Inhibition of NFkappaB and pancreatic cancer cell and tumor growth by curcumin is dependent on specificity protein down-regulation

Curcumin activates diverse anticancer activities that lead to inhibition of cancer cell and tumor growth, induction of apoptosis, and antiangiogenic responses. In this study, we observed that curcumin inhibits Panc28 and L3.6pL pancreatic cancer cell and tumor growth in nude mice bearing L3.6pL cells as xenografts. In addition, curcumin decreased expression of p50 and p65 proteins and NFkappaB-dependent transactivation and also decreased Sp1, Sp3, and Sp4 transcription factors that are overexpressed in pancreatic cancer cells. Because both Sp transcription factors and NFkappaB regulate several common genes such as cyclin D1, survivin, and vascular endothelial growth factor that contribute to the cancer phenotype, we also investigated interactions between Sp and NFkappaB transcription factors. Results of Sp1, Sp3, and Sp4 knockdown by RNA interference demonstrate that both p50 and p65 are Sp-regulated genes and that inhibition of constitutive or tumor necrosis factor-induced NFkappaB by curcumin is dependent on down-regulation of Sp1, Sp3, and Sp4 proteins by this compound. Curcumin also decreased mitochondrial membrane potential and induced reactive oxygen species in pancreatic cancer cells, and this pathway is required for down-regulation of Sp proteins in these cells, demonstrating that the mitochondriotoxic effects of curcumin are important for its anticancer activities.

Curcumin decreases specificity protein expression in bladder cancer cells

Curcumin is the active component of tumeric, and this polyphenolic compound has been extensively investigated as an anticancer drug that modulates multiple pathways and genes. In this study, 10 to 25 micromol/L curcumin inhibited 253JB-V and KU7 bladder cancer cell growth, and this was accompanied by induction of apoptosis and decreased expression of the proapoptotic protein survivin and the angiogenic proteins vascular endothelial growth factor (VEGF) and VEGF receptor 1 (VEGFR1). Because expression of survivin, VEGF, and VEGFR1 are dependent on specificity protein (Sp) transcription factors, we also investigated the effects of curcumin on Sp protein expression as an underlying mechanism for the apoptotic and antiangiogenic activity of this compound. The results show that curcumin induced proteasome-dependent down-regulation of Sp1, Sp3, and Sp4 in 253JB-V and KU7 cells. Moreover, using RNA interference with small inhibitory RNAs for Sp1, Sp3, and Sp4, we observed that curcumin-dependent inhibition of nuclear factor kappaB (NF-kappaB)-dependent genes, such as bcl-2, survivin, and cyclin D1, was also due, in part, to loss of Sp proteins. Curcumin also decreased bladder tumor growth in athymic nude mice bearing KU7 cells as xenografts and this was accompanied by decreased Sp1, Sp3, and Sp4 protein levels in tumors. These results show for the first time that one of the underlying mechanisms of action of curcumin as a cancer chemotherapeutic agent is due, in part, to decreased expression of Sp transcription factors in bladder cancer cells.

Nuclear receptor 4A (NR4A) family - orphans no more

The orphan nuclear receptors NR4A1, NR4A2 and NR4A3 are immediate early genes induced by multiple stressors, and the NR4A receptors play an important role in maintaining cellular homeostasis and disease. There is increasing evidence for the role of these receptors in metabolic, cardiovascular and neurological functions and also in inflammation and inflammatory diseases and in immune functions and cancer. Despite the similarities of NR4A1, NR4A2 and NR4A3 and their interactions with common cis-genomic elements, they exhibit unique activities and cell-/tissue-specific functions. Although endogenous ligands for NR4A receptors have not been identified, there is increasing evidence that structurally-diverse synthetic molecules can directly interact with the ligand binding domain of NR4A1 and act as agonists or antagonists, and ligands for NR4A2 and NR4A3 have also been identified. Since NR4A receptors are key factors in multiple diseases, there are opportunities for the future development of NR4A ligands for clinical applications in treating multiple health problems including metabolic, neurologic and cardiovascular diseases, other inflammatory conditions, and cancer.

Nur77 agonists induce proapoptotic genes and responses in colon cancer cells through nuclear receptor-dependent and nuclear receptor-independent pathways

Nerve growth factor-induced Balpha (NGFI-Balpha, Nur77) is an orphan nuclear receptor with no known endogenous ligands; however, recent studies on a series of methylene-substituted diindolylmethanes (C-DIM) have identified 1,1-bis(3'-indolyl)-1-(phenyl)methane (DIM-C-Ph) and 1,1-bis(3'-indolyl)-1-(p-anisyl)methane (DIM-C-pPhOCH3) as Nur77 agonists. Nur77 is expressed in several colon cancer cell lines (RKO, SW480, HCT-116, HT-29, and HCT-15), and we also observed by immunostaining that Nur77 was overexpressed in colon tumors compared with normal colon tissue. DIM-C-Ph and DIM-C-pPhOCH3 decreased survival and induced apoptosis in RKO colon cancer cells, and this was accompanied by induction of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) protein. The induction of apoptosis and TRAIL by DIM-C-pPhOCH3 was significantly inhibited by a small inhibitory RNA for Nur77 (iNur77); however, it was evident from RNA interference studies that DIM-C-pPhOCH3 also induced Nur77-independent apoptosis. Analysis of DIM-C-pPhOCH3-induced gene expression using microarrays identified several proapoptotic genes, and analysis by reverse transcription-PCR in the presence or absence of iNur77 showed that induction of programmed cell death gene 1 was Nur77 dependent, whereas induction of cystathionase and activating transcription factor 3 was Nur77 independent. DIM-C-pPhOCH3 (25 mg/kg/d) also inhibited tumor growth in athymic nude mice bearing RKO cell xenografts. These results show that Nur77-active C-DIM compounds represent a new class of anti-colon cancer drugs that act through receptor-dependent and receptor-independent pathways.

Polychlorinated biphenyls (PCBs), dibenzo-p-dioxins (PCDDs), and dibenzofurans (PCDFs) as antiestrogens in MCF-7 human breast cancer cells: quantitative structure-activity relationships

The concentration-dependent effects of several PCB, PCDD, and PCDF congeners and several commercial PCB preparations as antiestrogens were determined in the aryl hydrocarbon (Ah)-responsive MCF-7 human breast cancer cell lines. The inhibition of the 17 beta-estradiol-induced secretion of the 52-kDa protein (procathepsin D) was measured using a combination of polyacrylamide gel electrophoresis, double-staining of the protein bands with ISS ProBlue and silver stain, and quantitation by densitometric analysis. For the PCBs, the order of antiestrogenic potency was 3,3',4,4',5-pentachlorobiphenyl > 3,3',4,4',5,5'-hexachlorobiphenyl approximately 3,3',4,4'-tetrachlorobiphenyl > 2,3,3',4,4',5'-hexa, 2,3,3',4,4'- and 2,3,4,4',5-pentachlorobiphenyl > Aroclors 1221, 1232, 1248, 1254, and 1260 were inactive as antiestrogens at the highest concentrations used in this study (10(-6) M). For the PCDDs and PCDFs, the order of antiestrogenic potency was 2,3,7,8-tetrachlorodibenzo-p-dioxin > 2,3,7,8-tetrachlorodibenzofuran > 2,3,4,7,8-pentachlorodibenzofuran > 1,2,3,7,9-pentachlorodibenzofuran > 1,3,6,8-tetrachlorodibenzofuran. With few exceptions, the order of potency for all these congeners and mixtures paralleled their relative activities as agonists for other Ah receptor-mediated responses and their competitive binding affinities for the Ah receptor. The results of this study support the role for the Ah receptor in mediating the inhibition of the 17 beta-estradiol-induced secretion of the 52-kDa protein in MCF-7 cells and also points out the utility of this technique as a bioassay for this class of compounds.

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.

Toxicological and pathophysiological roles of reactive oxygen and nitrogen species

'Oxidative and Nitrative Stress in Toxicology and Disease' was the subject of a symposium held at the EUROTOX meeting in Dresden 15th September 2009. Reactive oxygen (ROS) and reactive nitrogen species (RNS) produced during tissue pathogenesis and in response to viral or chemical toxicants, induce a complex series of downstream adaptive and reparative events driven by the associated oxidative and nitrative stress. As highlighted by all the speakers, ROS and RNS can promote diverse biological responses associated with a spectrum of disorders including neurodegenerative/neuropsychiatric and cardiovascular diseases. Similar pathways are implicated during the process of liver and skin carcinogenesis. Mechanistically, reactive oxygen and nitrogen species drive sustained cell proliferation, cell death including both apoptosis and necrosis, formation of nuclear and mitochondrial DNA mutations, and in some cases stimulation of a pro-angiogenic environment. Here we illustrate the pivotal role played by oxidative and nitrative stress in cell death, inflammation and pain and its consequences for toxicology and disease pathogenesis. Examples are presented from five different perspectives ranging from in vitro model systems through to in vivo animal model systems and clinical outcomes.

Tolfenamic acid and pancreatic cancer growth, angiogenesis, and Sp protein degradation

BACKGROUND

Sp1, Sp3, and Sp4 are transcription factors that regulate cell proliferation and vascular endothelial growth factor (VEGF) expression and are overexpressed in many cancer cell lines. For some cancers, Sp1 overexpression is associated with poor survival. Cyclooxygenase inhibitors decrease Sp1 expression in cancer cells, and therefore different structural classes of nonsteroidal anti-inflammatory drugs (NSAIDs) were screened for their ability to decrease levels of Sp1, Sp3, and Sp4 and to decrease pancreatic tumor growth and metastasis in an in vivo model.

METHODS

Levels of Sp1, Sp3, Sp4, and VEGF proteins in pancreatic cancer cell lines were assessed by immunoblot analysis. mRNA was assessed by reverse transcription-polymerase chain reaction. Panc-1 pancreatic cancer cells transfected with VEGF promoter constructs were used to assess VEGF promoter activation. Pancreatic tumor weight and size and liver metastasis were assessed in an orthotopic mouse model of pancreatic cancer (groups of 10 mice). Protein expression in tumors was assessed immunohistochemically.

RESULTS

Tolfenamic acid and structurally related biaryl derivatives induced degradation of Sp1, Sp3, and Sp4 in pancreatic cancer cells. Tolfenamic acid also inhibited VEGF mRNA and protein expression in pancreatic cancer cells; this inhibition was associated with the decreased Sp-dependent activation of the VEGF promoter. In the mouse model for pancreatic cancer, treatment with tolfenamic acid (50 mg/kg of body weight), compared with control treatment, statistically significantly decreased tumor growth and weight (P = .005), liver metastasis (P = .027), and levels of Sp3 and VEGF (P = .009) and Sp1 and Sp4 (P = .006) proteins in tumors. For example, tumors from mice treated with tolfenamic acid (50 mg/kg) had statistically significantly lower VEGF levels (45%, 95% confidence interval = 39% to 51%; P = .009) than tumors from control mice.

CONCLUSIONS

Tolfenamic acid is a new antipancreatic cancer NSAID that activates degradation of transcription factors Sp1, Sp3, and Sp4; reduces VEGF expression; and decreases tumor growth and metastasis.

Xenoestrogen-induced regulation of EZH2 and histone methylation via estrogen receptor signaling to PI3K/AKT

Although rapid, membrane-activated estrogen receptor (ER) signaling is no longer controversial, the biological function of this nongenomic signaling is not fully characterized. We found that rapid signaling from membrane-associated ER regulates the histone methyltransferase enhancer of Zeste homolog 2 (EZH2). In response to both 17beta-estradiol (E2) and the xenoestrogen diethylstilbestrol, ER signaling via phosphatidylinositol 3-kinase/protein kinase B phosphorylates EZH2 at S21, reducing levels of trimethylation of lysine 27 on histone H3 in hormone-responsive cells. During windows of uterine development that are susceptible to developmental reprogramming, activation of this ER signaling pathway by diethylstilbestrol resulted in phosphorylation of EZH2 and reduced levels of trimethylation of lysine 27 on histone H3 in chromatin of the developing uterus. Furthermore, activation of nongenomic signaling reprogrammed the expression profile of estrogen-responsive genes in uterine myometrial cells, suggesting this as a potential mechanism for developmental reprogramming caused by early-life exposure to xenoestrogens. These data demonstrate that rapid ER signaling provides a direct linkage between xenoestrogen-induced nuclear hormone receptor signaling and modulation of the epigenetic machinery during tissue development.

Mechanisms of inhibitory aryl hydrocarbon receptor-estrogen receptor crosstalk in human breast cancer cells

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that forms a functional heterodimeric complex with the AhR nuclear translocator (Arnt) protein. The environmental toxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), is a high affinity ligand for the AhR and has been extensively used to investigate AhR-mediated biochemical and toxic responses. TCDD modulates several endocrine pathways including inhibition of 17beta-estradiol-induced responses in the immature and ovariectomized rodent uterus and mammary gland and in human breast cancer cell lines. TCDD inhibits formation and growth of mammary tumors in carcinogen-induced rodent models and relatively nontoxic selective AhR modulators (SAhRMs) are being developed for treatment of breast cancer. The mechanisms of inhibitory AhR-estrogen receptor (ER) crosstalk have been investigated in MCF-7 breast cancer cells by analysis of promoter regions of genes induced by E2 and inhibited by TCDD. AhR-mediated inhibition of E2-induced cathepsin D, pS2, c-fos, and heat shock protein 27 gene expression involves direct interaction of the AhR complex with inhibitory pentanucleotide (GCGTG) dioxin responsive elements (iDREs) resulting in disruption of interactions between proteins binding DNA elements required for ER action and the basal transcription machinery. Mechanisms of inhibitory AhR-ER crosstalk indicate that functional iDREs are required for inhibition of some genes; however, results indicate that other interaction pathways are important including AhR-mediated proteasome-dependent degradation of the ER.

Curcumin and synthetic analogs induce reactive oxygen species and decreases specificity protein (Sp) transcription factors by targeting microRNAs

BACKGROUND

Curcumin inhibits growth of several cancer cell lines, and studies in this laboratory in bladder and pancreatic cancer cells show that curcumin downregulates specificity protein (Sp) transcription factors Sp1, Sp3 and Sp4 and pro-oncogenic Sp-regulated genes. In this study, we investigated the anticancer activity of curcumin and several synthetic cyclohexanone and piperidine analogs in colon cancer cells.

METHODS

The effects of curcumin and synthetic analogs on colon cancer cell proliferation and apoptosis were determined using standardized assays. The changes in Sp proteins and Sp-regulated gene products were analysed by western blots, and real time PCR was used to determine microRNA-27a (miR-27a), miR-20a, miR-17-5p and ZBTB10 and ZBTB4 mRNA expression.

RESULTS

The IC50 (half-maximal) values for growth inhibition (24 hr) of colon cancer cells by curcumin and synthetic cyclohexanone and piperidine analogs of curcumin varied from 10 μM for curcumin to 0.7 μM for the most active synthetic piperidine analog RL197, which was used along with curcumin as model agents in this study. Curcumin and RL197 inhibited RKO and SW480 colon cancer cell growth and induced apoptosis, and this was accompanied by downregulation of specificity protein (Sp) transcription factors Sp1, Sp3 and Sp4 and Sp-regulated genes including the epidermal growth factor receptor (EGFR), hepatocyte growth factor receptor (c-MET), survivin, bcl-2, cyclin D1 and NFκB (p65 and p50). Curcumin and RL197 also induced reactive oxygen species (ROS), and cotreatment with the antioxidant glutathione significantly attenuated curcumin- and RL197-induced growth inhibition and downregulation of Sp1, Sp3, Sp4 and Sp-regulated genes. The mechanism of curcumin-/RL197-induced repression of Sp transcription factors was ROS-dependent and due to induction of the Sp repressors ZBTB10 and ZBTB4 and downregulation of microRNAs (miR)-27a, miR-20a and miR-17-5p that regulate these repressors.

CONCLUSIONS

These results identify a new and highly potent curcumin derivative and demonstrate that in cells where curcumin and RL197 induce ROS, an important underlying mechanism of action involves perturbation of miR-ZBTB10/ZBTB4, resulting in the induction of these repressors which downregulate Sp transcription factors and Sp-regulated genes.

Anthocyanin fraction from potato extracts is cytotoxic to prostate cancer cells through activation of caspase-dependent and caspase-independent pathways

Polyphenols from fruits and vegetables exhibit anticancer properties both in vitro and in vivo and specialty potatoes are an excellent source of dietary polyphenols, including phenolic acids and anthocyanins. This study investigated the effects of specialty potato phenolics and their fractions on LNCaP (androgen dependent) and PC-3 (androgen independent) prostate cancer cells. Phenolic extracts from four specialty potato cultivars CO112F2-2, PATX99P32-2, ATTX98462-3 and ATTX98491-3 and organic acid, phenolic acid and anthocyanin fractions (AF) were used in this study. CO112F2-2 cultivar extracts and their AF at 5 mug chlorogenic acid eq/ml were more active and inhibited cell proliferation and increased the cyclin-dependent kinase inhibitor p27 levels in both LNCaP and PC-3 cells. Potato extract and AF induced apoptosis in both the cells and, however, the effects were cell context dependent. Cell death pathways induced by potato extract and AF were associated with mitogen-activated protein kinase and c-jun N-terminal kinase activation and these kinases activated caspase-independent apoptosis through nuclear translocation of endonuclease G (Endo G) and apoptosis-inducing factor in both cell lines. Induction of caspase-dependent apoptosis was also kinase dependent but was observed only in LNCaP cells. Kinase inhibitors reversed this nuclear translocation of endonuclease G and apoptosis-inducing factor. This is the first report showing that the cytotoxic activities of potato extract/AF in cancer cells were due to activation of caspase-independent apoptosis. Current studies are focused on identifying individual components of the AF responsible for the induction of cell death pathways in prostate and other cancer cell lines and developing potato cultivars that overexpress these active compounds.

Inactivation of the orphan nuclear receptor TR3/Nur77 inhibits pancreatic cancer cell and tumor growth

Activation of the orphan nuclear receptor TR3/Nur77 (NR4A1) promotes apoptosis and inhibits pancreatic tumor growth, but its endogenous function and the effects of its inactivation have yet to be determined. TR3 was overexpressed in human pancreatic tumors compared with nontumor tissue. Small interfering RNA-mediated knockdown of TR3 or cell treatment with the TR3 antagonist 1,1-bis(3'-indolyl)-1-(p-hydroxyphenyl)methane (DIM-C-pPhOH) decreased proliferation, induced apoptosis, and decreased expression of antiapoptotic genes including Bcl-2 and survivin in pancreatic cancer cells. Survivin suppression was mediated by formation of a TR3-Sp1-p300 DNA binding complex on the proximal GC-rich region of the survivin promoter. When administered in vivo, DIM-C-pPhOH induced apoptosis and inhibited tumor growth in an orthotopic model of pancreatic cancer, associated with inhibition of the same antiapoptotic markers observed in vitro. Our results offer preclinical validation of TR3 as a drug target for pancreatic cancer chemotherapy, based on the ability of TR3 inhibitors to block the growth of pancreatic tumors.

The long non-coding RNA HOTTIP enhances pancreatic cancer cell proliferation, survival and migration

HOTTIP is a long non-coding RNA (lncRNA) transcribed from the 5' tip of the HOXA locus and is associated with the polycomb repressor complex 2 (PRC2) and WD repeat containing protein 5 (WDR5)/mixed lineage leukemia 1 (MLL1) chromatin modifying complexes. HOTTIP is expressed in pancreatic cancer cell lines and knockdown of HOTTIP by RNA interference (siHOTTIP) in Panc1 pancreatic cancer cells decreased proliferation, induced apoptosis and decreased migration. In Panc1 cells transfected with siHOTTIP, there was a decrease in expression of 757 genes and increased expression of 514 genes, and a limited gene analysis indicated that HOTTIP regulation of genes is complex. For example, Aurora kinase A, an important regulator of cell growth, is coregulated by MLL and not WDR5 and, in contrast to previous studies in liver cancer cells, HOTTIP does not regulate HOXA13 but plays a role in regulation of several other HOX genes including HOXA10, HOXB2, HOXA11, HOXA9 and HOXA1. Although HOTTIP and the HOX-associated lncRNA HOTAIR have similar pro-oncogenic functions, they regulate strikingly different sets of genes in Panc1 cells and in pancreatic tumors.

Polychlorinated dibenzofurans (PCDFs): effects of structure on binding to the 2,3,7,8-TCDD cytosolic receptor protein, AHH induction and toxicity

The effects of structure on the activity of 26 polychlorinated dibenzofurans (PCDFs) as competitive ligands for the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) rat hepatic cytosolic receptor protein were determined in a dose-response fashion. The ED50 values for these compounds varied 100 000-fold and the most active PCDFs were substituted in the 2,3,7 and 8 lateral positions; the ED50 for the most active PCDF, 2,3,4,7,8-pentachlorodibenzofuran was 1.5 X 10(-8) M which was only slightly less active than 2,3,7,8-TCDD (1.0 X 10(-8) M). A comparison of the binding affinities of several isomer pairs also indicated the relative importance of chlorine substitution at C-4 (or C-6) compared to C-1 (or C-9). Moreover, for some isomers it is apparent that C-4 (or C-6) substituents are more active than lateral substituents for facilitating ligand binding to the receptor protein. This is illustrated by the relative binding potencies of the following isomer pairs: 1,2,4,6,7-/1,2,4,7,8 = 19.2; 2,6,7-/2,3,8- = 2.2; 1,3,6-/1,3,8- = 19. Most of the PCDF structure-activity effects noted above were also observed for the induction of aryl hydrocarbon hydroxylase (AHH) and ethoxyresorufin O-deethylase (EROD) in rat hepatoma H-4-II-E cells in culture. The most active compounds were also substituted in the lateral 2,3,7 and 8 positions and a comparison of C-4 (or C-6) vs. C-1 (or C-9) substituted PCDFs confirmed the higher induction potencies for most of the former group of compounds. The in vitro quantitative structure-activity data were complemented by in vivo studies which determined the relative activities of selected PCDFs as inducers of hepatic microsomal cytochrome P-448 dependent monooxygenases and their effects on body weight gain and thymus weights in immature male Wistar rats. The results indicated that for 2 series of isomers, namely the 2,3,4,7,8-, 1,2,4,7,8- and 1,2,4,7,9-pentachlorodibenzofurans and the 2,3,7,8-, 2,3,4,8- and 1,2,4,8-tetrachlorodibenzofurans, their biologic and toxic potencies were dependent on one major structural factor, the number of lateral chloro substituents. These results support the proposed role of the cytosolic receptor protein in mediating the biologic and toxic effects of the PCDFs.

Aryl hydrocarbon receptor gene silencing with small inhibitory RNA differentially modulates Ah-responsiveness in MCF-7 and HepG2 cancer cells

Sequence-specific small interfering RNA (siRNA) duplexes can be used for gene silencing in mammalian cells and as mechanistic probes for determining gene function. Transfection of siRNAs for the aryl hydrocarbon receptor (AhR) and AhR nuclear translocator (ARNT) mRNAs in MCF-7 breast cancer cells resulted in a 60 to 80% decrease in levels of AhR and ARNT proteins in whole-cell extracts and decreased binding of nuclear extracts to 32P-labeled dioxin-responsive element. siRNA for the AhR also decreased 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced CYP1A1 protein, CYP1A1-dependent activity, and luciferase activity in cells transfected with an Ah-responsive construct. 17beta-estradiol (E2) induces proliferation of MCF-7 cells through enhanced G0/G1 --> S phase progression, and this response is inhibited in cells cotreated with E2 plus TCDD. The effects of TCDD on E2-induced cell-cycle progress were partially blocked in MCF-7 cells transfected with siRNA for AhR. The results also indicated that siRNA-dependent decreases in AhR protein in MCF-7 cells were accompanied by increased G0/G1 --> S phase progression, suggesting a growth-inhibitory role for the "endogenous" AhR. Surprisingly, TCDD alone induced G0/G1 --> S phase progression and exhibited estrogenic activity in MCF-7 cells transfected with siRNA for the AhR. In contrast, degradation of the AhR in HepG2 liver cancer cells resulted in decreased G0/G1 --> S phase progression, and this was accompanied by decreased expression of cyclin D1, cyclin E, cyclin-dependent kinase 2 (cdk2), and cdk4. In the absence of ligand, the AhR exhibits growth-inhibitory (MCF-7) and growth-promoting (HepG2) activity that is cell context-dependent.

Role of the aryl hydrocarbon receptor in carcinogenesis and potential as an anti-cancer drug target

The aryl hydrocarbon receptor (AhR) was initially identified as the receptor that binds and mediates the toxic effects induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and structurally related halogenated aromatics. Other toxic compounds including some polynuclear aromatic hydrocarbons act through the AhR; however, during the last 25 years, it has become apparent that the AhR plays an essential role in maintaining cellular homeostasis. Moreover, the scope of ligands that bind the AhR includes endogenous compounds such as multiple tryptophan metabolites, other endogenous biochemicals, pharmaceuticals and health-promoting phytochemicals including flavonoids, indole-3-carbinol and its metabolites. It has also been shown that like other receptors, the AhR is a drug target for multiple diseases including cancer, where both AhR agonists and antagonists effectively block many of the critical hallmarks of cancer in multiple tumor types. This review describes the anti-cancer activities of AhR ligands and demonstrates that it is time to separate the AhR from TCDD and exploit the potential of the AhR as a novel target for cancer chemotherapy.

Betulinic acid inhibits colon cancer cell and tumor growth and induces proteasome-dependent and -independent downregulation of specificity proteins (Sp) transcription factors

BACKGROUND

Betulinic acid (BA) inhibits growth of several cancer cell lines and tumors and the effects of BA have been attributed to its mitochondriotoxicity and inhibition of multiple pro-oncogenic factors. Previous studies show that BA induces proteasome-dependent degradation of specificity protein (Sp) transcription factors Sp1, Sp3 and Sp4 in prostate cancer cells and this study focused on the mechanism of action of BA in colon cancer cells.

METHODS

The effects of BA on colon cancer cell proliferation and apoptosis and tumor growth in vivo were determined using standardized assays. The effects of BA on Sp proteins and Sp-regulated gene products were analyzed by western blots, and real time PCR was used to determine microRNA-27a (miR-27a) and ZBTB10 mRNA expression.

RESULTS

BA inhibited growth and induced apoptosis in RKO and SW480 colon cancer cells and inhibited tumor growth in athymic nude mice bearing RKO cells as xenograft. BA also decreased expression of Sp1, Sp3 and Sp4 transcription factors which are overexpressed in colon cancer cells and decreased levels of several Sp-regulated genes including survivin, vascular endothelial growth factor, p65 sub-unit of NFκB, epidermal growth factor receptor, cyclin D1, and pituitary tumor transforming gene-1. The mechanism of action of BA was dependent on cell context, since BA induced proteasome-dependent and proteasome-independent downregulation of Sp1, Sp3 and Sp4 in SW480 and RKO cells, respectively. In RKO cells, the mechanism of BA-induced repression of Sp1, Sp3 and Sp4 was due to induction of reactive oxygen species (ROS), ROS-mediated repression of microRNA-27a, and induction of the Sp repressor gene ZBTB10.

CONCLUSIONS

These results suggest that the anticancer activity of BA in colon cancer cells is due, in part, to downregulation of Sp1, Sp3 and Sp4 transcription factors; however, the mechanism of this response is cell context-dependent.

Estrogen receptor-mediated activation of the serum response element in MCF-7 cells through MAPK-dependent phosphorylation of Elk-1

17beta-Estradiol (E2) induces c-fos protooncogene expression in MCF-7 human breast cancer cells, and deletion analysis of the c-fos promoter showed that the serum response element (SRE) at -325 to -296 was E2-responsive. The mechanism of ligand-activated estrogen receptor alpha (ERalpha)-dependent activation of gene expression through the SRE was determined by mutational analysis of the promoter, analysis of mitogen-activated protein kinase (MAPK) pathway activation by E2, and transforming growth factor alpha (TGF-alpha) as a positive control. In addition, ERalpha-negative MDA-MB-231 breast cancer and Chinese hamster ovary cells were used as reference cell lines. The results showed that transcriptional activation of the SRE by E2 was due to ERalpha activation of the MAPK pathway and increased binding of the serum response factor and Elk-1 to the SRE. Subsequent studies with dominant negative Elk-1, wild type, and variant GAL4-Elk-1 fusion proteins confirmed that phosphorylation of Elk-1 at serines 383 and 389 in the C-terminal region of Elk-1 is an important downstream target associated with activation of an SRE by E2. Both E2 (ERalpha-dependent) and growth factors (ERalpha-independent) activated the SRE in breast cancer cells via the Ras/MAPK pathway; however, in ER-negative CHO cells that do not express a receptor for TGF-alpha, only hormone-induced activation was observed in cells transfected with ERalpha.

Antiestrogenic activity of hydroxylated polychlorinated biphenyl congeners identified in human serum

Several hydroxylated polychlorinated biphenyls (PCBs) identified in human serum have been synthesized and these include 2,2',3,4',5,5'-hexachloro-4-biphenylol; 2,3,3',4',5-pentachloro-4-biphenylol; 2',3,3',4',5-pentachloro-4-biphenylol; 2,2',3,3',4',5-hexachloro-4-biphenylol; 2,2',3,3',4',5,5'-heptachloro-4-biphenylol; 2,2',3,4',5,5',6-heptachloro-4-biphenylol; and 2,2',3',4,4',5,5'-heptachloro-3-biphenylol. The hydroxy-PCBs exhibited minimal binding to the rat uterine cytosolic estrogen receptor (ER) and did not induce proliferation of estrogen-responsive MCF-7 human breast cancer cells at concentrations ranging from 10(-5) to 10(-8) M. The estrogenic activity of these compounds was further investigated utilizing two estrogen-responsive in vitro bioassays, namely, (i) HeLa cells stably transfected with a Gal4:human ER chimera and a 17-mer-regulated luciferase reporter gene, and (ii) MCF-7 cells transiently transfected with a full-length human ER expression plasmid and a plasmid containing an estrogen-responsive vitellogenin A2 promoter linked to a chloramphenicol acetyl transferase (CAT) reporter gene. None of the hydroxy-PCBs significantly induced luciferase activity in the stably transfected HeLa cells or CAT activity in MCF-7 cells at concentrations as high as 10(-5) M. The antiestrogenic effects of the hydroxy-PCBs were also investigated using the same bioassays in which the cells were cotreated with 17beta-estradiol plus the hydroxy-PCBs. All of the hydroxy-PCB congeners inhibited one or more estrogenic response, and one congener, 2,2',3,4',5,5',6-heptachloro-4-biphenylol, inhibited 17beta-estradiol-induced cell proliferation and CAT activity in MCF-7 cells and luciferase activity in HeLa cells.

Estrogen-induced c-fos protooncogene expression in MCF-7 human breast cancer cells: role of estrogen receptor Sp1 complex formation

17Beta-estradiol (E2) induces c-fos protooncogene expression in MCF-7 human breast cancer cells, and previous studies in HeLa cells identified an imperfect palindromic estrogen-responsive element (-1212 to -1200) that was required for trans-activation. In contrast, the estrogen-responsive element was not required for E2 responsiveness in MCF-7 cells, and using a series of constructs containing wild-type (pF1) and mutant 5'-flanking sequences (-1220 to -1155) from the c-fos protooncogene promoter in transient transfection assays, it was shown that a GC-rich motif (5'-GGGGCGTGG) containing an imperfect Sp1-binding site was required for hormone-induced activity. This sequence also bound Sp1 protein in gel mobility shift assays, and coincubation with the estrogen receptor (ER) enhanced Sp1-DNA binding. E2 and 4'-hydroxytamoxifen, but not ICI 164,384, induced reporter gene activity in cells transiently transfected with pF1. E2 induced reporter gene activity in MDA-MB-231 breast cancer cells transiently cotransfected with pF1 and wild-type ER or variant ER in which the DNA-binding domain was deleted (HE11); plasmids expressing N-terminal or C-terminal domains of the ER containing activator function-1 or -2, respectively, were inactive in these assays. In contrast, only wild-type ER mediated 4'-hydroxytamoxifen-induced activity. Induction of c-fos protooncogene expression by E2 in MCF-7 cells is dependent on the formation of a transcriptionally active ER/Sp1 complex that binds to a GC-rich enhancer element.

MicroRNA-223 is a crucial mediator of PPARγ-regulated alternative macrophage activation

Polarized activation of adipose tissue macrophages (ATMs) is crucial for maintaining adipose tissue function and mediating obesity-associated cardiovascular risk and metabolic abnormalities; however, the regulatory network of this key process is not well defined. Here, we identified a PPARγ/microRNA-223 (miR-223) regulatory axis that controls macrophage polarization by targeting distinct downstream genes to shift the cellular response to various stimuli. In BM-derived macrophages, PPARγ directly enhanced miR-223 expression upon exposure to Th2 stimuli. ChIP analysis, followed by enhancer reporter assays, revealed that this effect was mediated by PPARγ binding 3 PPARγ regulatory elements (PPREs) upstream of the pre-miR-223 coding region. Moreover, deletion of miR-223 impaired PPARγ-dependent macrophage alternative activation in cells cultured ex vivo and in mice fed a high-fat diet. We identified Rasa1 and Nfat5 as genuine miR-223 targets that are critical for PPARγ-dependent macrophage alternative activation, whereas the proinflammatory regulator Pknox1, which we reported previously, mediated miR-223-regulated macrophage classical activation. In summary, this study provides evidence to support the crucial role of a PPARγ/miR-223 regulatory axis in controlling macrophage polarization via distinct downstream target genes.

Minireview: role of orphan nuclear receptors in cancer and potential as drug targets

The nuclear orphan receptors for which endogenous ligands have not been identified include nuclear receptor (NR)0B1 (adrenal hypoplasia congenita critical region on chromosome X gene), NR0B2 (small heterodimer partner), NR1D1/2 (Rev-Erbα/β), NR2C1 (testicular receptor 2), NR2C2 (testicular receptor 4), NR2E1 (tailless), NR2E3 (photoreceptor-specific NR [PNR]), NR2F1 chicken ovalbumin upstream promoter transcription factor 1 (COUP-TFI), NR2F2 (COUP-TFII), NR2F6 (v-erbA-related protein), NR4A1 (Nur77), NR4A2 (Nurr1), NR4A3 (Nor1), and NR6A1 (GCNF). These receptors play essential roles in development, cellular homeostasis, and disease including cancer where over- or underexpression of some receptors has prognostic significance for patient survival. Results of receptor knockdown or overexpression in vivo and in cancer cell lines demonstrate that orphan receptors exhibit tumor-specific pro-oncogenic or tumor suppressor-like activity. For example, COUP-TFII expression is both a positive (ovarian) and negative (prostate and breast) prognostic factor for cancer patients; in contrast, the prognostic activity of adrenal hypoplasia congenita critical region on chromosome X gene for the same tumors is the inverse of COUP-TFII. Functional studies show that Nur77 is tumor suppressor like in acute leukemia, whereas silencing Nur77 in pancreatic, colon, lung, lymphoma, melanoma, cervical, ovarian, gastric, and some breast cancer cell lines induces one or more of several responses including growth inhibition and decreased survival, migration, and invasion. Although endogenous ligands for the orphan receptors have not been identified, there is increasing evidence that different structural classes of compounds activate, inactivate, and directly bind several orphan receptors. Thus, the screening and development of selective orphan receptor modulators will have important clinical applications as novel mechanism-based agents for treating cancer patients overexpressing one or more orphan receptors and also for combined drug therapies.

Identification of a motif within the 5' regulatory region of pS2 which is responsible for AP-1 binding and TCDD-mediated suppression

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related compounds modulate several endocrine systems by altering hormone synthesis, enhancing ligand metabolism, and down-regulating receptor levels/binding activity. Previous studies have demonstrated that TCDD inhibits the 17beta-estradiol (E2)-induction of pS2, a human breast cancer prognostic marker. This inhibition occurs at the gene expression level and is Ah receptor (AhR)-mediated. Analysis of the 5' regulatory region has identified three motifs which resemble dioxin response element (DRE) core sequences. pS2-regulated luciferase deletion constructs identified the DRE-like motif located at -527 to -514 as being required for TCDD-mediated suppression. A point mutation within this core motif (T-518C) abolished the inhibition by TCDD while UV-induced protein-DNA cross-linking and competitive gel retardation assays demonstrated AhR complex binding to this motif. Further study of this region also revealed an adjacent putative AP-1 site, diverging by one base pair from the consensus sequence. Gel retardation assays using TPA-treated MCF-7 cell nuclear extracts showed an induced complex binding to the AP-1-like site. Competition studies and antibody supershifts confirmed that the retarded complex consists of AP-1-like proteins. pS2-regulated luciferase constructs containing mutations specific to the AP-1-like motif greatly diminished the inducibility in response to E2. These results suggest that an interaction between AhR complexes and AP-1-like proteins may be responsible for TCDD-mediated inhibition of E2-induced pS2 expression.