention has recently been given to non-genomic activities of NR4A1. The first description of a non-genomic action of NR4A1 referred to the conversion of anti-apoptotic Bcl-2 into a pro-apoptotic protein by direct interaction with NR4A1. In response to certain apoptotic stimuli, NR4A1 translocates from the nucleus to the mitochondrial outer membrane (MOM) where it associates with Bcl-2 and thereby causes apoptosis. Afterwards, it appeared that NR4A1 could also bind and convert other anti-apoptotic Bcl-2 family members. The latest studies indicate a significant role of NR4A1 in the process of autophagy. For example, a new NR4A1-mediated pathway specific for melanoma cells has been described where NR4A1 interacts with the adenine nucleotide translocase 1 (ANT1) on the mitochondrial inner membrane (MIM) leading to induction of the autophagy pathway. Moreover, NR4A1 interaction with cytoplasmic p53 may also contribute to the induction of autophagy. In addition to mitochondria, NR4A1 could be translocated to the outer membrane of the endoplasmic reticulum (ER) and associate with Bcl-2 or translocon-associated protein subunit γ (TRAPγ) causing ER stress-induced apoptosis. NR4A1 also contributes to the proteasomal degradation of β-catenin in colon cancer cells in vitro and in vivo, as well as to the stabilization of hypoxia-inducible factor-1α (HIF-1α) under non-hypoxic conditions. This review summarizes research findings on non-genomic effects of NR4A1 in normal and cancer cells.

Combination of fenretinide and selenite inhibits proliferation and induces apoptosis in ovarian cancer cells

The combination of fenretinide and selenite on ovarian cancer cells was investigated to assess its effects on proliferation and ability to induce apoptosis. Our results showed that fenretinide and selenite in combination significantly suppress the proliferation of ovarian cancer cells and induced apoptosis (including reactive oxygen species generation, and the loss of mitochondrial membrane potential) compared with either drug used alone. The caspase3/9-dependent pathway was triggered significantly in combination treatment, and moreover, the AMPK pathway also mediated the apoptosis induction in fenretinide and selenite combination. Fenretinide and selenite combination treatment was demonstrated to suppress tumor growth in vivo, this drug combination has been thus found to have an enhanced anti-tumor effect on ovarian cancers cells.

The HPV E2-Host Protein-Protein Interactions: A Complex Hijacking of the Cellular Network

Over 100 genotypes of human papillomaviruses (HPVs) have been identified as being responsible for unapparent infections or for lesions ranging from benign skin or genital warts to cancer. The pathogenesis of HPV results from complex relationships between viral and host factors, driven in particular by the interplay between the host proteome and the early viral proteins. The E2 protein regulates the transcription, the replication as well as the mitotic segregation of the viral genome through the recruitment of host cell factors to the HPV regulatory region. It is thereby a pivotal factor for the productive viral life cycle and for viral persistence, a major risk factor for cancer development. In addition, the E2 proteins have been shown to engage numerous interactions through which they play important roles in modulating the host cell. Such E2 activities are probably contributing to create cell conditions appropriate for the successive stages of the viral life cycle, and some of these activities have been demonstrated only for the oncogenic high-risk HPV. The recent mapping of E2-host protein-protein interactions with 12 genotypes representative of HPV diversity has shed some light on the large complexity of the host cell hijacking and on its diversity according to viral genotypes. This article reviews the functions of E2 as they emerge from the E2/host proteome interplay, taking into account the large-scale comparative interactomic study.

The use of retinoids in ovarian cancer: a review of the literature

Ovarian cancer is the deadliest of all gynecologic malignancies. The search for novel treatment modalities to augment traditional chemotherapy and improve quality of life is ongoing. Retinoids, a class of compounds composed of vitamin A, its natural derivatives, and synthetic analogs, have been studied extensively in both the prevention and treatment of gynecologic malignancies. In this article, we reviewed preclinical studies and clinical trials conducted using retinoids in ovarian cancer.

Fenretinide (4-HPR): a preventive chance for women at genetic and familial risk?

The incidence and mortality of breast cancer have been recently influenced by several new therapeutic strategies. In particular our knowledge on cancer precursors, risk biomarkers, and genetics has considerably increased, and prevention strategies are being successfully explored. Since their discovery, retinoids, the natural and synthetic derivatives of vitamin A, have been known to play a crucial role in cell and tissue differentiation and their ability to inhibit carcinogenesis has made them the ideal chemopreventive agents studied in several preclinical and clinical trials. Fenretinide (4-HPR) is the most studied retinoid in breast cancer chemoprevention clinical trials due to its selective accumulation in breast tissue and its favorable toxicological profile. This agent showed a significative reduction of the incidence of second breast tumors in premenopausal women confirmed after 15-year followups. Considering Fenretinide protective action, a similar trend on ovarian cancer, this drug warrants reevaluations as a preventive agent for high-risk young women, such as BRCA-1 and 2 mutation carriers or with a high familial risk. This favorable effect therefore provides a strong rationale for a primary prevention trial in these unaffected cohort of women.

The impact of novel retinoids in combination with platinum chemotherapy on ovarian cancer stem cells

OBJECTIVE

Retinoids are important modulators of cell growth, differentiation, and proliferation. 9cUAB30, 9cUAB124, and 9cUAB130 are three novel retinoid compounds that show cytotoxic effects in other malignancies. We evaluated these novel retinoids in combination with chemotherapy against ovarian cancer stem cells (CSCs) in vitro and in an ex vivo model.

METHODS

A2780 cells were plated in 96-well plates and treated with retinoid, carboplatin, or combination therapy. Cell viability was evaluated using ATPLite assay. The A2780 cell line was also analyzed for CSCs by evaluating ALDH activity using flow cytometry. A2780 cells treated ex vivo with retinoids and chemotherapy were injected into the flank of athymic nude mice in order to evaluate subsequent tumor initiating capacity.

RESULTS

A2780 cells were sensitive to treatment with retinoids and carboplatin. The best treatment resulted from the combination of retinoid 9cUAB130 and carboplatin. Untreated A2780 cells demonstrated ALDH activity in 3.3% of the cell population. Carboplatin treatment enriched ALDH activity to 27.3%, while 9cUAB130±carboplatin maintained the ALDH positive levels similar to untreated controls (2.3% and 6.7%, respectively). Similar results were found in tumorsphere-forming conditions. Flank injections of ex vivo treated A2780 cells resulted in 4/4 mice developing tumors at 40 days in the untreated group, while 0/4 tumors developed in the 9cUAB130 and carboplatin treated group.

CONCLUSION

Combination treatment with carboplatin and retinoids reduced cell-viability, reduced CSC marker expression, and inhibited tumorigenicity, making it a more effective treatment when compared with carboplatin alone.

Enhanced anti-neuroblastoma activity of a fenretinide complexed form after intravenous administration

OBJECTIVES

The major limitation to successful chemotherapy of neuroblastoma (NB) is the toxicity and the poor bioavailability of traditional drugs.

METHODS

We synthesised an amphiphilic dextrin derivative (DX-OL) able to host fenretinide (4-HPR) by complexation. In this study, we have investigated the effects of 4-HPR-loaded amphipilic dextrin (DX-OL/4-HPR) in comparison with 4-HPR alone both in vitro on human NB cells and in vivo in pseudometastatic NB models. The haemolysis assay was used as a measure of the potential damage caused by the pharmaceutical formulation in vivo. Pharmacokinetic experiments were performed to assess drug plasma levels in mice treated with free or complexed 4-HPR.

KEY FINDINGS

DX-OL/4-HPR exerted a more potent cytotoxic activity on NB cells. Complexed 4-HPR significantly increased the proportion of sub-G1 cells with respect to free 4-HPR. Dextrin derivatives showed no haemolytic activity, indicating their suitability for parenteral administration. DX-OL/4-HPR increased the lifespan and the long-term survival of treated mice over controls. The analysis of drug plasma levels indicates that the complexed drug has a higher AUC due to a reduced clearance from the blood.

CONCLUSIONS

Our data suggest that DX-OL/4-HPR is an injectable formulation that is able to improve drug aqueous solubility and bioavailability.

Enhanced cell cycle perturbation and apoptosis mediate the synergistic effects of ST1926 and ATRA in neuroblastoma preclinical models

Retinoic acid therapy is nowadays an important component of treatment for residual disease of stage IV neuroblastoma after multimodal therapy. Nevertheless, arising resistance and treatment toxicity could represent relevant limiting factors. In the present study, we show that retinoic acid enhances the cytostatic and apoptogenic properties of the novel adamantyl retinoid ST1926 in a panel of neuroblastoma cells with different p53 status and caspase 8 expression, resulting in synergistic effects as assessed by Combination Index and Isobologram analysis. Under conditions where the two drugs alone produced no toxic effects, their combination resulted in enhanced G2-M arrest and sub-G1 population as shown by BrdU pulse-chase and labeling experiments. PARP cleavage, caspase 3, 8 and 9 activation and modulation of DR4 and FAS were indicative of enhanced apoptosis triggered by the co-incubation of the two drugs whereas neither ST1926-mediated genotoxic damage nor ATRA-differentiating effects were affected by the combined treatment. Caspase-3 and 8-mediated apoptosis appeared to play an important role in the drugs synergism. In fact, the addition of a pan-caspase inhibitor ZVAD-FMK reverted this effect in SK-N-DZ cells, and synergism was confined to limited drugs doses in HTLA cells not expressing caspase-8. Although not modulated, p53 appeared to enhance cells responsiveness to retinoid/ATRA combination. In vivo studies in the most sensitive neuroblastoma model SK-N-DZ, confirmed enhanced activity of the drugs combination vs single treatments. The study provides important lines of evidence that such a drugs combination could represent a less toxic and more effective approach for maintenance treatment in children with neuroblastoma.

Preferential involvement of both ROS and ceramide in fenretinide-induced apoptosis of HL60 rather than NB4 and U937 cells

Leukemic cells responding to apoptosis-inducing drugs can be varied in terms of the mechanisms of action. Fenretinide, a synthetic retinoid, is worth of study as a promising candidate for apoptosis-based therapy of leukemia. Yet, it remains unclear whether this drug exerts the similar mechanisms on different leukemic cells. Here, we report a comparative analysis of fenretinide-induced apoptosis in three acute myeloid leukemic (AML) cell lines including HL60, NB4 and U937. Through a series of antagonist assays, we revealed similarities and differences of mechanisms involved in these three cell lines. Antioxidant vitamin C completely abrogated fenretinide-induced apoptosis in all cell lines, demonstrating that ROS is an essential and common mediator. However, the apoptotic effects of fenretinide could be blocked by ceramide synthase inhibitor fumonisin B1 only in HL60 rather than the other two. Moreover, fumonisin B1 was unable to inhibit the generation of ROS in fenretinide-treated HL60 cells, indicating that ROS may function as upstream stimulus of ceramide-mediated apoptosis. These comparative results strongly suggest that the apoptotic response induced by fenretinide in HL60 involves both ROS and ceramide, whereas drug-induced apoptosis in NB4 and U937 requires ROS but is independent of ceramide. Differentiated modes of action exerting on AML may guide the use of this apoptosis-inducing drug, and hence advance our knowledge about the nature of cancer-specific responses to this drug.

Targeting NR4A1 (TR3) in cancer cells and tumors

INTRODUCTION

Nuclear receptor 4A1(NR4A1) (testicular receptor 3 (TR3), nuclear hormone receptor (Nur)77) is a member of the nuclear receptor superfamily of transcription factors and is highly expressed in multiple tumor types. RNA interference studies indicate that NR4A1 exhibits growth-promoting, angiogenic and prosurvival activity in most cancers.

AREAS COVERED

Studies on several apoptosis-inducing agents that activate nuclear export of NR4A1, which subsequently forms a mitochondrial NR4A1-bcl-2 complex that induces the intrinsic pathway for apoptosis are discussed. Cytosporone B and related compounds that induce NR4A1-dependent apoptosis in cancer cells through both modulation of nuclear NR4A1 and nuclear export are discussed. A relatively new class of diindolylmethane analogs (C-DIMs) including 1,1-bis(3'-indolyl)-1-(p-methoxyphenyl)methane (DIM-C-pPhOCH(3)) (NR4A1 activator) and 1,1-bis(3'-indolyl)-1-(p-hydroxyphenyl)methane (DIM-C-pPhOH) (NR4A1 deactivator) are discussed in more detail. These anticancer drugs (C-DIMs) act strictly through nuclear NR4A1 and induce apoptosis in cancer cells and tumors.

EXPERT OPINION

It is clear that NR4A1 plays an important pro-oncogenic role in cancer cells and tumors, and there is increasing evidence that this receptor can be targeted by anticancer drugs that induce cell death via NR4A1-dependent and -independent pathways. Since many of these compounds exhibit relatively low toxicity, they represent an important class of mechanism-based anticancer drugs with excellent potential for clinical applications.

Retinoid pathway and cancer therapeutics

The retinoids are a class of compounds that are structurally related to vitamin A. Retinoic acid, which is the active metabolite of retinol, regulates a wide range of biological processes including development, differentiation, proliferation, and apoptosis. Retinoids exert their effects through a variety of binding proteins including cellular retinol-binding protein (CRBP), retinol-binding proteins (RBP), cellular retinoic acid-binding protein (CRABP), and nuclear receptors i.e. retinoic acid receptor (RAR) and retinoid x receptor (RXR). Because of the pleiotropic effects of retinoids, understanding the function of these binding proteins and nuclear receptors assists us in developing compounds that have specific effects. This review summarizes our current understanding of how retinoids are processed and act with an emphasis on the application of retinoids in cancer treatment and prevention.

Activation of nerve growth factor-induced B alpha by methylene-substituted diindolylmethanes in bladder cancer cells induces apoptosis and inhibits tumor growth

Nerve growth factor-induced B (NGFI-B) genes are orphan nuclear receptors, and NGFI-B alpha (Nur77, TR3) is overexpressed in bladder tumors and bladder cancer cells compared with nontumorous bladder tissue. 1,1-Bis(3'-indolyl)-1-(p-methoxyphenyl)-methane (DIM-C-pPhOCH(3)) and 1,1-bis(3'-indolyl)-1-(p-phenyl)methane have previously been identified as activators of Nur77, and both compounds inhibited growth and induced apoptosis of UC-5 and KU7 bladder cancer cells. The proapoptotic effects of methylene-substituted diindolylmethanes (C-DIMs) were unaffected by cotreatment with leptomycin B and were dependent on nuclear Nur77, and RNA interference with a small inhibitory RNA for Nur77 (iNur77) demonstrated that C-DIM-induced activation of apoptosis was Nur77-dependent. Microarray analysis of DIM-C-pPhOCH(3)-induced genes in UC-5 bladder cancer cells showed that this compound induced multiple Nur77-dependent proapoptotic or growth inhibitory genes including tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), cystathionase, p21, p8, and sestrin-2. DIM-C-pPhOCH(3) (25 mg/kg/d) also induced apoptosis and inhibited tumor growth in athymic nude mice bearing KU7 cells as xenografts, demonstrating that Nur77-active C-DIMs exhibit potential for bladder cancer chemotherapy by targeting Nur77, which is overexpressed in this tumor type.

Unraveling insulin-like growth factor binding protein-3 actions in human disease

The IGF system plays critical roles in somatic growth in an endocrine fashion (somatomedin hypothesis) as well as proliferation and differentiation of normal and malignant cells in a paracrine/autocrine fashion. IGFBP-3 is known to modulate the actions of IGFs in circulation as well as the immediate extracellular environment. Interestingly, apart from the ability to inhibit or enhance IGF actions, IGFBP-3 also exhibits very clear, distinct biological effects independent of the IGF/IGF-I receptor axis. Over the past decade it has become widely appreciated that IGF/IGF-IR-independent actions of IGFBP-3 (antiproliferative and proapoptotic effects) contribute to improving the pathophysiology of a variety of human diseases, such as cancer, diabetes, and malnutrition. Recent studies have implicated interaction of IGFBP-3 with a variety of proteins or signaling cascades critical to cell cycle control and apoptosis; however, the actual mechanism of IGFBP-3 action is still unclear. This review reinforces the concept in support of the IGF/IGF-IR axis-independent actions of IGFBP-3 and delineates potential underlying mechanisms involved and subsequent biological significance, focusing in particular on functional binding partners and the clinical significance of IGFBP-3 in the assessment of cancer risk.

Chromodomain helicase/adenosine triphosphatase DNA binding protein 1-like (CHD1l) gene suppresses the nucleus-to-mitochondria translocation of nur77 to sustain hepatocellular carcinoma cell survival

Amplification of 1q21 has been detected in 58% to 78% of primary hepatocellular carcinoma cases, suggesting that one or more oncogenes within the amplicon play a critical role in the development of this disease. The chromodomain helicase/adenosine triphosphatase DNA binding protein 1-like gene (CHD1L) is a recently identified oncogene localized at 1q21. Our previous studies have demonstrated that CHD1L has strong tumorigenic ability and confers high susceptibility to spontaneous tumors in a CHD1L-transgenic mouse model. In this study, we demonstrate that the antiapoptotic ability of CHD1L is associated with its interaction with Nur77, a critical member of a p53-independent apoptotic pathway. As the first cellular protein identified to bind Nur77, CHD1L is able to inhibit the nucleus-to-mitochondria translocation of Nur77, which is the key step of Nur77-mediated apoptosis, resulting in the hindrance of the release of cytochrome c and the initiation of apoptosis. Knock-down of CHD1L expression by RNA interference could rescue the mitochondrial targeting of Nur77 and the subsequent apoptosis. Further studies found that the C-terminal Macro domain of CHD1L is responsible for the interaction with Nur77, and a CHD1L mutant lacking residues 600-897 failed to interact with Nur77 and prevented Nur77-mediated apoptosis. More importantly, we found that the inhibition of Nur77-mediated apoptosis by endogenous CHD1L is a critical biological cellular process in hepatocarcinogenesis.

CONCLUSION

We demonstrate in this study that overexpression of CHD1L could sustain tumor cell survival by preventing Nur77-mediated apoptosis.

Perspectives in mammalian IGFBP-3 biology: local vs. systemic action

Insulin-like growth factor (IGF) binding protein (IGFBP)-3 has traditionally been defined by its role as a binding protein and its association with IGF delivery and availability. Development of non-IGF binding IGFBP-3 analogs and the use of cell lines devoid of type 1 IGF receptors (IGF-R) have led to critical advances in the field of IGFBP-3 biology. These studies show that IGFBP-3 has IGF-independent roles in inhibiting cell proliferation in cancer cell lines. Nuclear transcription factor, retinoid X receptor (RXR)-alpha, and IGFBP-3 functionally interact to reduce prostate tumor growth and prostate-specific antigen in vivo. Moreover, IGFBP-3 inhibits insulin-stimulated glucose uptake into adipocytes independent of IGF. The purpose of this review is to highlight IGFBP-3 as a novel effector molecule and not just another "binding protein" by discussing its IGF-independent actions on metabolism and cell growth. Although this review presents studies that assume the role of IGFBP-3 as either an endocrine or autocrine/paracrine molecule, these systems may not exist as distinct entities, justifying the examination of IGFBP-3 in an integrated model. Also, we provide an overview of factors that regulate IGFBP-3 availability, including its production, methylation, and ubiquitination. We conclude with the role of IGFBP-3 in whole body systems and possible future applications of IGFBP-3 in physiology.

Cancer chemoprevention: a radical perspective

Cancer chemopreventive agents block the transformation of normal cells and/or suppress the promotion of premalignant cells to malignant cells. Certain agents may achieve these objectives by modulating xenobiotic biotransformation, protecting cellular elements from oxidative damage, or promoting a more differentiated phenotype in target cells. Conversely, various cancer chemopreventive agents can encourage apoptosis in premalignant and malignant cells in vivo and/or in vitro, which is conceivably another anticancer mechanism. Furthermore, it is evident that many of these apoptogenic agents function as prooxidants in vitro. The constitutive intracellular redox environment dictates a cell's response to an agent that alters this environment. Thus, it is highly probable that normal cells, through adaption, could acquire resistance to transformation via exposure to a chemopreventive agent that promotes oxidative stress or disrupts the normal redox tone of these cells. In contrast, transformed cells, which typically endure an oxidizing intracellular environment, would ultimately succumb to apoptosis due to an uncontrollable production of reactive oxygen species caused by the same agent. Here, we provide evidence to support the hypothesis that reactive oxygen species and cellular redox tone are exploitable targets in cancer chemoprevention via the stimulation of cytoprotection in normal cells and/or the induction of apoptosis in transformed cells.

Cancer chemotherapy with indole-3-carbinol, bis(3'-indolyl)methane and synthetic analogs

Indole-3-carbinol (I3C) conjugates are phytochemicals expressed in brassica vegetables and have been associated with the anticancer activities of vegetable consumption. I3C and its metabolite bis(3'-indolyl)methane (DIM) induce overlapping and unique responses in multiple cancer cell lines and tumors, and these include growth inhibition, apoptosis and antiangiogenic activities. The mechanisms of these responses are complex and dependent on cell context. I3C and/or DIM activate or inactivate multiple nuclear receptors, induce endoplasmic reticulum stress, decrease mitochondrial membrane potential, and modulate multiple signaling pathways including kinases. DIM has been used as a template to synthesize a series of 1,1-bis(3'indolyl)-1-(substituted aromatic)methanes (i.e. C-DIMs) which are also cytotoxic to cancer cells and tumors. Some of the effects of C-DIMs resemble those reported for DIM analogs; however, structure-activity studies with the aromatic ring has resulted in generation of highly unique receptor agonists. For example, p-trifluoromethylphenyl, p-t-butylphenyl and p-biphenyl analogs activate peroxisome proliferator-activated receptor gamma (PPARgamma), and p-methoxyphenyl and p-phenyl compounds activate nerve growth factor-induced-Balpha (NGFI-Balpha, Nur77) orphan nuclear receptor. The effects of C-DIMs on PPARgamma and Nur77 coupled with their receptor-independent activities has resulted in the development of a novel group of multi-targeted anticancer drugs with excellent potential for clinical treatment of cancer.

1,1-bis(3'-indolyl)-1-(p-methoxyphenyl)methane activates Nur77-independent proapoptotic responses in colon cancer cells

1,1-Bis(3'-indolyl)-1-(p-methoxyphenyl)methane (DIM-C-pPhOCH(3)) is a methylene-substituted diindolylmethane (C-DIM) analog that activates the orphan receptor nerve growth factor-induced-Balpha (NGFI-Balpha, Nur77). RNA interference studies with small inhibitory RNA for Nur77 demonstrate that DIM-C-pPhOCH(3) induces Nur77-dependent and -independent apoptosis, and this study has focused on delineating the Nur77-independent proapoptotic pathways induced by the C-DIM analog. DIM-C-pPhOCH(3) induced caspase-dependent apoptosis in RKO colon cancer cells through decreased mitochondrial membrane potential which is accompanied by increased mitochondrial bax/bcl-2 ratios and release of cytochrome c into the cytosol. DIM-C-pPhOCH(3) also induced phosphatidylinositol-3-kinase-dependent activation of early growth response gene-1 which, in turn, induced expression of the proapoptotic nonsteroidal anti-inflammatory drug-activated gene-1 (NAG1) in RKO and SW480 colon cancer cells. Moreover, DIM-C-pPhOCH(3) also induced NAG-1 expression in colon tumors in athymic nude mice bearing RKO cells as xenografts. DIM-C-pPhOCH(3) also activated the extrinsic apoptosis pathway through increased phosphorylation of c-jun N-terminal kinase which, in turn, activated C/EBP homologous transcription factor (CHOP) and death receptor 5 (DR5). Thus, the effectiveness of DIM-C-pPhOCH(3) as a tumor growth inhibitor is through activation of Nur77-dependent and -independent pathways.

Retinoids in biological control and cancer

More than 80 years ago, Wolbach and Howe provided the first evidence suggesting a link between alterations within human cells that lead to malignancies and vitamin A deficiencies (Wolbach and Howe 1925 Nutr. Rev. 36: 16-19). Since that time, epidemiological, preclinical and clinical studies have established a causative relationship between vitamin A deficiency and cancer. Laboratory research has provided insight into the intracellular targets, various signaling cascades and physiological effects of the biologically-active natural and synthetic derivatives of vitamin A, known as retinoids. Collectively, this body of research supports the concept of retinoids as chemopreventive and chemotherapeutic agents that can prevent epithelial cell tumorigenesis by directing the cells to either differentiate, growth arrest, or undergo apoptosis, thus preventing or reversing neoplasia. Continued refinement of the retinoid signaling pathway is essential to establishing their use as effective therapeutics for tumor subtypes whose oncogenic intracellular signaling pathways can be blocked or reversed by treatment with retinoids.

Induction of apoptosis in PA-1 ovarian cancer cells by vitamin K2 is associated with an increase in the level of TR3/Nur77 and its accumulation in mitochondria and nuclei

PURPOSE

We examined the growth-inhibitory and apoptosis-inducing effects of vitamin K(2) (VK(2); menaquinone-4) on various lines of human ovarian cancer cells to study the mechanism of induction of apoptosis by VK(2).

METHODS

Cell proliferation was determined by XTT method, and apoptotic cells were detected by Hoechst staining. TR3, also known as Nur77 and NGFI-B, was detected by immunoblotting and immunofluorescence analysis. Role of TR3 on induction of apoptosis was examined by a siRNA experiment.

RESULTS AND CONCLUSIONS

We found that PA-1 cells were the most sensitive to VK(2) (IC(50) = 5.0 +/- 0.7 microM), while SK-OV-3 cells were resistant to VK(2). Immunoblotting and immunofluorescence analyses indicated that levels of TR3 were elevated in cell lysates 48 h after the start of treatment with 30 microM VK(2). In the VK(2)-treated cells, TR3 accumulated at significant levels in mitochondria, as well as in the nuclei of PA-1 cells. No similar changes were observed in SK-OV-3 cells under the same conditions. Treatment of PA-1 cells with small interfering RNA (siRNA) directed against TR3, and with cycloheximide or SP600125 (an inhibitor of c-jun N-terminal kinase; JNK), separately, inhibited the VK(2)-induced synthesis of TR3 and apoptosis. From these results, we can conclude that an increase in the synthesis of TR3 and the accumulation of TR3 in mitochondria and in nuclei might be involved in the induction of apoptosis by VK(2) and that the synthesis of TR3 might be regulated through a JNK signaling pathway.

Galactomutarotase and other galactose-related genes are rapidly induced by retinoic acid in human myeloid cells

Aldose-1-epimerase (mutarotase) catalyzes the interconversion of alpha and beta hexoses, which is essential for normal carbohydrate metabolism and the production of complex oligosaccharides. Galactose mutarotase (GALM) has been well characterized at the protein level, but information is lacking on the regulation of GALM gene expression. We report herein that all-trans-retinoic acid (RA), an active metabolite of vitamin A that is known to induce myeloid lineage cell differentiation into macrophage-like cells, induces a rapid and robust regulation of GALM mRNA expression in human myeloid cells. all-trans-RA at a physiological concentration (20 nM), or Am580, a ligand selective for the nuclear retinoid receptor RARalpha, increased GALM mRNA in THP-1 cells, with significantly increased expression in 2 h, increasing further to an approximately 8-fold elevation after 6-40 h (P < 0.005). In contrast, tumor necrosis factor-alpha did not increase GALM mRNA expression, although it is capable of inducing cell differentiation. RA also increased GALM mRNA in U937 and HL-60 cells. The increase in GALM mRNA by RA was blocked by pretreating THP-1 cells with actinomycin D but not by cycloheximide. GALM protein and mutarotase activity were also increased time dependently in RA-treated THP-1 cells. In addition to GALM, several other genes in the biosynthetic pathway of galactosyl-containing complex oligosaccharides were more highly expressed in RA-treated THP-1 cells, including B4GALT5, ST3GAL3, ST6GALNAC5, and GALNAC4S-6ST. Thus, the results of this study identify RA as a significant regulator of GALM and other galactose-related genes in myeloid-monocytic cells, which could affect energy utilization and synthesis of cell-surface glycoproteins or glycolipids involved in cell motility, adhesion, and/or functional properties.

The unhydrolyzable fenretinide analogue 4-hydroxybenzylretinone induces the proapoptotic genes GADD153 (CHOP) and Bcl-2-binding component 3 (PUMA) and apoptosis that is caspase- dependent and independent of the retinoic acid receptor

The synthetic retinoid N-(4-hydroxyphenyl)retinamide (4-HPR) induces apoptosis in a variety of cell lines and has shown promise as an anticancer agent both in vitro and in vivo. The clinical dose of 4-HPR, however, is limited by residual-associated toxicities, indicating a need for a less toxic drug. In this study, we show that 4-hydroxybenzylretinone (4-HBR), the unhydrolyzable analogue of 4-HPR, is effective in producing apoptosis in a variety of 4-HPR-sensitive cell lines, including breast cancer, neuroblastoma, and leukemia cells. We also show through the use of a pan-caspase inhibitor that this 4-HBR-induced apoptosis is dependent, at least in part, on caspase activity. 4-HBR is shown to exhibit binding to the retinoic acid receptors (RAR) at concentrations necessary to induce cell death and induces expression of all-trans-retinoic acid-responsive genes that can be blocked by a RAR pan-antagonist. However, through the use of this RAR pan-antagonist, 4-HBR-induced apoptosis and cell death is shown to be independent of the RAR signaling pathway. To further characterize the mechanism of action of 4-HBR, expression of the endoplasmic reticulum stress-induced genes GADD153 and Bcl-2-binding component 3 was examined. These mRNAs are shown to be rapidly induced in 4-HBR-treated and 4-HPR-treated breast cancer cells, and this up-regulation is also shown to be independent of the RARs. These results suggest that a stress-mediated apoptotic cascade is involved in the mechanism of action of these retinoids.

Targeting Nur77 translocation

The ultimate growth of a tumour depends on not only the rate of tumour cell proliferation, but also the rate of tumour cell death (apoptosis). Nur77 (also known as TR3 or NGFI-B), an orphan member of the nuclear receptor superfamily, controls both survival and death of cancer cells. A wealth of recent experimental data demonstrates that the Nur77 activities are regulated through its subcellular localisation. In the nucleus, Nur77 functions as an oncogenic survival factor, promoting cancer cell growth. In contrast, it is a potent killer when migrating to mitochondria, where it binds to Bcl-2 and converts its survival phenotype, triggering cytochrome c release and apoptosis. Agents, such as 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid (AHPN/CD437), which induce Nur77 migration from the nucleus to mitochondria, effectively induce apoptosis of cancer cells. Moreover, Nur77 translocation is highly controlled by retinoid X receptor (RXR), suggesting a role of RXR ligands in regulating the process. Thus, translocation of Nur77 from the nucleus to mitochondria represents a new paradigm in cancer cell apoptosis, and targeting the Nur77 translocation by AHPN/CD437 or RXR ligands promises to effectively restrict cancer cell growth by simultaneously promoting cancer cell death and suppressing cancer cell proliferation.

GADD45A is a mediator of CD437 induced apoptosis in ovarian carcinoma cells

Ovarian cancer is one of the leading causes of cancer death in women. A number of studies have suggested that synthetic retinoids may play an important role as an ovarian cancer chemotherapeutic agent. The synthetic retinoid CD 437 induces apoptosis in ovarian tumor cells by a mechanism that is not completely understood. In this study we demonstrate that CD437 treatment leads to an increase in GADD45A and GADD45B mRNA expression in CA-OV3 cells but not in CA-CD437R cells, a cell line which is resistant to CD437. This induction is specific to CD437 since no change in expression of either GADD45A or GADD45B was observed with either all-trans-RA or 4-HPR treatment. Western blot analysis demonstrated that the induction of GADD45A mRNA in the CA-OV3 cell line by CD437 was accompanied by an increase in GADD45A protein. Upregulation of GADD45A by CD437 is regulated at least in part at the post-transcriptional level. In contrast, CD437 regulates GADD45B expression by different mechanisms. The importance of GADD45A induction in mediating apoptosis was demonstrated in CA-OV3 cells overexpressing GADD45A antisense RNA (GADD45A-AS cells). Our results suggest that induction of GADD45A expression might play a role in mediating the apoptotic response of ovarian cancer cells to the synthetic retinoid CD437.

Mechanisms of fenretinide-induced apoptosis

Fenretinide, a synthetic retinoid, has emerged as a promising anticancer agent based on numerous in vitro and animal studies, as well as chemoprevention clinical trials. In vitro observations suggest that the anticancer activity of fenretinide may arise from its ability to induce apoptosis in tumor cells. Diverse signaling molecules including reactive oxygen species, ceramide, and ganglioside GD3 can mediate apoptosis induction by fenretinide in transformed, premalignant, and malignant cells. In many cell types, these signaling intermediates appear to be induced by mechanisms that are independent of retinoic acid receptor activation, and ultimately initiate the intrinsic or mitochondrial-mediated pathway of cell elimination. Numerous investigations conducted during the past 10 years have discovered a great deal about the apoptogenic activity of fenretinide. In this review we explore the mechanisms associated with fenretinide-induced apoptosis and highlight certain mechanistic underpinnings of fenretinide-induced cell death that remain poorly understood and thus warrant further characterization.

p53 mediates the negative regulation of MDM2 by orphan receptor TR3

MDM2 is an oncoprotein whose transforming potential is activated by overexpression. The expression level of MDM2 is negatively regulated by orphan receptor TR3 that mainly acts as a transcriptional factor to regulate gene expression. However, the underlying mechanism is largely unclear. Here, we present the first evidence that inhibition of TR3 on MDM2 is mediated by p53. We found that TR3 directly interacts with p53 but not MDM2, and such interaction is critical for TR3 to inhibit MDM2 expression. TR3 downregulates p53 transcriptional activity by blocking its acetylation, leading to a decrease on the transcription level of MDM2. Furthermore, TR3 binding to p53 obstructs its ubiquitination and degradation induced by MDM2, resulting in the MDM2 ubiquitination and degradation. In addition, TR3 could enhance p53-mediated apoptosis induced by UV irradiation. Taken together, our findings demonstrate that p53 mediates the suppression of TR3 on MDM2 at both transcriptional and post-transcriptional level and suggest TR3 as a potential target to develop new anticancer agents that restrict MDM2-induced tumor progression.

The novel atypical retinoid ST1926 is active in ATRA resistant neuroblastoma cells acting by a different mechanism

E-3-(4'-Hydroxy-3'-adamantylbiphenyl-4-yl)acrylic acid (ST1926) is a novel orally available compound belonging to the class of synthetic atypical retinoids. These agents are attracting growing attention because of their unique mechanism of antitumor action that appears different from that of classical retinoic acid. This study aims at investigating the antitumor activity of ST1926 in neuroblastoma (NB) preclinical models. In vitro, ST1926 was more cytotoxic than both its prototype, CD437 and all-trans-retinoic acid (ATRA) and it was active in the SK-N-AS cell line, which is refractory to ATRA. We showed that unlike ATRA, ST1926 does not induce morphological differentiation in NB cells where it produces indirect DNA damage, cell cycle arrest in late S-G2 phases and p53-independent programmed cell death. DNA damage was not mediated by oxidative stress and was repaired by 24h after drug removal. The SK-N-DZ cell line appeared the most sensitive to the proapoptotic activity of ST1926, probably because both the extrinsic and intrinsic pathways appear involved in the process. Studies with Z-VAD-FMK, suggested that ST1926 might also mediate caspase-independent apoptosis in NB cells. In vivo, orally administered ST1926, appeared to inhibit tumor growth of NB xenografts with tolerable toxicity. Overall, our results support the view that ST1926 might represent a good drug candidate in this pediatric tumor.

p53 and Nur77/TR3 - transcription factors that directly target mitochondria for cell death induction

The complex apoptotic functions of the p53 tumor suppressor are central to its antineoplastic activity in vivo. Conversely, p53 function is altered or attenuated in one way or another in the majority of human cancers. Besides its well-understood action as a transcriptional regulator of multiple apoptotic genes, p53 also exerts a direct pro-apoptotic role at the mitochondria by engaging in protein-protein interactions with anti- and pro-apoptotic Bcl2 family members, thereby executing the shortest known circuitry of p53 death signaling. Nur77, also known as TR3 or NGFI-B, is a unique transcription factor belonging to the orphan nuclear receptor superfamily. Even more extreme than p53, Nur77 can exert opposing biological activities of survival and death. Its activities are regulated by subcellular distribution, expression levels, protein modification and heterodimerization with retinoid X receptor. In cancer cells, Nur77 functions in the nucleus as an oncogenic survival factor, but becomes a potent killer when certain death stimuli induce its migration to mitochondria, where it binds to Bcl2 and conformationally converts it to a killer that triggers cytochrome c release and apoptosis. This review focuses on their unexpected transcription-independent pro-death programs at mitochondria and highlights the remarkable mechanistic similarities between them. Moreover, an accumulating body of evidence provides ample rationale to further investigate how these mitochondrial p53 and Nur77 pathways could become exploitable targets for new cancer therapeutics.

Rb2/p130 and protein phosphatase 2A: key mediators of ovarian carcinoma cell growth suppression by all-trans retinoic acid

Despite a number of attempts to improve treatment of ovarian cancer, it remains the most common cause of death from gynecological cancers. Thus, it is very important to identify more effective drugs for treatment and prevention of ovarian cancer. All-trans-retinoic acid (ATRA) has been shown to arrest the growth of ovarian carcinoma cells in G0/G1 and to significantly elevate levels of Rb2/p130 protein, a member of the retinoblastoma family of tumor suppressors. As ATRA treatment leads to a significant increase in the amount of Rb2/p130 protein but not mRNA, the elevated levels of Rb2/p130 protein is likely the result of increased stability. In studies to elucidate the mechanism by which ATRA alters Rb2/p130 stability in ovarian cancer cells, it was determined that PP2A, a serine/threonine phosphatase, binds and dephosphorylates Rb2/p130. Dephosphorylated Rb2/p130 exhibits decreased ubiquitination and thus is not degraded by the proteasome. The sites at which PP2A catalytic subunit (PP2Ac) interacts with Rb2/p130 have been localized to the NLS in the C-terminus of Rb2/p130. These sites are also involved in the interaction of Rb/p130 with importin beta and importin alpha, members of the nuclear transport machinery. It is known that importin alpha recognizes a NLS on a target protein and importin beta binds the nuclear pore complex. Moreover, it has been shown that the binding of importin alpha to NLS significantly decreases with phosphorylation of NLS. In ATRA-treated ovarian carcinoma cells, PP2A binds to Rb2/p130 and dephosphorylates the NLS of Rb2/p130 leading to the interaction of importin alpha with Rb2/p130. Importin beta then binds to the importin alpha-Rb2/p130 complex, leading to the translocation of the Rb2/p130 to the nucleus where it acts to arrest ovarian cancer cells in G1 and suppress proliferation.

Regulation of Nur77 nuclear export by c-Jun N-terminal kinase and Akt

Proapoptotic nuclear receptor family member Nur77 translocates from the nucleus to the mitochondria, where it interacts with Bcl-2 to trigger apoptosis. Nur77 translocation is induced by certain apoptotic stimuli, including the synthetic retinoid-related 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalenecarboxylic acid (AHPN)/CD437 class. In this study, we investigated the molecular mechanism by which AHPN/CD437 analog (E)-4-[3-(1-adamantyl)-4-hydroxyphenyl]-3-chlorocinnamic acid (3-Cl-AHPC) induces Nur77 nuclear export. Our results demonstrate that 3-Cl-AHPC effectively activated Jun N-terminal kinase (JNK), which phosphorylates Nur77. Inhibition of JNK activation by a JNK inhibitor suppressed 3-Cl-AHPC-induced Nur77 nuclear export and apoptosis. In addition, several JNK upstream activators, including the phorbol ester TPA, anisomycin and MAPK kinase kinase-1 (MEKK1), phosphorylated Nur77 and induced its nuclear export. However, Nur77 phosphorylation by JNK, although essential, was not sufficient for inducing Nur77 nuclear export. Induction of Nur77 nuclear export by MEKK1 required a prolonged MEKK1 activation and was attenuated by Akt activation. Expression of constitutively active Akt prevented MEKK1-induced Nur77 nuclear export. Conversely, transfection of dominant-negative Akt or treatment with a phosphatidylinositol 3-kinase (PI3-K) inhibitor accelerated MEKK1-induced Nur77 nuclear export. Furthermore, mutation of an Akt phosphorylation residue Ser351 in Nur77 abolished the effect of Akt or the PI3-K inhibitor. Together, our results demonstrate that both activation of JNK and inhibition of Akt play a role in translocation of Nur77 from the nucleus to the cytoplasm.

Development of resistance to the atypical retinoid, ST1926, in the lung carcinoma cell line H460 is associated with reduced formation of DNA strand breaks and a defective DNA damage response

Atypical retinoids are potent inducers of apoptosis, but activation of the apoptotic pathway seems to be independent of retinoid receptors. Previous studies with a novel adamantyl retinoid, ST1926, have shown that apoptosis induction is associated with an early genotoxic stress. To better understand the relevance of these events, we have selected a subline of the H460 lung carcinoma cell line resistant to ST1926. Resistant cells exhibited cross-resistance to a related molecule, CD437, but not cross-resistance to agents with different mechanisms of action. In spite of a lack of defects in intracellular drug accumulation, induction of DNA strand breaks in resistant cells required exposure to a substantially higher concentration, which was consistent with the degree of resistance. At drug concentrations causing a similar antiproliferative effect (IC80) and a comparable extent of DNA lesions in sensitive and resistant cells, the apoptotic response was a delayed and less marked event in resistant cells, thus indicating a reduced susceptibility to apoptosis. In spite of recognition of DNA lesions in resistant cells, as supported by phosphorylation of p53 and histone H2AX, resistant cells exhibited no activation of the mitochondrial pathways of apoptosis. Following exposure to equitoxic drug concentrations, only sensitive cells exhibited a typical stress/DNA damage response, with activation of the S-phase checkpoint. The cellular resistance to ST1926 reflects alterations responsible for a reduced generation of DNA lesions and for an enhanced tolerance of the genotoxic stress, resulting in lack of activation of the intrinsic pathway of apoptosis. The defective DNA damage response, accompanied by a reduced susceptibility to apoptosis in resistant cells, provides further support to the involvement of genotoxic stress as a critical event in mediating apoptosis induction by ST1926.

Mitochondria: A novel target for the chemoprevention of cancer

The mitochondria have emerged as a novel target for anticancer chemotherapy. This tenet is based on the observations that several conventional and experimental chemotherapeutic agents promote the permeabilization of mitochondrial membranes in cancerous cells to initiate the release of apoptogenic mitochondrial proteins. This ability to engage mitochondrial-mediated apoptosis directly using chemotherapy may be responsible for overcoming aberrant apoptosis regulatory mechanisms commonly encountered in cancerous cells. Interestingly, several putative cancer chemopreventive agents also possess the ability to trigger apoptosis in transformed, premalignant, or malignant cells in vitro via mitochondrial membrane permeabilization. This process may occur through the regulation of Bcl-2 family members, or by the induction of the mitochondrial permeability transition. Thus, by exploiting endogenous mitochondrial-mediated apoptosis-inducing mechanisms, certain chemopreventive agents may be able to block the progression of premalignant cells to malignant cells or the dissemination of malignant cells to distant organ sites as means of modulating carcinogenesis in vivo. This review will examine cancer chemoprevention with respect to apoptosis, carcinogenesis, and the proapoptotic activity of various chemopreventive agents observed in vitro. In doing so, I will construct a paradigm supporting the notion that the mitochondria are a novel target for the chemoprevention of cancer.

A novel retinoid-related molecule inhibits pancreatic cancer cell proliferation by a retinoid receptor independent mechanism via suppression of cell cycle regulatory protein function and induction of caspase-associated apoptosis

Retinoid-related molecules are important potential agents for the treatment of cancer. In the present study, we test the effect of a novel retinoid-related ligand, AGN193198 (4-[3-(1-heptyl-4,4-dimethyl-2-oxo-1,2,3,4-tetrahydroquinolin-6-yl)-3-oxo-prophenyl] benzoic acid), on pancreatic cancer cell proliferation and survival. AGN193198 treatment reduces BxPC-3 cell proliferation more efficiently than high-affinity retinoid acid receptor (RAR)- or retinoid X receptor (RXR)-selective retinoids. Moreover, AGN193198 does not activate transcription from RAR or RXR response elements and its effects on cell survival are not reversed by treatment with RAR- or RXR receptor-selective antagonists. These results suggest that the AGN193198-dependent inhibition of BxPC-3 cell function is not mediated via activation of the classical retinoid receptors. Cell cycle analysis of AGN193198-treated BxPC-3 cells indicates that AGN193198 causes accumulation of cells in G2/M. This change is associated with a marked reduction in regulators of S (cyclin A, cyclin-dependent kinase (cdk)2), G2/M (cyclin B1, cdk1, cdc25c) and G1 (cyclin D1, cyclin E, cdk2, cdk4) phase, and an increase in p21 and p27 level. Kinases assays reveal that cdk1, cdk2 and cdk4 activity are suppressed in AGN193198-treated cells. In addition, reduced cell proliferation is associated with enhanced procaspase (3, 8 and 9) and PARP cleavage. Z-VAD-FMK, a pancaspase inhibitor, inhibits AGN193198-dependent caspase activation and attenuates cell death. Z-VAD-FMK inhibits PARP cleavage, but does not alter the AGN193198-dependent reduction in cell cycle regulatory protein expression and activity, suggesting that caspase activation and suppression of cell cycle regulatory protein levels are independent processes. AGN193198 produces similar responses in other pancreatic cancer cell lines including AsPC-1 and MIA PaCa-2. These studies suggest that AGN193198 may be useful for the treatment of pancreatic cancer.

Activation of Nur77 by selected 1,1-Bis(3'-indolyl)-1-(p-substituted phenyl)methanes induces apoptosis through nuclear pathways

Nur77 is an orphan receptor and a member of the nerve growth factor-I-B subfamily of the nuclear receptor family of transcription factors. Based on the results of transactivation assays in pancreatic and other cancer cell lines, we have now identified for the first time Nur77 agonists typified by 1,1-bis(3-indolyl)-1-(p-anisyl)methane that activate GAL4-Nur77 chimeras expressing wild-type and the ligand binding domain (E/F) of Nur77. In Panc-28 pancreatic cancer cells, Nur77 agonists activate the nuclear receptor, and downstream responses include decreased cell survival and induction of cell death pathways, including tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and poly(ADP-ribose) polymerase (PARP) cleavage. Moreover, the transactivation and apoptotic responses are also induced in other pancreatic, prostate, and breast cancer cells that express Nur77. In Panc-28 cells, small inhibitory RNA for Nur77 reverses ligand-dependent transactivation and induction of TRAIL and PARP cleavage. Nur77 agonists also inhibit tumor growth in vivo in athymic mice bearing Panc-28 cell xenografts. These results identify compounds that activate Nur77 through the ligand binding domain and show that ligand-dependent activation of Nur77 through nuclear pathways in cancer cells induces cell death and these compounds are a novel class of anticancer agents.

Rapid Apoptosis Induction by IGFBP-3 Involves an Insulin-like Growth Factor-independent Nucleomitochondrial Translocation of RXRα/Nur77

Insulin-like growth factor-binding protein-3 (IGFBP-3) induces apoptosis by its ability to bind insulin-like growth factors (IGFs) as well as its IGF-independent effects involving binding to other molecules including the retinoid X receptor-alpha (RXRalpha). Here we describe that in response to IGFBP-3, the RXRalpha binding partner nuclear receptor Nur77 rapidly undergoes translocation from the nucleus to the mitochondria, initiating an apoptotic cascade resulting in caspase activation within 6 h. This translocation is a type 1 IGF receptor-signaling independent event as IGFBP-3 induces Nur77 translocation in R-cells. IGFBP-3 and Nur77 are additive in inducing apoptosis. GFP-Nur77 transfection into RXRalpha wild-type and knock-out mouse embryonic fibroblasts and subsequent treatment with IGFBP-3 show that RXRalpha is required for IGFBP-3-induced Nur77 translocation and apoptosis. Addition of IGFBP-3 to 22RV1 cell lysates enhanced the ability of GST-RXRalpha to "pull down" Nur77, and overexpression of IGFBP-3 enhanced the accumulation of mitochondrial RXRalpha. This unique nongenotropic nuclear pathway supports an emerging role for IGFBP-3 as a novel, multicompartmental signaling molecule involved in induction of apoptosis in malignant cells.

Effect of the Synthetic Retinoid Fenretinide on Circulating Free Prostate-Specific Antigen, Insulin-Like Growth Factor-I, and Insulin-Like Growth Factor Binding Protein-3 Levels in Men with Superficial Bladder Cancer

PURPOSE

Fenretinide (4-HPR) is a synthetic retinoid that has shown a preventive activity in prostate cancer animal models.

EXPERIMENTAL DESIGN

We measured the changes in total and free prostate-specific antigen (PSA) and its association with insulin-like growth factor I (IGF-I) and IGFBP-3 levels after 1 year of treatment in 24 subjects given 4-HPR and 24 control subjects enrolled in a randomized bladder cancer prevention trial.

RESULTS

No significant effect of 4-HPR was observed on total and free fraction of PSA levels. The median percentage [95 confidence interval (95% CI)] change for % free PSA and total PSA in the 4-HPR and the control group were, respectively, 7.6 (95% CI, -4.0 to 69.3) versus 5.1 (95% CI, -21.4 to 59.8) and -7.8 (95% CI, -18.2 to 52.5) versus -12.3 (95% CI, -44.6 to 9.6). However, in patients ages <60 years, there was a trend to an increase of total free PSA and % free PSA after treatment with 4-HPR that was different from a trend to a decrease in the control group (P = 0.002 and 0.052, respectively). The interaction between age and treatment was statistically significant on free PSA (P = 0.001). A similar pattern was noted with smoking status (P = 0.011 for the interaction on free PSA). No association was observed between PSA levels and IGF-I or IGFBP-3 levels.

CONCLUSIONS

We conclude that 4-HPR has no significant effect on circulating PSA, but it increases significantly free PSA levels in subjects younger than 60 years and in nonsmokers. These effects might support an activity in prostate cancer prevention but further studies are required.

Retinoid-related molecule AGN193198 potently induces G2M arrest and apoptosis in bladder cancer cells

The novel synthetic retinoid-related molecule 4-[3-(1-heptyl-4,4-dimethyl-2-oxo-1,2,3,4-tetrahydroquinolin-6-yl)-3-oxo-propenyl]benzoic acid (AGN193198) neither binds effectively to retinoic acid receptors (RARs) and retinoid X receptors (RXRs) nor transactivates in RAR- and RXR-mediated reporter assays. Even so, AGN193198 is potent in inducing apoptosis in human prostate and breast carcinoma cells (Keedwell et al., Cancer Res 2004;64:3302-12). Here, we extend these findings to show that AGN193198 potently and rapidly induces apoptosis in bladder carcinoma cell lines. One micromolar of AGN193198 completely abolished the growth of the transitional cell carcinoma lines UM-UC-3 and J82, and the squamous cell carcinoma line SCaBER; the transitional cell papilloma line RT-4 was slightly less sensitive to the growth inhibitory effect of AGN193198. Treated cells accumulated in the G2M phase of the cell cycle. This was accompanied by apoptosis, as revealed by staining cells for exposure of phosphatidylserine at their surface (binding of Annexin V) and FACS analysis of propidium iodide labeled cells. As reported for prostate cancer cells, AGN193198 provoked rapid activation of caspases-3 (by 6 hr), -8 (by 16 hr) and -9 (by 6 hr) in bladder cancer cells. These findings suggest that AGN193198 and related compounds, whose mechanism of action does not appear to involve RARs and RXRs, may be useful in the treatment of bladder cancer.

Retinoid X receptor regulates Nur77/TR3-dependent apoptosis [corrected] by modulating its nuclear export and mitochondrial targeting

Retinoid X receptor (RXR) plays a central role in the regulation of intracellular receptor signaling pathways by acting as a ubiquitous heterodimerization partner of many nuclear receptors, including the orphan receptor Nur77 (also known as TR3 [corrected] or NGFI-B), which translocates from the nucleus to mitochondria, where it interacts with Bcl-2 to induce apoptosis. Here, we report that RXRalpha is required for nuclear export and mitochondrial targeting of Nur77 through their unique heterodimerization that is mediated by dimerization interfaces located in their DNA-binding domain. The effects of RXRalpha are attributed to a putative nuclear export sequence (NES) present in its carboxyl-terminal region. RXRalpha ligands suppress NES activity by inducing RXRalpha homodimerization or altering RXRalpha/Nur77 heterodimerization. The RXRalpha NES is also silenced by RXRalpha heterodimerization with retinoic acid receptor or vitamin D receptor. Consistently, we were able to show that the mitochondrial targeting of the RXRalpha/Nur77 heterodimer and its induction of apoptosis are potently inhibited by RXR ligands. Together, our results reveal a novel nongenotropic function of RXRalpha and its involvement in the regulation of the Nur77-dependent apoptotic pathway [corrected]

An antagonist of retinoic acid receptors more effectively inhibits growth of human prostate cancer cells than normal prostate epithelium

Screening of synthetic retinoids for activity against prostate carcinoma cell lines has identified antagonists of retinoic acid receptors (RARs) as potent growth inhibitors (Hammond et al, 2001, Br J Cancer 85, 453–462). Here we report that 5 days of exposure to a high-affinity pan-RAR antagonist (AGN194310) abolished growth of prostate carcinoma cells from 14 out of 14 patients, with half-maximal inhibition between 200 and 800 nM. It had similar effects (at ∼250 nM) on the prostate carcinoma lines LNCaP, DU-145 and PC-3. AGN194310 inhibited the growth of normal prostate epithelium cells less potently, by 50% at ∼1 μM. The growth of tumour cells was also inhibited more than that of normal cells when RARβ together with RARγ, but not RARα alone, were antagonised. Treatment of LNCaP cells with AGN194310 arrested them in G1 of cell cycle within 12 h, with an accompanying rise in the level of p21^waf1. The cells underwent apoptosis within 3 days, as indicated by mitochondrial depolarisation, Annexin V binding and DNA fragmentation. Apoptosis was caspase-independent: caspases were neither cleaved nor activated, and DNA fragmentation was unaffected by the pan-caspase inhibitor Z-VAD-FMK. The ability of AGN 194310 to induce apoptosis of prostate cancer cells and its differential effect on malignant and normal prostate epithelial cells suggests that this compound may be useful in the treatment of prostate cancer.

Antagonist Analogue of 6-[3‘-(1-Adamantyl)-4‘-hydroxyphenyl]-2-naphthalenecarboxylic Acid (AHPN) Family of Apoptosis Inducers That Effectively Blocks AHPN-Induced Apoptosis but Not Cell-Cycle Arrest

The retinoid 6-[3'-(1-adamantyl)-4'-hydroxyphenyl]-2-naphthalenecarboxylic acid (AHPN) and its active analogues induce cell-cycle arrest and programmed cell death (apoptosis) in cancer cells independently of retinoic acid receptor (RAR) interaction. Its analogue, (E)-4-[3'-(1-adamantyl)-4'-hydroxyphenyl]-3-(3'-acetamidopropyloxy)cinnamic acid (3-A-AHPC) selectively antagonized cell apoptotic events (TR3/nur77/NGFI-B expression and nuclear-to-mitochondrial translocation) but not the proliferative events (cell-cycle arrest and p21(WAF1/CIP1) expression) induced by proapoptotic AHPN and its analogues. The syntheses of 3-A-AHPC and proapoptotic (E)-6-[3'-(1-adamantyl)-4'-hydroxyphenyl]-5-chloronaphthalenecarboxylic acid (5-Cl-AHPN) are described. Computational studies on AHPN, AHPC, and three substituted analogues (5-Cl-AHPN, 3-Cl-AHPC, and 3-A-AHPC) suggested reasons for their diametric effects on RAR activation. Density functional theory studies indicated that the 1-adamantyl (1-Ad) groups of the AHPN and AHPC configurations assumed positions that were nearly planar with the aromatic rings of their polar termini. In contrast, in the configurations of the substituted analogues having chloro and 3-acetamidopropyloxy groups, rather than a hydrogen, ortho to the diaryl bonds, the diaryl bond torsion angles increased so that the 1-Ad groups were oriented out of this plane. Docking and molecular dynamics of AHPN, AHPC, and these substituted analogues in the RARgamma ligand-binding domain illustrated how specific substituents on the AHPN and AHPC scaffolds modulated the positions and dynamics of the 1-Ad groups. As a result, the position of RARgamma helix H12 in forming the coactivator-binding site was impacted in a manner consistent with the experimental effect of each analogue on RARgamma transcriptional activation.

A Retinoid-Related Molecule that Does Not Bind to Classical Retinoid Receptors Potently Induces Apoptosis in Human Prostate Cancer Cells through Rapid Caspase Activation

Synthetic retinoid-related molecules, such as N-(4-hydroxyphenyl)retinamide (fenretinide) and 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid (CD437) induce apoptosis in a variety of malignant cells. The mechanism(s) of action of these compounds does not appear to involve retinoic acid receptors (RARs) and retinoid X receptors (RXRs), although some investigators disagree with this view. To clarify whether some retinoid-related molecules can induce apoptosis without involving RARs and/or RXRs, we used 4-[3-(1-heptyl-4,4-dimethyl-2-oxo-1,2,3,4-tetrahydroquinolin-6-yl)-3-oxo-E-propenyl] benzoic acid (AGN193198) that neither binds effectively to RARs and RXRs nor transactivates in RAR- and RXR-mediated reporter assays. AGN193198 potently induced apoptosis in prostate, breast, and gastrointestinal carcinoma cells and in leukemia cells. AGN193198 also abolished growth (by 50% at 130-332 nM) and induced apoptosis in primary cultures established from prostatic carcinoma (13 patients) and gastrointestinal carcinoma (1 patient). Apoptosis was induced rapidly, as indicated by mitochondrial depolarization and DNA fragmentation. Molecular events provoked by AGN193198 included activation of caspase-3, -8, -9, and -10 (by 4-6 h) and the production of BID/p15 (by 6 h). These findings show that caspase-mediated induction of apoptosis by AGN193198 is RAR/RXR-independent and suggest that this compound may be useful in the treatment of prostate cancer.

Conversion of Bcl-2 from protector to killer by interaction with nuclear orphan receptor Nur77/TR3

The Bcl-2 family proteins are key regulators of apoptosis in human diseases and cancers. Though known to block apoptosis, Bcl-2 promotes cell death through an undefined mechanism. Here, we show that Bcl-2 interacts with orphan nuclear receptor Nur77 (also known as TR3), which is required for cancer cell apoptosis induced by many antineoplastic agents. The interaction is mediated by the N-terminal loop region of Bcl-2 and is required for Nur77 mitochondrial localization and apoptosis. Nur77 binding induces a Bcl-2 conformational change that exposes its BH3 domain, resulting in conversion of Bcl-2 from a protector to a killer. These findings establish the coupling of Nur77 nuclear receptor with the Bcl-2 apoptotic machinery and demonstrate that Bcl-2 can manifest opposing phenotypes, induced by interactions with proteins such as Nur77, suggesting novel strategies for regulating apoptosis in cancer and other diseases.

Synthetic retinoids as inducers of apoptosis in ovarian carcinoma cell lines

Apoptosis is also known as programmed cell death. Apoptosis plays an essential role in maintaining normal tissue and cell physiology in multicellular organisms. Clearance of aberrant or pre-cancerous cells occurs through the induction of apoptosis. It has been reported that many tumors and tumor cell lines have dysfunctional apoptosis signaling, causing these tumors to escape immune monitoring and internal cellular control mechanisms. One potential cause of this dysfunctional apoptosis is the tumor suppressor p53, an important regulator of growth arrest and apoptosis that is mutated in over 50% of all cancers. Retinoids have great potential in the areas of cancer therapy and chemoprevention. While some tumor cells are sensitive to the growth inhibitory effects of natural retinoids such as all-trans-retinoic acid (ATRA), many ovarian tumor cells are not. 6-[3-(1-Admantyl)]-4-hydroxyphenyl]-2-naphthalene carboxylic acid (CD437) and fenretinide N-[4-hydroxyphenyl] retinamide (4-HPR) are conformationally restricted synthetic retinoids that induce growth arrest and apoptosis in both ATRA-sensitive and ATRA-resistant ovarian tumor cell lines. Recently, we have identified the molecular pathways of apoptosis induced by treatment of ovarian carcinoma cells with mutated p53 by CD437 and 4-HPR.