Transgenic mice demonstrate AP-1 (activator protein-1) transactivation is required for tumor promotion

A dominant-negative c-jun mutant inhibits lung carcinogenesis in mice

Lung cancer is the leading cause of cancer mortality in the United States and worldwide. The identification of key regulatory and molecular mechanisms involved in lung tumorigenesis is therefore critical to increase our understanding of this disease and could ultimately lead to targeted therapies to improve prevention and treatment. Induction of members of the activator protein-1 (AP-1) transcription factor family has been described in human non-small cell lung carcinoma. Activation of AP-1 can either stimulate or repress transcription of multiple gene targets, ultimately leading to increased cell proliferation and inhibition of apoptosis. In the present study, we show induction of AP-1 in carcinogen-induced mouse lung tumors compared with surrounding normal lung tissue. We then used a transgenic mouse model directing conditional expression of the dominant-negative c-jun mutant TAM67 in lung epithelial cells to determine the effect of AP-1 inhibition on mouse lung tumorigenesis. Consistent with low AP-1 activity in normal lung tissue, TAM67 expression had no observed effects in adult mouse lung. TAM67 decreased tumor number and overall lung tumor burden in chemically induced mouse lung tumor models. The most significant inhibitory effect was observed on carcinoma burden compared with lower-grade lesions. Our results support the concept that AP-1 is a key regulator of mouse lung tumorigenesis, and identify AP-1-dependent transcription as a potential target to prevent lung tumor progression.

AC3-33, a novel secretory protein, inhibits Elk1 transcriptional activity via ERK pathway

The transcription factor AP-1 plays an important role in cellular proliferation, transformation and death. In this study, we report a novel human gene, AC3-33 (GenBank name: c3orf33, FLJ31139), which encodes a secretory protein that can inhibit Elk1 transcriptional activity via ERK1/2 pathway. The AC3-33 mRNA encodes a protein of 251 amino acids, which is a classical secretory protein. Functional investigation reveals that overexpression of AC3-33 significantly inhibit AP-1 activity and DNA-binding ability. Further investigation indicated that overexpression of AC3-33 significantly inhibit transcriptional activity of Elk1 and c-jun, but not c-fos. As for the upstream of signaling pathway of Elk-1, our study demonstrated that overexpression of AC3-33 significantly down-regulates phosphorylation of ERK1/2, but not JNK/SAPK or p38 MAPK. These results clearly indicate that AC3-33 is a novel member of the secretory family and inhibits Elk1 transcriptional activity via ERK1/2 MAPK.

The role of neurofibromin in N-Ras mediated AP-1 regulation in malignant peripheral nerve sheath tumors

Plexiform neurofibromas commonly found in patients with Neurofibromatosis type I (NF1) have a 5% risk of being transformed into malignant peripheral nerve sheath tumors (MPNST). Germline mutations in the NF1 gene coding for neurofibromin, which is a Ras GTPase activating protein (RasGAP) and a negative regulator of Ras, result in an upregulation of the Ras pathway. We established a direct connection between neurofibromin deficiency and downstream effectors of Ras in cell lines from MPNST patients by demonstrating that knockdown of NF1 expression using siRNA in a NF1 wild type MPNST cell line, STS-26T, activates the Ras/ERK1,2 pathway and increases AP-1 binding and activity. We believe this is the first time the transactivation of AP-1 has been linked directly to neurofibromin deficiency in a disease relevant MPNST cell line. Previously, we have shown that N-Ras is constitutively activated in cell lines derived from independent MPNSTs from NF1 patients. We therefore sought to analyze the role of the N-Ras pathway in deregulating AP-1 transcriptional activity. We show that STS-26T clones conditionally expressing oncogenic N-Ras show increased phosphorylated ERK1,2 and phosphorylated JNK expression concomitant with increased AP-1 activity. MAP kinase pathways (ERK1,2 and JNK) were further examined in ST88-14, a neurofibromin-deficient MPNST cell line. The basal activity of ERK1,2 but not JNK was found to increase AP-1 activity. These experiments further confirmed the link between the loss of neurofibromin and increased activity of Ras/MAP kinase pathways and the activation of downstream transcriptional mechanisms in MPNSTs from NF1 patients.

Growth factor signaling pathways as targets for prevention of epithelial carcinogenesis

Growth factor receptor (GFR) signaling controls epithelial cell growth by responding to various endogenous or exogenous stimuli and subsequently activating downstream signaling pathways including Stat3, PI3K/Akt/mTOR, MAPK, and c-Src. Environmental chemical toxicants and UVB irradiation cause enhanced and prolonged activation of GFR signaling and downstream pathways that contributes to epithelial cancer development including skin cancer. Recent studies, especially those with tissue-specific transgenic mouse models, have demonstrated that GFRs and their downstream signaling pathways contribute to all three stages of epithelial carcinogenesis by regulating a wide variety of biological functions including proliferation, apoptosis, angiogenesis, cell adhesion, and migration. Inhibiting these signaling pathways early in the carcinogenic process results in reduced cell proliferation and survival, leading to decreased tumor formation. Collectively, these studies suggest that GFR signaling and subsequent downstream signaling pathways are potential targets for the prevention of epithelial cancers including skin cancer.

Role of NEK6 in tumor promoter-induced transformation in JB6 C141 mouse skin epidermal cells

NEK6 (NIMA-related kinase 6) is a homologue of the Aspergillus nidulans protein NIMA (never in mitosis, gene A). We demonstrate that overexpression of NEK6 induces anchorage-independent transformation of JB6 Cl41 mouse epidermal cells. Tissue arrays and Western immunoblot analysis show that NEK6 is overexpressed in malignant tissues and several cancer cell lines. Our data also show that NEK6 interacts with STAT3, an oncogenic transcription factor, and phosphorylates STAT3 on Ser(727), which is important for transcriptional activation. Additional studies using NEK6 mutants suggested that the phosphorylation on both Ser(206) and Thr(210) of NEK6 is critical for STAT3 phosphorylation and anchorage-independent transformation of mouse epidermal cells. Notably, knockdown of NEK6 decreased colony formation and STAT3 Ser(727) phosphorylation. Based on our findings, the most likely mechanism that can account for this biological effect involves the activation of STAT3 through the phosphorylation on Ser(727). Because of the critical role that STAT3 plays in mediating oncogenesis, the stimulatory effects of NEK6 on STAT3 and cell transformation suggest that this family of serine/threonine kinases might represent a novel chemotherapeutic target.

Quercetin potentiates UVB-Induced c-Fos expression: implications for its use as a chemopreventive agent

Quercetin (Qu) is currently being investigated as a chemopreventive agent for several cancers, including nonmelanoma skin cancer induced by UV light. We previously reported that Qu degradation has important consequences on signaling and cell biology. In the current study, we report that Qu induces c-Fos mRNA and protein expression through activation of p38 and cAMP-responsive element binding protein (CREB), and Qu potentiates UVB-induced c-Fos expression. Inclusion of ascorbic acid (AA) in cell culture medium stabilizes Qu and completely prevents both Qu- and UVB-induced p38 and CREB activation, leading to a blockade of c-fos gene expression through reduced CREB/cAMP-responsive element binding. AA stabilizes c-Fos mRNA, increasing steady-state levels even when c-fos gene expression is suppressed, but this has no effect on c-Fos protein levels in either mock- or UVB-irradiated cells. We report that Qu blocks mammalian target of rapamycin signaling and inhibits c-Fos protein expression directly through this mechanism because cotreatment with Qu and AA resulted in the complete suppression of UVB-induced c-Fos protein expression even in the presence of significantly increased mRNA levels. We further confirmed that this was not due to increased protein turnover because inhibition of proteasome activity with MG-132 did not raise c-Fos protein levels in Qu+AA-treated cells. Together, these data indicate that although Qu has been reported to have some beneficial properties as a chemopreventive agent, it is also capable of inducing c-fos expression, a cellular event important for the promotion phase of tumor development, if it is not stabilized.

STAT2 contributes to promotion of colorectal and skin carcinogenesis

Signal transducer and activator of transcription 2 (STAT2) is an essential transcription factor in the type I IFN (IFN-alpha/beta) signal transduction pathway and known for its role in mediating antiviral immunity and cell growth inhibition. Unlike other members of the STAT family, IFNs are the only cytokines known to date that can activate STAT2. Given the inflammatory and antiproliferative dual nature of IFNs, we hypothesized that STAT2 prevents inflammation-induced colorectal and skin carcinogenesis by altering the inflammatory immune response. Contrary to our hypothesis, deletion of STAT2 inhibited azoxymethane/dextran sodium sulfate-induced colorectal carcinogenesis as measured by prolonged survival, lower adenoma incidence, smaller polyps, and less chronic inflammation. STAT2 deficiency also inhibited 7,12-dimethylbenz(a)anthracene/12-O-tetradecanoylphorbol-13-acetate-induced skin carcinogenesis as indicated by reduced papilloma multiplicity. A potential mechanism by which STAT2 promotes carcinogenesis is through activation of proinflammatory mediators. Deletion of STAT2 decreased azoxymethane/dextran sodium sulfate-induced expression and release of proinflammatory mediators, such as interleukin-6 and CCL2, and decreased interleukin-6 release from skin carcinoma cells, which then decreased STAT3 activation. Our findings identify STAT2 as a novel contributor to colorectal and skin carcinogenesis that may act to increase the gene expression and secretion of proinflammatory mediators, which in turn activate the oncogenic STAT3 signaling pathway.

The AP-1 repressor protein, JDP2, potentiates hepatocellular carcinoma in mice

Background

The AP-1 transcription factor plays a major role in cell proliferation, apoptosis, differentiation and developmental processes. AP-1 proteins are primarily considered to be oncogenic. Gene disruption studies placed c-Jun as an oncogene at the early stage of a mouse model of hepatocellular carcinoma. Mice lacking c-Jun display reduced number and size of hepatic tumors attributed to elevated p53 expression and increased apoptosis. This suggests that c-Jun inhibition may serve as a therapeutic target for liver cancer. The c-Jun dimerization protein 2, JDP2 is an AP-1 repressor protein that potently inhibits AP-1 transcription. On the other hand, the JDP2 locus was found at a recurring viral integration site in T-cell lymphoma. We sought to examine the potential of JDP2 to inhibit c-Jun/AP-1 oncogenic activity in mice. Towards this end, we generated a tetracycline inducible transgenic mouse expressing JDP2 specifically in the liver. We used diethylnitrosamine (DEN) injection to initiate liver cancer in mice and assessed the extent of liver cancer in JDP2-transgenic and wild type control mice by biochemical and molecular biology techniques.

Results

JDP2-transgenic mice display normal liver function. JDP2-transgenic mice displayed potentiation of liver cancer, higher mortality and increased number and size of tumors. The expression of JDP2 at the promotion stage was found to be the most critical for enhancing liver cancer severity.

Conclusions

This study suggests that JDP2 expression may play a critical role in liver cancer development by potentiating the compensatory proliferative response and increased inflammation in the DEN liver cancer model.

AP-1--The Jun proteins: Oncogenes or tumor suppressors in disguise?

Since its discovery more than two decades ago the involvement of the Activating protein 1 (AP-1) in proliferation, inflammation, differentiation, apoptosis, cellular migration and wound healing has been intensively studied. A model based on the early studies suggested antagonistic roles for the Jun proteins in proliferation and transformation. c-Jun was suggested to enhance transformation whereas JunB suggested to inhibit it in an antagonistic manner. Surprisingly, despite accumulation of data obtained from animal models regarding the role of Jun proteins in cancer and identification of oncogenic pathways regulating them, their involvement in human cancer was not demonstrated until recently. Here, we will describe the current knowledge about the roles of Jun proteins in human neoplasia. We will focus on the pathological examples demonstrating that the initial dogma has to be reexamined. For example, like c-Jun, JunB seems to play an oncogenic role in lymphomas, particularly in Hodgkin's lympomas. Furthermore, unlike the antagonistic activities of c-Jun and JunB in the transcription of genes coding for major cell cycle regulators such as CyclinD or p16INK4A, the transcription of other cell cycle regulating genes is modified similarly by c-Jun or JunB. Interestingly, some of these genes such as the ones coding for CyclinA or p19(ARF) are important players in either positive or negative regulation of cellular proliferation and survival. Finally, we will also discuss results posing JNK, known so far as the major activator of c-Jun, as a negative regulator of c-Jun level and activity. These recent findings suggest that the role of each Jun protein in neoplasia as well as in cellular survival should be examined in a context-dependent manner.

Emerging roles of ATF2 and the dynamic AP1 network in cancer

Cooperation among transcription factors is central for their ability to execute specific transcriptional programmes. The AP1 complex exemplifies a network of transcription factors that function in unison under normal circumstances and during the course of tumour development and progression. This Perspective summarizes our current understanding of the changes in members of the AP1 complex and the role of ATF2 as part of this complex in tumorigenesis.

The anticancer effects of actinoporin RTX-A from the sea anemone Heteractis crispa (=Radianthus macrodactylus)

Four isoforms of actinoporins were isolated in 2002-2004 from the tropical sea anemone Heteractis crispa (=Radianthus macrodactylus). Their potent hemolytic activities and effects on Ehrlich ascites carcinoma bearing mice were also studied. In this study, the individual actinoporin (RTX-A) demonstrated potential cancer-preventive activity at extremely low and non-cytotoxic concentrations. The substance suppressed the malignant transformation of mouse JB6 P(+) Cl41 cells stimulated by epidermal growth factor (EGF) in soft agar with the inhibition of number of the colonies C(50) (INCC(50))=0.034 nM. Actinoporin RTX-A also was shown to inhibit the phenotype expression of HeLa human cancer cells with an INCC(50)=0.03 nM. The cytotoxic effect of RTX-A against JB6 P(+) Cl41 cells and HeLa, THP-1, MDA-MB-231, and SNU-C4 human tumor cell lines was high (IC(50)=0.57, 2.26, 1.11, 30.0 and 4.66 nM), but significantly less than their capacity to suppress tumor cell colony formation or phenotype expression. RTX-A also induced apoptosis and inhibited basal AP-1, NF-kappaB, and p53-dependent transcriptional activity in JB6 Cl41 cells. These results confirmed that actinoporin RTX-A from H. crispa, at least partially, might exhibit cancer-preventive and anticancer cytotoxic properties through the induction of p53-independent apoptosis and inhibition of the oncogenic AP-1 and NF-kappaB nuclear factors activity.

The potential role of c-Jun activation in patients with cutaneous lichen planus

c-Jun, a component of the activating protein-1 transcription factor family, has been known to play an important role in the control of cell proliferation. It is also suspected to be a critical mediator of tumor promotion. However, investigations of c-Jun activation patterns in inflammatory and inflammatory transforming skin diseases have not been described so far. In this work, we show the c-Jun activation pattern in skin samples of patients with cutaneous lichen planus (LP), squamous cell carcinoma (SCC), psoriasis and normal skin using an immunohistochemical approach and Western blot analysis. In addition, we studied the c-Jun activation pattern in histological samples of three patients in whom LP transformed to SCC. We show that c-Jun is rarely activated in normal skin and psoriasis in contrast to LP and SCC. Our results suggest that c-Jun activation in human skin is involved in (1) proliferation and (2) could potentially participate in the transformation of LP from an inflammatory to a carcinogenic state. Nevertheless, JNK1/2, an important c-Jun activating kinase, was not found to be differentially regulated in LP and SCC compared with normal skin.

Effect of ultraviolet B radiation on activator protein 1 constituent proteins and modulation by dietary energy restriction in SKH-1 mouse skin

The study examined the timing of modulation of activator protein 1(AP-1):DNA binding and production of AP-1 constituent proteins by ultraviolet B (UVB) radiation and effect of dietary energy restriction [DER, 40% calorie reduction from fat and carbohydrate compared to control ad libitum (AL) diet] in SKH-1 mouse epidermis. AP-1:DNA binding by electromobility shift assay (EMSA) was increased in a biphasic manner after treatment with a tumor-promoting suberythemal dose (750 mJ/cm(2)) of UVB light (311-313 nm) with peaks at 3 and 18 h postirradiation. DER overall reduced AP-1:DNA binding in mock-treated and UVB-treated skin at 3 and 18 h after UVB treatment. The timing of modulation of production of AP-1 constituent proteins by Western blot analysis was examined at 0 h (mock treatment), 3, 9, 18, and 24 h. We found that c-jun (9 h), jun-B (9 and 18 h), phosphorylated c-jun (3 h), and fra-1 (18 h) protein levels were increased after UVB treatment compared to mock controls. In a follow-up diet experiment, animals were placed on DER or AL diet for 10-12 wk and treated with UVB as before. DER was found to completely block the UVB-induced increase in phosphorylated c-jun protein levels and decrease in fra-2 protein levels at 18 h. In addition, DER enhanced UVB-induced increase in jun B levels and lowered basal levels of c-fos seen 18 h after UVB. These data suggest that DER may be able to assist in the prevention of UVB-induced skin carcinogenesis by modulating AP-1:DNA binding and AP-1 constituent protein levels.

Identification and characterization of human LYPD6, a new member of the Ly-6 superfamily

The Ly-6 protein superfamily is usually identified as a group of proteins with a LU protein domain. LU domain is about 80 amino acids long and characterized by a conserved pattern of 10 cysteine residues. Here we report the cloning and characterization of a novel human LU domain containing gene, LYPD6, isolated from human testis cDNA library, and mapped to 2q23.1-23.2 by searching the UCSC genomic database. The LYPD6 cDNA sequence of 3,501 base pairs contains an open reading frame encoding 171 amino acids. Subcellular localization of LYPD6 demonstrated that the protein was localized in the cytoplasm when overexpressed in COS-7 cells. RT-PCR analysis showed that LYPD6 was widely expressed in human tissues and the expression levels in brain and heart were relatively high. Furthermore, the subsequent analysis based on reporter gene assays suggested that overexpression of LYPD6 in HEK 293T cells was able to suppress the transcriptional activities of AP1.

Deoxycholate induces COX-2 expression via Erk1/2-, p38-MAPK and AP-1-dependent mechanisms in esophageal cancer cells

Background

The progression from Barrett's metaplasia to adenocarcinoma is associated with the acquirement of an apoptosis-resistant phenotype. The bile acid deoxycholate (DCA) has been proposed to play an important role in the development of esophageal adenocarcinoma, but the precise molecular mechanisms remain undefined. The aim of this study was to investigate DCA-stimulated COX-2 signaling pathways and their possible contribution to deregulated cell survival and apoptosis in esophageal adenocarcinoma cells.

Methods

Following exposure of SKGT-4 cells to DCA, protein levels of COX-2, MAPK and PARP were examined by immunoblotting. AP-1 activity was assessed by mobility shift assay. DCA-induced toxicity was assessed by DNA fragmentation and MTT assay.

Results

DCA induced persistent activation of the AP-1 transcription factor with Fra-1 and JunB identified as the predominant components of the DCA-induced AP-1 complex. DCA activated Fra-1 via the Erk1/2- and p38 MAPK while Erk1/2 is upstream of JunB. Moreover, DCA stimulation mediated inhibition of proliferation with concomitant low levels of caspase-3-dependent PARP cleavage and DNA fragmentation. Induction of the anti-apoptotic protein COX-2 by DCA, via MAPK/AP-1 pathway appeared to balance the DCA mediated activation of pro-apoptotic markers such as PARP cleavage and DNA fragmentation. Both of these markers were increased upon COX-2 suppression by aspirin pretreatment prior to DCA exposure.

Conclusion

DCA regulates both apoptosis and COX-2-regulated cell survival in esophageal cells suggesting that the balance between these two opposing signals may determine the transformation potential of DCA as a component of the refluxate.

p38delta Mitogen-activated protein kinase is essential for skin tumor development in mice

Activating Ras mutations occur in a large portion of human tumors. Yet, the signaling pathways involved in Ras-induced tumor formation remain incompletely understood. The mitogen-activated protein kinase pathways are among the best studied Ras effector pathways. The p38 mitogen-activated protein kinase isoforms are important regulators of key biological processes including cell proliferation, differentiation, survival, inflammation, senescence, and tumorigenesis. However, the specific in vivo contribution of individual p38 isoforms to skin tumor development has not been elucidated. Recent studies have shown that p38delta, a p38 family member, functions as an important regulator of epidermal keratinocyte differentiation and survival. In the present study, we have assessed the effect of p38delta deficiency on skin tumor development in vivo by subjecting p38delta knockout mice to a two-stage 7,12-dimethylbenz(a)anthracene/12-O-tetradecanoylphorbol-13-acetate chemical skin carcinogenesis protocol. We report that mice lacking p38delta gene exhibited a marked resistance to development of 7,12-dimethylbenz(a)anthracene/12-O-tetradecanoylphorbol-13-acetate-induced skin papillomas, with increased latency and greatly reduced incidence, multiplicity, and size of tumors compared with wild-type mice. Our data suggest that the underlying mechanism for reduced susceptibility to skin carcinogenesis in p38delta-null mice involves a defect in proliferative response associated with aberrant signaling through the two major transformation-promoting pathways: extracellular signal-regulated kinase 1/2-activator protein 1 and signal transducer and activator of transcription 3. These findings strongly suggest an in vivo role for p38delta in promoting cell proliferation and tumor development in epidermis and may have therapeutic implication for skin cancer.

Inhibition of CREB function in mouse epidermis reduces papilloma formation

We used a double transgenic tetracycline system to conditionally express A-CREB, a dominant negative protein that prevents the DNA binding and function of cAMP-responsive element binding protein (CREB) family members, in mouse basal epidermis using the keratin 5 promoter. There was no phenotype in the adult. However, following a 7,12-dimethylbenz(a)anthracene (DMBA)/phorbol-12-myristate-13-acetate two-stage skin carcinogenesis experiment, A-CREB-expressing epidermis develop 5-fold fewer papillomas than wild-type controls. However, A-CREB expression one month after DMBA treatment does not prevent papilloma formation, suggesting that CREB functions at an early stage of papilloma formation. Oncogenic H-Ras genes with A-->T mutations in codon 61 were found in wild-type skin but not in A-CREB-expressing skin 2 days after DMBA treatment, suggesting that A-CREB either prevents DMBA mutagenesis or kills oncogenic H-Ras cells. In primary keratinocyte cultures, A-CREB expression induced apoptosis of v-Ras(Ha)-infected cells and suppressed the expression of cell cycle proteins cyclin B1 and cyclin D1. These results suggest that inhibiting CREB function is a valuable cancer prevention strategy.

Identification of the B-Raf/Mek/Erk MAP kinase pathway as a target for all-trans retinoic acid during skin cancer promotion

Background

Retinoids have been studied extensively for their potential as therapeutic and chemopreventive agents for a variety of cancers, including nonmelanoma skin cancer (NMSC). Despite their use for many years, the mechanism of action of retinoids in the prevention of NMSC is still unclear. In this study we have attempted to understand the chemopreventive mechanism of all-trans retinoic acid (ATRA), a primary biologically active retinoid, in order to more efficiently utilize retinoids in the clinic.

Results

We have used the 2-stage dimethylbenzanthracene (DMBA)/12-O-tetradecanoylphorbol-13-acetate (TPA) mouse skin carcinogenesis model to investigate the chemopreventive effects of ATRA. We have compared the gene expression profiles of control skin to skin subjected to the 2-stage protocol, with or without ATRA, using Affymetrix 430 2.0 DNA microarrays. Approximately 49% of the genes showing altered expression with TPA treatment are conversely affected when ATRA is co-administered. The activity of these genes, which we refer to as 'counter-regulated', may contribute to chemoprevention by ATRA. The counter-regulated genes have been clustered into functional categories and bioinformatic analysis has identified the B-Raf/Mek/Erk branch of the MAP kinase pathway as one containing several genes whose upregulation by TPA is blocked by ATRA. We also show that ATRA blocks signaling through this pathway, as revealed by immunohistochemistry and Western blotting. Finally, we found that blocking the B-Raf/Mek/Erk pathway with a pharmacological inhibitor, Sorafenib (BAY43-9006), induces squamous differentiation of existing skin SCCs formed in the 2-stage model.

Conclusion

These results indicate that ATRA targets the B-Raf/Mek/Erk signaling pathway in the 2-stage mouse skin carcinogenesis model and this activity coincides with its chemopreventive action. This demonstrates the potential for targeting the B-Raf/Mek/Erk pathway for chemoprevention and therapy of skin SCC in humans. In addition our DNA microarray results provide the first expression signature for the chemopreventive effect of ATRA in a mouse skin cancer model. This is a potential source for novel targets for ATRA and other chemopreventive and therapeutic agents that can eventually be tested in the clinic.

Linoleic acid metabolite suppresses skin inflammation and tumor promotion in mice: possible roles of programmed cell death 4 induction

(+/-)-13-Hydroxy-10-oxo-trans-11-octadecenoic acid (13-HOA) is one of the lipoxygenase metabolites of linoleic acid (LA) from corn germ. Recently, we reported that this metabolite suppressed the expression of lipopolysaccharide-induced proinflammatory genes in murine macrophages by disrupting mitogen-activated protein kinases and Akt pathways. In this study, we investigated the inhibitory effects of 13-HOA on 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced inflammation in ears and skin, as well as tumor promotion in female ICR mice. Pretreatment with 13-HOA (1600 nmol) inhibited ear edema formation by 95% (P < 0.05) in an inflammation test and reduced tumor incidence and the number of tumors per mouse by 40 and 64% (P < 0.05 each), respectively, in a two-stage skin carcinogenesis model. Histological examinations revealed that it decreased epidermal thickness, the number of infiltrated leukocytes and cell proliferation index. Furthermore, 13-HOA (8-40 muM) suppressed TPA-induced anchorage-independent growth of JB6 mouse epidermal cells by 70-100%, whereas LA was virtually inactive. 13-HOA (40 muM) inhibited TPA-induced activator protein-1 transactivation but not extracellular signal-regulated kinase1/2 activation. Interestingly, 13-HOA (40 muM and 1600 nmol in JB6 cells and mouse skin, respectively) induced expression of programmed cell death 4 (Pdcd4), a novel tumor suppressor protein. To our knowledge, this is the first report of a food factor that is able to induce Pdcd4 expression. Collectively, our results indicate that 13-HOA may be a novel anti-inflammatory and antitumor chemopreventive agent with a unique mode of action.

Quassinoid inhibition of AP-1 function does not correlate with cytotoxicity or protein synthesis inhibition

Several quassinoids were identified in a high-throughput screening assay as inhibitors of the transcription factor AP-1. Further biological characterization revealed that while their effect was not specific to AP-1, protein synthesis inhibition and cell growth assays were inconsistent with a mechanism of simple protein synthesis inhibition. Numerous plant extracts from the plant family Simaroubaceae were also identified in the same screen; bioassay-guided fractionation of one extract (Ailanthus triphylla) yielded two known quassinoids, ailanthinone (3) and glaucarubinone (4), which were also identified in the pure compound screening procedure.

Delphinidin attenuates neoplastic transformation in JB6 Cl41 mouse epidermal cells by blocking Raf/mitogen-activated protein kinase kinase/extracellular signal-regulated kinase signaling

Recent studies suggest that anthocyanidins play a pivotal role in the chemopreventive effects of fruits and vegetables. However, the underlying molecular mechanisms and cellular targets remain unknown. Neoplastic transformation of cells and inflammation are considered to be major events contributing to carcinogenesis. Here, we report that delphinidin, a major dietary anthocyanidin, inhibits tumor promoter-induced transformation and cyclooxygenase-2 (COX-2) expression in JB6 promotion-sensitive mouse skin epidermal (JB6 P+) cells by directly targeting Raf and mitogen-activated protein kinase kinase (MEK). Delphinidin inhibited 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced neoplastic transformation and COX-2 expression at both the protein and transcriptional levels. The activation of activator protein-1 and nuclear factor-kappaB induced by TPA was dose dependently inhibited by delphinidin treatment. Delphinidin strongly suppressed Raf1 and MEK1 kinase activities and subsequently attenuated TPA-induced phosphorylation of MEK, extracellular signal-regulated kinase (ERK), p90RSK, and MSK. Although delphinidin suppressed ERK and c-Jun NH(2)-terminal kinase activities, it was more effective at inhibiting Raf1 or MEK1 activities. Pull-down and competition assays revealed that delphinidin binds with Raf1 or MEK1 noncompetitively with ATP. Delphinidin also dose dependently suppressed JB6 P+ cell transformation induced by epidermal growth factor and H-Ras, both of which are involved in the activation of Raf/MEK/ERK signaling. Together, these findings suggested that the targeted inhibition of Raf1 and MEK activities and COX-2 expression by delphinidin contribute to the chemopreventive potential of fruits and vegetables.

A selective small-molecule nuclear factor-kappaB inhibitor from a high-throughput cell-based assay for "activator protein-1 hits"

NSC 676914 has been identified as a selective nuclear factor-kappaB (NF-kappaB) inhibitor that does not inhibit cell proliferation. This compound was originally identified in a high-throughput cell-based assay for activator protein-1 (AP-1) inhibitors using synthetic compound libraries and the National Cancer Institute natural product repository. NSC 676914 shows activity against NF-kappaB in luciferase reporter assays at concentrations much less than the IC50 for AP-1. A serum response element reporter used as a specificity control and indicator of cell proliferation was relatively insensitive to the compound. Pretreatment with NSC 676914 is here shown to repress 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced IkappaB-alpha phosphorylation and translocation of p65/50 to the nucleus but not the processing of p52 from p100, suggesting the inhibition of NF-kappaB regulator IKKbeta rather than IKKalpha. Inhibition of NF-kappaB activation occurred as a consequence of blocking phosphorylation of IKK. Induction of IkappaB-alpha phosphorylation by TPA was diminished by pretreatment of NSC 676914 even at 1.1 mumol/L. In contrast, kinases c-Jun-NH2-kinase and extracellular signal-regulated kinases 1 and 2, important for AP-1 activation, showed no significant repression by this compound. Furthermore, a Matrigel invasion assay with breast cancer cell lines and a transformation assay in mouse JB6 cells revealed that TPA-induced invasion and transformation responses were completely repressed by this compound. These results suggest that NSC 676914 could be a novel inhibitor having potential therapeutic activity to target NF-kappaB for cancer treatment or prevention.

Morelloflavone, a biflavonoid, inhibits tumor angiogenesis by targeting rho GTPases and extracellular signal-regulated kinase signaling pathways

Morelloflavone, a biflavonoid extracted from Garcinia dulcis, has shown antioxidative, antiviral, and anti-inflammatory properties. However, the function and the mechanism of this compound in cancer treatment and tumor angiogenesis have not been elucidated to date. In this study, we postulated that morelloflavone might have the ability to inhibit angiogenesis, the pivotal step in tumor growth, invasiveness, and metastasis. We showed that morelloflavone could inhibit vascular endothelial growth factor (VEGF)-induced cell proliferation, migration, invasion, and capillary-like tube formation of primary cultured human umbilical vascular endothelial cells in a dose-dependent manner. Morelloflavone effectively inhibited microvessel sprouting of endothelial cells in the mouse aortic ring assay and the formation of new blood microvessels induced by VEGF in the mouse Matrigel plug assay. Furthermore, morelloflavone inhibited tumor growth and tumor angiogenesis of prostate cancer cells (PC-3) in xenograft mouse tumor model in vivo, suggesting that morelloflavone inhibited tumorigenesis by targeting angiogenesis. To understand the underlying mechanism of morelloflavone on the inhibitory effect of tumor growth and angiogenesis, we showed that morelloflavone could inhibit the activation of both RhoA and Rac1 GTPases but have little effect on the activation of Cdc42 GTPase. Additionally, morelloflavone inhibited the phosphorylation and activation of Raf/mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) kinase/ERK pathway kinases without affecting VEGF receptor 2 activity. Together, our results indicate that morelloflavone exerts antiangiogenic action by targeting the activation of Rho-GTPases and ERK signaling pathways. These findings are the first to reveal the novel functions of morelloflavone in tumor angiogenesis and its molecular basis for the anticancer action.

Cooperation between GLI and JUN enhances transcription of JUN and selected GLI target genes

Sustained Hedgehog (HH) signaling is implicated in basal cell carcinoma of the skin and other types of cancer. Here we show that GLI1 and GLI2, the main transcriptional activators of the HH pathway, directly regulate expression of the activator protein 1 (AP-1) family member JUN, a transcription factor controlling keratinocyte proliferation and skin homeostasis. Activation of the JUN promoter by GLI is dependent on a GLI-binding site and the AP-1 sites known to be involved in self-activation of JUN. Transcription of JUN is greatly enhanced in the presence of GLI and requires activated JUN protein. GLI2act is a more potent activator than GLI1 in these experiments and physical interaction with phosphorylated JUN was only detected for GLI2act. The synergistic effect of GLI and JUN extends to the activation of further GLI target genes as shown by shRNA-mediated knockdown of JUN in human keratinocytes. Some of these cooperatively activated genes are involved in cell-cycle progression, which is consistent with a significant reduction of the proliferative potential of GLI in the absence of JUN. These results suggest a novel connection between HH/GLI pathway activity and JUN, which may contribute to the oncogenic activity of HH/GLI signaling in skin.

Loss of E-cadherin-mediated cell-cell contacts activates a novel mechanism for up-regulation of the proto-oncogene c-Jun

Loss of E-cadherin-mediated cell-cell contacts can elicit a signaling pathway that leads to acquisition of an invasive phenotype. Here, we show that at the receiving end of this pathway is the proto-oncogene c-Jun, a member of the activator protein-1 family of transcription factors that play a key role in stimulation of cell proliferation and tumor promotion. Cell separation or abrogation of E-cadherin-mediated cell-cell contacts both cause a dramatic increase in accumulation of the c-Jun protein. Unlike growth factors that enhance the expression of c-Jun by activating the transcription of the c-jun gene, the cell contact-dependent increase in c-Jun accumulation is not accompanied by a corresponding increase in c-Jun mRNA or c-Jun protein stability but rather in the translatability of the c-Jun transcript. Consistently, the increase in c-Jun accumulation is not dependent on activation of the mitogen-activated protein kinase or beta-catenin pathways but is mediated by signals triggered by the restructured cytoskeleton. Depolymerization of the cytoskeleton can mimic the effect of cell separation and cause a dramatic increase in c-Jun accumulation, whereas Taxol inhibits the cell contact-dependent increase. This novel mechanism of c-Jun regulation seems to underlie the robust overexpression of c-Jun in tumor cells of patients with colon carcinoma.

Cyclin-dependent kinase-3-mediated c-Jun phosphorylation at Ser63 and Ser73 enhances cell transformation

c-Jun is a component of the activator protein-1 (AP-1) complex, which plays a crucial role in the regulation of gene expression, cell proliferation, and cell transformation, as well as cancer development. Herein, we found that cyclin-dependent kinase (Cdk)-3, but not Cdk2 or c-Jun NH(2)-terminal kinase, is a novel kinase of c-Jun induced by stimulation with growth factors such as epidermal growth factor (EGF). Cdk3 was shown to phosphorylate c-Jun at Ser63 and Ser73 in vitro and ex vivo. EGF-induced Cdk3 activation caused c-Jun phosphorylation at Ser63 and Ser73, resulting in increased AP-1 transactivation. Ectopic expression of Cdk3 resulted in anchorage-independent cell transformation of JB6 Cl41 cells induced by EGF and foci formation stimulated by constitutively active Ras (Ras(G12V)), which was mediated by AP-1 in NIH3T3 cells. These results showed that the Cdk3/c-Jun signaling axis plays an important role in EGF-stimulated cell proliferation and cell transformation.

Activation of phosphatidylinositol 3-kinase is required for tumor necrosis factor-alpha-induced upregulation of matrix metalloproteinase-9: its direct inhibition by quercetin

Matrix metalloproteinases (MMPs) are involved in various skin disorders, including photoaging, dermatitis, and tumorigenesis. Tumor necrosis factor (TNF)-alpha is a key proinflammatory cytokine that acts to provoke inflammation, proliferation, and tumorigenesis. The present study investigated the possible inhibitory effects of red wine polyphenols on TNF-alpha-induced upregulation of MMP-9 and on the migratory phenotype of JB6 P+ mouse epidermal (JB6 P+) cells. Red wine extract (RWE) and quercetin, which is a major flavonoid present in red wine, inhibited significantly the TNF-alpha-induced upregulation of MMP-9 and cell migration, whereas resveratrol did not have significant inhibitory effects. The inhibitory effects of RWE and quercetin were mediated by suppression of the phosphorylation of Akt and the transactivation of activator protein-1 and nuclear factor-kappaB, as determined by Western blotting and luciferase assays, respectively. Aside from Akt, quercetin had no effect on the phosphorylation of other mitogen-activated protein kinases. Direct kinase assay data revealed that RWE and quercetin inhibited phosphatidylinositol 3-kinase (PI3K) activity. The results of direct and cell-based pull-down assays demonstrated that RWE and quercetin bound to PI3K, resulting in the inhibition of PI3K activity. Using chemical inhibitors, it was confirmed that the PI3K-dependent Akt pathway was involved in TNF-alpha-induced MMP-9 upregulation and migration in JB6 P+ cells. Collectively, these results indicate that TNF-alpha-induced MMP-9 upregulation and the migratory phenotype are associated with the PI3K/Akt pathway, and that these effects are inhibited strongly by RWE and quercetin.

Oral administration of blueberry inhibits angiogenic tumor growth and enhances survival of mice with endothelial cell neoplasm

Endothelial cell neoplasms are the most common soft tissue tumor in infants. Subcutaneous injection of spontaneously transformed murine endothelial (EOMA) cells results in development of hemangioendothelioma (HE). We have previously shown that blueberry extract (BBE) treatment of EOMA cells in vitro prior to injection in vivo can significantly inhibit the incidence and size of developing HE. In this study, we sought to determine whether oral BBE could be effective in managing HE and to investigate the mechanisms through which BBE exerts its effects on endothelial cells. A dose-dependent decrease in HE tumor size was observed in mice receiving daily oral gavage feeds of BBE. Kaplan-Meier survival curve showed significantly enhanced survival for mice with HE tumors given BBE, compared to control. BBE treatment of EOMA cells inhibited both c-Jun N-terminal kinase (JNK) and NF-kappaB signaling pathways that culminate in monocyte chemoattractant protein-1 (MCP-1) expression required for HE development. Antiangiogenic effects of BBE on EOMA cells included decreased proliferation by BrdU assay, decreased sprouting on Matrigel, and decreased transwell migration. Thus, this work provides first evidence demonstrating that BBE can limit tumor formation through antiangiogenic effects and inhibition of JNK and NF-kappaB signaling pathways. Oral administration of BBE represents a potential therapeutic antiangiogenic strategy for treating endothelial cell neoplasms in children.

Sulfiredoxin is an AP-1 target gene that is required for transformation and shows elevated expression in human skin malignancies

Previous studies have shown that a dominant negative form of c-Jun (TAM67) suppresses mouse skin carcinogenesis both in vitro and in vivo. The current study identifies Sulfiredoxin (Srx) as a unique target of activator protein-1 (AP-1) activation and TAM67 inhibition. Manipulation of Srx levels by ShRNA or over-expression demonstrates that Srx is critical for redox homeostasis through reducing hyperoxidized peroxiredoxins. In JB6 cells, knockdown of Srx abolishes tumor promoter-induced transformation and enhances cell sensitivity to oxidative stress. Knockdown of Srx also impairs c-Jun phosphorylation, implicating a role for Srx in the feedback regulation of AP-1 activity. Screening of patient tissues by tissue microarray reveals elevated Srx expression in several types of human skin cancers. Our study indicates that Srx is a functionally significant target of AP-1 blockade that may have value in cancer prevention or treatment.

Inhibitory effects of Baicalin on ultraviolet B-induced photo-damage in keratinocyte cell line

Baicalin, one kind of Chinese herbal medicine with anti-inflammatory and anti-oxidant property, has been commonly used as a clinical medicine. However, little has been known about the effects of Baicalin on ultraviolet (UV) induced photo-aging and photo-carcinogenesis. The photoproduct is critical to the initial event of UV-induced photo-carcinogenesis. The purpose of the present study was to investigate whether Baicalin, in immortalized human keratinocyte HaCaT cells, could inhibit ultraviolet-B (UVB) induced skin damage and its possible underlying mechanisms, such as inhibiting UVB-induced cytotoxicity and apoptosis, cyclobutane pyrimidine dimers (CPDs), down-regulating the expression of regulatory proteins which are related to cell apoptosis and DNA damage/repair. Our study revealed that Baicalin treatment could inhibit the UVB-induced cytotoxicity, apoptosis and CPD level. It also decreased the mRNA expression of apoptosis-regulatory genes (p53-p21 and c-fos), the protein levels of p53, proliferating cell nuclear antigen (PCNA) and repair protein A (RPA), and the secretion of cytokines [interleukin(IL)-6 and tumor necrosis factor (TNF-alpha)]. These results suggested that Baicalin may have an inhibitory effect on the UVB-induced photo-damage by blocking the relevant cytokine secretion and expression of p53-p21, c-fos, PCNA and RPA genes.

An autoregulatory loop mediated by miR-21 and PDCD4 controls the AP-1 activity in RAS transformation

The transcription factor AP-1 plays key roles in tumorigenesis, by regulating a variety of protein-coding genes, implicated in multiple hallmarks of cancer. Among non-coding genes, no AP-1 target has been described yet in tumorigenesis. MicroRNAs (miRNAs) are negative post-transcriptional regulators of protein-coding genes. miRNA expression signatures are highly relevant in cancer and several tumor-associated miRNAs (oncomirs) play critical roles in oncogenesis. Here, we show that the miRNA miR-21, which represents the most frequently upregulated oncomir in solid tumors, is induced by AP-1 in response to RAS. By analyzing validated miR-21 targets, we have found that the tumor suppressors PTEN and PDCD4 are downregulated by RAS in an AP-1- and miR-21-dependent fashion. We further show that, given the role of PDCD4 as negative regulator of AP-1, the miR-21-mediated downregulation of PDCD4 is essential for the maximal induction of AP-1 activity in response to RAS. Our data reveal a novel mechanism of positive autoregulation of the AP-1 complex in RAS transformation and disclose the function of oncomirs as critical targets and regulators of AP-1 in tumorigenesis.

Proapoptotic and Anticarcinogenic Activities of Leviusculoside G from the Starfish Henricia leviuscula and Probable Molecular Mechanism

Proapoptotic and anticarcinogenic properties of leviusculoside G (LSG) isolated from the starfish Henricia leviuscula and a probable mechanism of these effects were investigated in human leukemia HL-60, THP-1, and mouse skin JB6 Cl41 cells, using a variety of assessments, including a cell viability (MTS) assay, flow cytometry, anchorage-independent soft agar assay, luciferase assay, and western-blotting. Application of LSG at nontoxic doses induced apoptosis in cancer cells and decreased cell transformation. This compound exerted its actions, at least in part, through the induction of p53-dependent apoptosis and inhibition of AP-1, NF-κB, and ERKs activities.

Conformationally constrained analogues of diacylglycerol. 29. Cells sort diacylglycerol-lactone chemical zip codes to produce diverse and selective biological activities

Diacylglycerol-lactone (DAG-lactone) libraries generated by a solid-phase approach using IRORI technology produced a variety of unique biological activities. Subtle differences in chemical diversity in two areas of the molecule, the combination of which generates what we have termed "chemical zip codes", are able to transform a relatively small chemical space into a larger universe of biological activities, as membrane-containing organelles within the cell appear to be able to decode these "chemical zip codes". It is postulated that after binding to protein kinase C (PKC) isozymes or other nonkinase target proteins that contain diacylglycerol responsive, membrane interacting domains (C1 domains), the resulting complexes are directed to diverse intracellular sites where different sets of substrates are accessed. Multiple cellular bioassays show that DAG-lactones, which bind in vitro to PKCalpha to varying degrees, expand their biological repertoire into a larger domain, eliciting distinct cellular responses.

Myricetin suppresses UVB-induced skin cancer by targeting Fyn

Skin cancer is currently the most common type of human cancer in Americans. Myricetin, a naturally occurring phytochemical, has potent anticancer-promoting activity and contributes to the chemopreventive potential of several foods, including red wine. Here, we show that myricetin suppresses UVB-induced cyclooxygenase-2 (COX-2) expression in mouse skin epidermal JB6 P+ cells. The activation of activator protein-1 and nuclear factor-kappaB induced by UVB was dose-dependently inhibited by myricetin treatment. Western blot and kinase assay data revealed that myricetin inhibited Fyn kinase activity and subsequently attenuated UVB-induced phosphorylation of mitogen-activated protein kinases. Pull-down assays revealed that myricetin competitively bound with ATP to suppress Fyn kinase activity. Importantly, myricetin exerted similar inhibitory effects compared with 4-amino-5-(4-chloro-phenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine, a well-known pharmacologic inhibitor of Fyn. In vivo mouse skin data also revealed that myricetin inhibited Fyn kinase activity directly and subsequently attenuated UVB-induced COX-2 expression. Mouse skin tumorigenesis data clearly showed that pretreatment with myricetin significantly suppressed UVB-induced skin tumor incidence in a dose-dependent manner. Docking data suggest that myricetin is easily docked to the ATP-binding site of Fyn, which is located between the N and C lobes of the kinase domain. Overall, these results indicated that myricetin exerts potent chemopreventive activity mainly by targeting Fyn in skin carcinogenesis.

Glycogen synthase kinase 3beta (GSK3beta) in tumorigenesis and cancer chemotherapy

Glycogen synthase kinase 3beta (GSK3beta), a multifunctional serine/threonine kinase found in all eukaryotes, had been initially identified as a key regulator of insulin-dependent glycogen synthesis. It is now known that GSK3beta functions in diverse cellular processes including proliferation, differentiation, motility and survival. Aberrant regulation of GSK3beta has been implicated in a range of human pathologies including non-insulin-dependent diabetes mellitus, cardiovascular disease, some neurodegenerative diseases, and bipolar disorder. As a consequence, the therapeutic potential of GSK3beta inhibitors has become an important area of investigation. However, GSK3beta also participates in neoplastic transformation and tumor development. The role of GSK3beta in tumorigenesis and cancer progression remains controversial; it may function as a "tumor suppressor" for certain types of tumors, but promotes growth and development for some others. GSK3beta also mediates drug sensitivity/resistance in cancer chemotherapy. Therefore, although GSK3beta is an attractive therapeutic target for a number of human diseases, its potential impact on tumorigenesis and cancer chemotherapy needs to be carefully evaluated. This mini-review discusses the role of GSK3beta in tumorigenesis/cancer progression as well as its modulation of cancer chemotherapy.

Selectional and mutational scope of peptides sequestering the Jun-Fos coiled-coil domain

The activator protein-1 (AP-1) complex plays a crucial role in numerous pathways, and its ability to induce tumorigenesis is well documented. Thus, AP-1 represents an interesting therapeutic target. We selected peptides from phage display and compared their ability to disrupt the cFos/cJun interaction to a previously described in vivo protein-fragment complementation assay (PCA). A cJun-based library was screened to enrich for peptides that disrupt the AP-1 complex by binding to the cFos coiled-coil domain. Interestingly, phage display identified one helix, JunW(Ph1) [phage-selected winning peptide (clone 1) targeting cFos], which differs in only 2 out of 10 randomized positions to JunW (PCA-selected winning peptide targeting cFos). Phage-selected peptides revealed higher affinity to cFos than wild-type cJun, harboring a T(m) of 53 degrees C compared to 16 degrees C for cFos/cJun or 44 degrees C for cFos/JunW. In PCA growth assays in the presence of cJun as competitor, phage-selected JunW(Ph1) conferred shorter generation times than JunW. Bacterial growth was barely detectable, using JunW(Ph1) as a competitor for the wild-type cJun/cFos interaction, indicating efficient cFos removal from the dimeric wild-type complex. Importantly, all inhibitory peptides were able to interfere with DNA binding as demonstrated in gel shift assays. The selected sequences have consequently improved our 'bZIP coiled-coil interaction prediction algorithm' in distinguishing interacting from noninteracting coiled-coil sequences. Predicting and manipulating protein interaction will accelerate the systems biology field, and generated peptides will be valuable tools for analytical and biomedical applications.

Osteopontin: regulation in tumor metastasis

Osteopontin is a secreted phosphoprotein that has been implicated as an important mediator of tumor metastasis and has been investigated for use as a biomarker for advanced disease and as a potential therapeutic target in the regulation of cancer metastasis. The OPN DNA sequence is highly conserved and the protein contains several important functional domains including alpha(v)beta integrin and CD44 binding sites. High levels of OPN expression correlate with tumor invasion, progression or metastasis in multiple cancer. Studies demonstrate that osteopontin mediates the molecular mechanisms which determine metastatic spread, such as prevention of apoptosis, extracellular matrix proteolysis and remodeling, cell migration, evasion of host-immune cells and neovascularization. Transcriptional regulation of OPN is complex and involves multiple pathways, including AP-1, Myc, v-Src, Runx/CBF, TGF-B/BMPs/Smad/Hox, and Wnt/ss-catenin/APC/GSK-3ss/Tcf-4. The current state of knowledge of OPN biology suggests that it is an attractive target for therapeutic modulation of metastatic disease.

Polymeric black tea polyphenols inhibit mouse skin chemical carcinogenesis by decreasing cell proliferation

OBJECTIVES

The aim of this study was to investigate the antitumour promoting effects and possible mechanisms of action of the most abundant polymeric black tea polyphenols (PBPs 1-5) or thearubigins, in vivo.

MATERIALS AND METHODS

Effect of PBP pre-treatments on 12-O-tetradecanoylphorbol-13-acetate (TPA) promoted skin papillomas was studied in 7,12-dimethylbenz(a)anthracene initiated mice over 40 weeks. Cell proliferation and apoptosis, in epidermis of the skin, were measured using appropriate immunohistochemical staining. Mitogen-activated protein kinase signalling studies were conducted with Western blot analysis at 10, 20, 30 and 40 weeks of promotion.

RESULTS

Pre-treatments with PBP fractions differentially altered latency, multiplicity and incidence of skin papillomas as compared to TPA treatments thereby exhibiting antipromoting effects. Most PBP fractions decreased TPA-induced cell proliferation by decreasing activation of signalling kinases (c-Jun N-terminal protein kinase, extracellular signal-regulated protein kinase, p38 protein kinase and Akt), transcription factors (activator protein-1 and nuclear factor kappa B) and inflammatory protein (cyclooxygenase 2). TPA-induced epidermal cell apoptosis was also decreased by pre-treatment with most PBP fractions. Higher levels of p53 and p21 in skin cells pre-treated with PBP fractions followed by TPA treatment as compared to only TPA-treated animals suggested possible activation of a cell cycle checkpoint.

CONCLUSIONS

PBP-2 was observed to be the most potent polymeric polyphenol fraction and PBP-4 and PBP-5 showed only marginal activity, whereas PBP-1 and PBP-3 displayed intermediate efficacies. In conclusion, the protective effects of PBP fractions could be attributed to inhibition of TPA-induced cellular proliferation.

Targeting the activator protein 1 transcription factor for the prevention of estrogen receptor-negative mammary tumors

The oncogene erbB2 is overexpressed in 20% to 30% human breast cancers and is most commonly overexpressed in estrogen receptor (ER)-negative breast cancers. Transgenic mice expressing erbB2 develop ER-negative mammary tumors, mimicking human breast carcinogenesis. Previously, we have shown that activator protein 1 (AP-1) regulates proliferation of ER-negative breast cancer cells. We hypothesized that blockade of AP-1 in mouse mammary epithelial cells will suppress ER-negative tumorigenesis induced by erbB2. Trigenic erbB2 mice were generated by crossing a bigenic pUHD-Tam67/MMTV-rtTA mouse to a MMTV-erbB2 mouse. The resulting trigenic mice develop tumors and express a doxycycline-inducible c-Jun dominant negative mutant (Tam67) in the mammary glands. In vivo AP-1 blockade by Tam67 expression started delayed mammary tumor formation in MMTV-erbB2 mice by more than 11 weeks. By 52 weeks of age, 100% (18 of 18) of the untreated animals had developed mammary tumors, whereas 56% (9 of 16) of the doxycycline-treated trigenic mice developed tumors. In addition, the tumors that arose in the AP-1-blocked erbB2 mice failed to express Tam67. Twenty-five percent of the doxycycline-treated MMTV-erbB2 mice survived more than 72 weeks of age without developing mammary tumors. Examination of normal-appearing mammary glands from these mice showed that AP-1 blockade by Tam67 also significantly prevents the development of premalignant lesions in these glands. The expression of erbB2 either in normal mammary tissue or in mammary tumors was not altered. Our results show that blocking the AP-1 signaling in mammary cells suppresses erbB2-induced transformation, and show that the AP-1 transcription factor is a critical transducer of erbB2. These results provide a scientific rationale to develop targeted drugs that inhibit AP-1 to prevent the development of ER-negative breast cancer.

Raf and MEK protein kinases are direct molecular targets for the chemopreventive effect of quercetin, a major flavonol in red wine

Considerable attention has focused on the health-promoting effects of red wine and its nonflavonoid polyphenol compound resveratrol. However, the underlying molecular mechanisms and molecular target(s) of red wine or other potentially active ingredients in red wine remain unknown. Here, we report that red wine extract (RWE) or the red wine flavonoid quercetin inhibited 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced transformation of JB6 promotion-sensitive mouse skin epidermal (JB6 P+) cells. The activation of activator protein-1 and nuclear factor-kappaB induced by TPA was dose dependently inhibited by RWE or quercetin treatment. Western blot and kinase assay data revealed that RWE or quercetin inhibited mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) kinase (MEK) 1 and Raf1 kinase activities and subsequently attenuated TPA-induced phosphorylation of ERK/p90 ribosomal S6 kinase. Although either RWE or quercetin suppressed Raf1 kinase activity, they were more effective in inhibiting MEK1 activity. Importantly, quercetin exerted stronger inhibitory effects than PD098059, a well-known pharmacologic inhibitor of MEK. Resveratrol did not affect either MEK1 or Raf1 kinase activity. Pull-down assays revealed that RWE or quercetin (but not resveratrol) bound with either MEK1 or Raf1. RWE or quercetin also dose dependently suppressed JB6 P+ cell transformation induced by epidermal growth factor or H-Ras, both of which are involved in the activation of MEK/ERK signaling. Docking data suggested that quercetin, but not resveratrol, formed a hydrogen bond with the backbone amide group of Ser(212), which is the key interaction for stabilizing the inactive conformation of the activation loop of MEK1.

Suppressor role of activating transcription factor 2 (ATF2) in skin cancer

Activating transcription factor 2 (ATF2) regulates transcription in response to stress and growth factor stimuli. Here, we use a mouse model in which ATF2 was selectively deleted in keratinocytes. Crossing the conditionally expressed ATF2 mutant with K14-Cre mice (K14.ATF2(f/f)) resulted in selective expression of mutant ATF2 within the basal layer of the epidermis. When subjected to a two-stage skin carcinogenesis protocol [7,12-dimethylbenz[a]anthracene/phorbol 12-tetradecanoate 13-acetate (DMBA/TPA)], K14.ATF2(f/f) mice showed significant increases in both the incidence and prevalence of papilloma development compared with the WT ATF2 mice. Consistent with these findings, keratinocytes of K14.ATF2(f/f) mice exhibit greater anchorage-independent growth compared with ATF2 WT keratinocytes. Papillomas of K14.ATF2(f/f) mice exhibit reduced expression of presenilin1, which is associated with enhanced beta-catenin and cyclin D1, and reduced Notch1 expression. Significantly, a reduction of nuclear ATF2 and increased beta-catenin expression were seen in samples of squamous and basal cell carcinoma, as opposed to normal skin. Our data reveal that loss of ATF2 transcriptional activity serves to promote skin tumor formation, thereby indicating a suppressor activity of ATF2 in skin tumor formation.

Ultraviolet B regulation of transcription factor families: roles of nuclear factor-kappa B (NF-kappaB) and activator protein-1 (AP-1) in UVB-induced skin carcinogenesis

Prolonged and repeated exposure of the skin to ultraviolet light (UV) leads not only to aging of the skin but also increases the incidence of non-melanoma skin cancer (NMSC). Damage of cells induced by ultraviolet B (UVB) light both at the DNA level and molecular level initiates the activation of transcription factor pathways, which in turn regulate the expression of a number of genes termed the "UV response genes". Two such transcription factor families that are activated in this way are those of the nuclear factor-kappaB (NF-kappaB) and activator protein-1 (AP-1) families. These two transcription factor families have been identified to be involved in the processes of cell proliferation, cell differentiation and cell survival and therefore play important roles in tumorigenesis. The study of these two transcription factor pathways and the cross-talk between them in response to UVB exposure may help with the development of new chemopreventive strategies for the prevention of UVB-induced skin carcinogenesis.

P-glycoprotein: so many ways to turn it on

Expression of the ABC transporter P-glycoprotein (P-gp or ABCB1) is associated with resistance to chemotherapy in cancer. However, early investigations into the regulation of ABCB1 expression revealed that the process is not a classical induction as observed for certain metabolizing enzymes. The process involves the cellular stress response pathway initiated by either inflicted (e.g., chemotherapy damage) or endogenous (e.g., hypoxia) factors. However, ABCB1 is also expressed in a number of noncancerous tissues. In particular, the protein is found at tissues providing a barrier or secretory function. The localization of ABCB1 in normal tissues will impact significantly on drug pharmacokinetics, in particular the absorption and elimination processes. This review also describes the mechanism underlying ABCB1 expression in noncancerous tissue, a process that does not involve the stress response.

The effect of cyclooxygenase-2 overexpression on skin carcinogenesis is context dependent

The up-regulation of the inducible form of cyclooxygenase (COX-2), a central enzyme in the prostaglandin (PG) biosynthetic pathway, occurs in many epithelial tumors and has been associated with tumor cell proliferation and angiogenesis. To better understand the role of COX-2 in skin tumor development, we generated transgenic mice that overexpress COX-2 under the control of the keratin 14 promoter. We previously reported (Cancer Res. 62: 2516, 2002) that these mice, referred to as keratin 14 (K14).COX2 mice, were unexpectedly very resistant to 12-O-tetradecanoylphorbol 13-acetate (TPA) tumor promotion. The current studies were undertaken to determine the mechanism of this resistance and determine if it was restricted to TPA promotion. Transgenic and wild-type mice were subjected to a complete carcinogenesis protocol using 7,12-dimethylbenz[a]anthracene (DMBA) only, as well as a two-stage protocol using DMBA plus an unrelated tumor promoter, anthralin. In addition, the responses of transgenic and wild-type mice to TPA in terms of induction of proliferation and various down-stream mediators were examined. The TPA resistance phenotype correlated with a reduced ability to induce ornithine decarboxylase, interleukin-1alpha, and tumor necrosis factor-alpha and a reduced proliferation response. This resistance phenotype appears to be restricted to phorbol ester promotion because K14.COX2 mice developed six times more tumors than wild-type mice when anthralin was used as the tumor promoter. Additionally, K14.COX2 mice treated only with DMBA developed approximately 3.5 times more tumors than wild-type mice, suggesting that PGs have intrinsic tumor promoting activity. We conclude that the role of PGs in skin tumorigenesis is context dependent.