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

Inhibition of Jun NH2-terminal kinases suppresses the growth of experimental head and neck squamous cell carcinoma

PURPOSE

This study was carried out to investigate whether c-Jun NH2-terminal kinases (JNK) are potential targets for treating head and neck squamous cell carcinoma (HNSCC).

EXPERIMENTAL DESIGN

JNK activity was first evaluated in 20 paired samples of human HNSCC. The antitumor activity of SP600125, a reversible nonselective ATP-competitive inhibitor of JNKs, was then investigated both in an HNSCC xenograft model and in vitro using immunohistochemistry, immunoblotting, enzyme immunoassay, flow cytometry, and a Matrigel assay of capillary tube formation. Complementary studies were carried out using small interfering RNA to JNK1/2.

RESULTS

JNK activity was increased in human HNSCC compared with normal-appearing epithelium. Treatment of mice bearing HNSCC xenografts with SP600125 resulted in >60% inhibition of tumor growth relative to vehicle-treated animals. Inhibition of tumor growth was associated with significant reductions in both cell proliferation and microvessel density. SP600125 inhibited tumor cell proliferation by causing delays in both the S and G2-M phases of the cell cycle. Inhibition of angiogenesis seemed to reflect effects on both tumor and endothelial cells. The JNK inhibitor suppressed the production of vascular endothelial growth factor and interleukin-8 by tumor cells and also inhibited endothelial cell proliferation and capillary tube formation. Reduced amounts and phosphorylation of epidermal growth factor receptor were found in tumor cells after treatment with SP600125. Small interfering RNA-mediated suppression of JNK1/2 led to reduced tumor cell proliferation and decreased levels of epidermal growth factor receptor, vascular endothelial growth factor, and interleukin-8.

CONCLUSIONS

JNK activity is commonly increased in HNSCC. Our preclinical results provide a rationale for evaluating JNK inhibition as an approach to treating HNSCC.

SAG/ROC2 E3 ligase regulates skin carcinogenesis by stage-dependent targeting of c-Jun/AP1 and IkappaB-alpha/NF-kappaB

Sensitive to apoptosis gene (SAG)/regulator of cullins-2-Skp1-cullin-F-box protein (SCF) E3 ubiquitin ligase regulates cellular functions through ubiquitination and degradation of protein substrates. We report that, when expressed in mouse epidermis driven by the K14 promoter, SAG inhibited TPA-induced c-Jun levels and activator protein-1 (AP-1) activity in both in vitro primary culture, in vivo transgenic mice, and an AP-1- luciferase reporter mouse model. After AP-1 inactivation, epidermal proliferation induced by 7,12-dimethylbenz(a)-anthracene/12-O-tetradecanoylphorbol-13-acetate at the early stage of carcinogenesis was substantially inhibited. Later stage tumor formation was also substantially inhibited with prolonged latency and reduced frequency of tumor formation. Interestingly, SAG expression increased tumor size, not because of accelerated proliferation, but caused by reduced apoptosis resulting, at least in part, from nuclear factor kappaB (NF-kappaB) activation. Thus, SAG, in a manner depending on the availability of F-box proteins, demonstrated early-stage suppression of tumor formation by promoting c-Jun degradation, thereby inhibiting AP-1, and later-stage enhancement of tumor growth, by promoting inhibitor of kappaBalpha degradation to activate NF-kappaB and inhibit apoptosis.

The role of glycogen synthase kinase 3beta in the transformation of epidermal cells

Glycogen synthase kinase 3beta (GSK3beta) is a multifunctional serine/threonine kinase. We showed that the expression of GSK3beta was drastically down-regulated in human cutaneous squamous cell carcinomas and basal cell carcinomas. Due to its negative regulation of many oncogenic proteins, we hypothesized that GSK3beta may function as a tumor suppressor during the neoplastic transformation of epidermal cells. We tested this hypothesis using an in vitro model system, JB6 mouse epidermal cells. In response to epidermal growth factor (EGF) or 12-O-tetradecanoylphorbol-13-acetate (TPA), the promotion-sensitive JB6 P+ cells initiate neoplastic transformation, whereas the promotion-resistant JB6 P- cells do not. JB6 P- cells expressed much higher levels of GSK3beta than JB6 P+ cells; JB7 cells, the transformed derivatives of JB6, had the least amount of GSK3beta. The activity of GSK3beta is negatively regulated by its phosphorylation at Ser9. EGF and TPA induced strong Ser9 phoshorylation in JB6 P+ cells, but phosphorylation was seen at a much lesser extent in JB6 P- cells. EGF and TPA-stimulated Ser9 phosphorylation was mediated by phosphoinositide-3-kinase (PI3K)/Akt and protein kinase C (PKC) pathways. Inhibition of GSK3beta activation significantly stimulated activator protein-1 (AP-1) activity. Overexpression of wild-type (WT) and S9A mutant GSK3beta in JB6 P+ cells suppressed EGF and TPA-mediated anchorage-independent growth in soft agar and tumorigenicity in nude mice. Overexpression of a kinase-deficient (K85R) GSK3beta, in contrast, potentiated anchorage-independent growth and drastically enhanced in vivo tumorigenicity. Together, these results indicate that GSK3beta plays an important role in skin tumorigenesis.

Effects of polyphenols derived from fruit of Crataegus pinnatifida on cell transformation, dermal edema and skin tumor formation by phorbol ester application

The dried fruits of Crataegus pinnatifida have been used traditionally as oriental medicine and local soft drink material recently. Previously, we demonstrated that C. pinnatifida exhibited anti-oxidation and anti-inflammatory potential. To clarify the active components in anti-transformation and anti-tumor promotion, we collected the polyphenol fraction (CF-TP) of hot-water extracts from dried fruits of C. pinnatifida for the following study. By anchorage-independent transformation assay, CF-TP significantly inhibited 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced cell transformation in JB6 P(+) cells. Moreover, we found that CF-TP inhibited the expression of osteopontin (OPN), a transformational marker, and the activation of NF-kappaB and AP-1 induced by TPA in JB6 P(+) cells. In addition, we evaluated the effect of CF-TP on TPA application to ICR mouse skin with measurement of H(2)O(2) production, myeloperoxidase (MPO) activity, edema formation, epidermal thickness and leukocyte infiltration. As a result, CF-TP significantly inhibited the generation of reactive oxygen species (ROS) and the phenomena of inflammation induced by TPA. It also suppressed the expression of COX-2 and iNOS, and the activation of ornithine decarboxylase (ODC). Furthermore, CF-TP inhibited benzo[a]pyrene (B[a]P)/TPA-induced skin tumor formation and decreased the incidence of tumor. These results indicate that CF-TP possesses potential as a cancer chemopreventive agent against tumor promotion.

Cancer chemoprevention by phytochemicals: potential molecular targets, biomarkers and animal models

Recent studies have strongly indicated that certain daily-consumed dietary phytochemicals could have cancer protective effects against transgenic mice cancer models and cancers mediated by carcinogens, irradiations and carcinogenic metabolites derived from exogenous or endogenous sources. The cancer-protective effects elicited by these dietary compounds are believed to be due at least in part to the induction of cellular defense systems including the detoxifying and antioxidant enzymes system, as well as the inhibition of anti-inflammatory and anti-cell growth signaling pathways culminating in cell cycle arrest and/or celldeath. In this review, we summarize the potential mechanisms including the modulation of nuclear factor kappaB (NF-kappaB), cyclooxygenases-2 (COX-2), activator protein-1 (AP-1), mitogen-activated protein kinases (MAPKs) and the induction of phase II cellular detoxifying and antioxidant enzymes mediated mainly by the antioxidant response elements (ARE) within the promoter regions of these genes through nuclear factor-erythroid 2-related factor 2 (Nrf2), a member of the Cap "n" collar (CNC) family of the basic region-leucine zipper transcription factor. In addition, we also review several animal models of carcinogenesis and cancer chemopreventive efficacy studies of these animal models using dietary chemopreventive compounds. Finally, we discuss the cellular signaling cascades mediated by Nrf2, NF-kappaB, AP-1, MAPKs and COX-2, which have been considered to play pivotal roles in tumor initiation, promotion and progression processes, and could be promising molecular targets for the design of drugs targeting cancer prevention and therapy.

Epidermal hyperplasia and oral carcinoma in mice overexpressing the transcription factor ATF3 in basal epithelial cells

ATF3 is a highly conserved eukaryotic transcription factor that is ubiquitously upregulated transcriptionally during cellular responses to a variety of stresses, in particular DNA damage. However, the role of ATF3 in the DNA damage response is unclear. Transgenic mice that overexpress human ATF3 in basal epithelial cells under the control of the bovine keratin 5 (K5) promoter were constructed and characterized for epidermal alterations. Strong, nuclear expression of the exogenous ATF3 protein was seen in basal cells of the epidermis, hair follicles, and oral mucosa. Hyperplastic changes in the K5-expressing, outer root sheath (ORS) cells of the hair follicle were observed in young mice, resulting in multiple layers of ORS cells in the mature follicle and large aberrantly shaped follicles. Mild hyperplasia of the interfollicular epidermis was also noted, increasing with age. However, no epidermal tumors were identified in BK5.ATF3 mice observed for 16 mo. At 16 mo of age, most transgenic mice exhibited multi-focal areas of hyperplasia and dysplasia in the oral mucosa, with cellular atypia and underlying acute inflammatory changes. Neoplastic lesions were also seen in the oral cavity of BK5.ATF3 mice, including oral squamous cell carcinoma (60% incidence) and basal cell tumors with follicular differentiation (70% incidence), but not in non-transgenic FVB/N littermates. Heterogeneous nuclear expression (or stabilization) of p53 protein was seen in some oral dysplasias, with a patchy distribution primarily in the least differentiated layers of the lesions. This represents the first indication that ATF3 may have oncogenic properties in epithelial cells.

SAG/ROC2/Rbx2 is a novel activator protein-1 target that promotes c-Jun degradation and inhibits 12-O-tetradecanoylphorbol-13-acetate-induced neoplastic transformation

SAG (sensitive to apoptosis gene) was first identified as a stress-responsive protein that, when overexpressed, inhibited apoptosis both in vitro and in vivo. SAG was later found to be the second family member of ROC1 or Rbx1, a RING component of SCF and DCX E3 ubiquitin ligases. We report here that SAG/ROC2/Rbx2 is a novel transcriptional target of activator protein-1 (AP-1). AP-1 bound both in vitro and in vivo to two consensus binding sites in a 1.3-kb region of the mouse SAG promoter. The SAG promoter activity, as measured by luciferase reporter assay, was dependent on these sites. Consistently, endogenous SAG is induced by 12-O-tetradecanoylphorbol-13-acetate (TPA) with an induction time course following the c-Jun induction in both mouse epidermal JB6-Cl.41 and human 293 cells. TPA-mediated SAG induction was significantly reduced in JB6-Cl.41 cells overexpressing a dominant-negative c-Jun, indicating a requirement of c-Jun/AP-1. On the other hand, SAG seemed to modulate the c-Jun levels. When overexpressed, SAG remarkably reduced both basal and TPA-induced c-Jun levels, whereas SAG small interfering RNA (siRNA) silencing increased substantially the levels of both basal and TPA-induced c-Jun. Consistently, SAG siRNA silencing reduced c-Jun polyubiquitination and blocked c-Jun degradation induced by Fbw7, an F-box protein of SCF E3 ubiquitin ligase. Finally, SAG overexpression inhibited, whereas SAG siRNA silencing enhanced, respectively, the TPA-induced neoplastic transformation in JB6-Cl.41 preneoplastic model. Thus, AP-1/SAG establishes an autofeedback loop, in which on induction by AP-1, SAG promotes c-Jun ubiquitination and degradation, thus inhibiting tumor-promoting activity of AP-1.

BRD7 suppresses the growth of Nasopharyngeal Carcinoma cells (HNE1) through negatively regulating beta-catenin and ERK pathways

BRD7 is a novel gene which involved NPC in our lab. Our previous studies showed that BRD7 was expressed at high level in normal nasopharyngeal epithelial tissues, but at low level in nasopharyngeal carcinoma biopsies and cell lines. In these papers, we found that ectopic expression of BRD7 can decrease cell proliferation and capability to form colonies in soft agar. FCM (Flow cytometry) assay indicated that the cell cycle progression from G1 to S phase was inhibited and the expression of cyclinD1 was significantly decreased after being transfected with BRD7 in HNE1 cells (NPC cells). To further investigate the molecular mechanism of BRD7 suppression of NPC cells growth, the cDNA microarray was performed to detect difference in gene expression profile induced by BRD7. The results indicated that 21 genes expression were changed after being transfected with BRD7 and the differentially expressed gene including alpha-catenin, cyclinD1, E2F3 was confirmed by western-blot. Next, we found that even though no obvious changes of the total expression of beta-catenin were observed, the accumulation of beta-catenin in nucleus was blocked. In addition, it was found that the expression of beta-catenin was up-regulated in the complex composed of beta-catenin and alpha-catenin in HNE1 cells induction of BRD7. So, we concluded that over-expression of BRD7 increased the expression of alpha-catenin which "hold" beta-catenin in the complex and inhibited its accumulating in nucleus. At last, we demonstrated the c-jun, p-MEK, and p-ERK1/2 expression were down-regulated, and the Ap-1 promoter activity was inactive after being transfected with BRD7. We also found that over-expression of BRD7 can inactivate the c-jun and p-ERK1/2 after being treated with EGF in HNE1 cells. These results indicated that BRD7 played a negative role in ERK1/2 pathway. Taken together, our present results provide new insights for BRD7 function to inhibit NPC cells growth through negative regulating beta-catenin and ERK1/2 pathways.

Dominant-negative activator protein 1 (TAM67) targets cyclooxygenase-2 and osteopontin under conditions in which it specifically inhibits tumorigenesis

Activation of activator protein 1 (AP-1) and nuclear factor kappaB (NFkappaB)-dependent transcription is required for tumor promotion in cell culture models and transgenic mice. Dominant-negative c-Jun (TAM67) blocks AP-1 activation by dimerizing with Jun or Fos family proteins and blocks NFkappaB activation by interacting with NFkappaB p65. Two-stage [7,12-dimethylbenz(a)anthracene (DMBA)/12-O-tetradecanoylphorbol-13-acetate (TPA)] skin carcinogenesis experiments in a model relevant to human cancer risk, transgenic mice expressing human papillomavirus 16 E7 oncogene (K14-HPV16-E7), show E7-enhanced tumor promotion. A cross to K14-TAM67-expressing mice results in dramatic inhibition of tumor promoter-induced AP-1 luciferase reporter activation and papillomagenesis. Epithelial specific TAM67 expression inhibits tumorigenesis without affecting TPA- or E7-induced hyperproliferation of the skin. Thus, the mouse model enriches for TAM67 targets relevant to tumorigenesis rather than to general cell proliferation or hyperplasia, implicating a subset of AP-1- and/or NFkappaB-dependent genes. The aim of the present study was to identify target genes responsible for TAM67 inhibition of DMBA-TPA-induced tumorigenesis. Microarray expression analysis of epidermal tissues revealed small sets of genes in which expression is both up-regulated by tumor promoter and down-regulated by TAM67. Among these, cyclooxygenase-2 (Cox-2/Ptgs2) and osteopontin (Opn/Spp1) are known to be functionally significant in driving carcinogenesis. Results identify both Cox-2 and Opn as transcriptional targets of TAM67 with CRE, but not NFkappaB sites important in the Cox-2 promoter and an AP-1 site important in the Opn promoter.

Suppression of keratinocyte stratification by a dominant negative JunB mutant without blocking cell proliferation

Keratinocytes make a stratified epidermoid structure when cultured at an air-liquid interface. The three-dimensional (3D) culture of keratinocytes has been successfully used for more than 25 years, but it is still unclear why keratinocytes stratify in response to air exposure. AP-1 proteins are ubiquitous transcription factors that regulate many biological processes, including cell proliferation, differentiation and apoptosis. We established HaCaT-JunBDeltaN, a human keratinocyte cell line that expressed a mutant JunB with a dominant negative effect on AP-1 activity. Stratification of HaCaT-JunBDeltaN cells was markedly suppressed in a 3D culture condition, in which HaCaT cells stratified similarly to stratified squamous epithelia. However, HaCaT-JunBDeltaN cells had proliferation activities that were closely equivalent to those of HaCaT cells, under both two-dimensional (2D) and 3D culture conditions. To screen for the candidate gene responsible for the different stratification ability, we examined the gene expression profile of HaCaT cells before and after air exposure. Several genes with an antioxidative function, such as aldo-keto reductase and selenoprotein P were highly expressed after air exposure in HaCaT cells but not in HaCaT-JunBDeltaN cells. Our findings indicate the presence of a novel role of AP-1 activity when HaCaT cells make a stratified epidermoid structure under 3D culture conditions.

A high-throughput cell-based assay to identify specific inhibitors of transcription factor AP-1

The oncogenic transcription factor AP-1 (activator protein-1) is required for tumor promotion and progression. Identification of novel and specific AP-1 inhibitors would be beneficial for cancer prevention and therapy. The authors have developed a high-throughput assay to screen synthetic and natural product libraries for noncytotoxic inhibitors of mitogen-activated AP-1 activity. The cell-based high-throughput screen is conducted in a 384-well format using a fluorescent resonance energy transfer (FRET) substrate to quantify the activity of a beta-lactamase reporter under the control of an AP-1-dependent promoter. The ratiometric FRET readout makes this assay extremely robust and reproducible, particularly for use with natural product extracts. To eliminate false positives due to cell killing, a cytotoxicity assay was incorporated. The AP-1 beta-lactamase reporter was validated with inhibitors of kinases located upstream of AP-1 and with known natural product inhibitors of AP-1 (nordihydroguaiaretic acid and curcumin). The assay was able to identify other known AP-1 inhibitors and protein kinase C modulators, as well as a number of chemically diverse compounds with unknown mechanisms of action from natural products libraries. Application to natural product extracts identified hits from a range of taxonomic groups. Screening of synthetic compounds and natural products should identify novel AP-1 inhibitors that may be useful in the prevention and treatment of cancers.

Evaluation of cancer-preventive activity and structure-activity relationships of 3-demethylubiquinone Q2, isolated from the ascidian Aplidium glabrum, and its synthetic analogs

PURPOSE

3-Demethylubiquinone Q2 was isolated from the ascidian Aplidium glabrum. The cancer-preventive properties and the structure-activity relationship for 3-demethylubiquinone Q2 and 12 of its synthetic analogs are reported.

METHODS

Compounds, having one or several di- or triprenyl substitutions and quinone moieties with methoxyls in different positions, were synthesized. The cancer-preventive properties of compounds and were tested in JB6 Cl41 mouse skin cells, using a variety of assessments, including the methanethiosulfonate (MTS) assay, flow cytometry, and soft agar assay. Statistical nonparametric methods were used to confirm statistical significance.

RESULTS

All quinones tested were shown to inhibit JB6 Cl41 cell transformation, to induce apoptosis, AP-1, and NF-kappaB activity, and to inhibit p53 activity. The most promising effects were indicated for compounds containing two isoprene units in a side chain and a methoxyl group at the para-position to a polyprenyl substitution.

CONCLUSIONS

Quinones and demonstrated cancer-preventive activity in JB6 Cl41 cells, which may be attributed to the induction of p53-independent apoptosis. These activities depended on the length of side chains and on the positions of the methoxyl groups in the quinone part of the molecule.

Activator protein-1 activity regulates epithelial tumor cell identity

To examine the consequences of inhibiting activator protein-1 (AP-1) transcription factors in skin, transgenic mice were generated, which use the tetracycline system to conditionally express A-FOS, a dominant negative that inhibits AP-1 DNA binding. Older mice develop mild alopecia and hyperplasia of sebaceous glands, particularly around the eyes. When A-FOS was expressed during chemical-induced skin carcinogenesis, mice do not develop characteristic benign and malignant squamous lesions but instead develop benign sebaceous adenomas containing a signature mutation in the H-ras proto-oncogene. Inhibiting AP-1 activity after tumor formation caused squamous tumors to transdifferentiate into sebaceous tumors. Furthermore, reactivating AP-1 in sebaceous tumors results in a reciprocal transdifferentiation into squamous tumors. In both cases of transdifferentiation, individual cells express molecular markers for both cell types, indicating individual tumor cells have the capacity to express multiple lineages. Molecular characterization of cultured keratinocytes and tumor material indicates that AP-1 regulates the balance between the wnt/beta-catenin and hedgehog signaling pathways that determine squamous and sebaceous lineages, respectively. Chromatin immunoprecipitation analysis indicates that c-Jun binds several wnt promoters, which are misregulated by A-FOS expression, suggesting that members of the wnt pathway can be a primary targets of AP-1 transcriptional regulation. Thus, AP-1 activity regulates tumor cell lineage and is essential to maintain the squamous tumor cell identity.

Papilloma development is delayed in osteopontin-null mice: implicating an antiapoptosis role for osteopontin

Osteopontin is a secreted, adhesive glycoprotein, whose expression is markedly elevated in several types of cancer and premalignant lesions, implicating its association with carcinogenesis. To test the hypothesis that induced osteopontin is involved in tumor promotion in vivo, osteopontin-null and wild-type (WT) mice were subjected to a two-stage skin chemical carcinogenesis protocol. Mice were initiated with 7,12-dimethylbenz(a)anthracene (DMBA) applied on to the dorsal skin followed by twice weekly application of 12-O-tetradecanoylphorbol-13-acetate (TPA) for 27 weeks. Osteopontin-null mice showed a marked decrease both in tumor/papilloma incidence and multiplicity compared with WT mice. Osteopontin is minimally expressed in normal epidermis, but on treatment with TPA its expression is highly induced. To determine the possible mechanism(s) by which osteopontin regulates tumor development, we examined cell proliferation and cell survival. Epidermis from osteopontin-null and WT mice treated with TPA thrice or with DMBA followed by TPA for 11 weeks showed a similar increase in epidermal hyperplasia, suggesting that osteopontin does not mediate TPA-induced cell proliferation. Bromodeoxyuridine staining of papillomas and adjacent epidermis showed no difference in cell proliferation between groups. However, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling analyses indicated a greater number of apoptotic cells in DMBA-treated skin and papillomas from osteopontin-null versus WT mice. These studies are the first to show that induction of the matricellular protein osteopontin facilitates DMBA/TPA-induced cutaneous carcinogenesis most likely through prevention of apoptosis.

Resveratrol inhibits proliferation of human epidermoid carcinoma A431 cells by modulating MEK1 and AP-1 signalling pathways

Resveratrol (trans-3,4',5-trihydroxystilbene) is a naturally occurring polyphenolic phytoalexin found in grapes, and has been shown to inhibit the growth of various types of cancer cells. We investigated the mechanism of the antiproliferative effect of resveratrol in A431-transformed keratinocytes harbouring mutant p53, and show that it is accompanied by G1 cell cycle arrest, which coincides with a marked inhibition of G1 cell cycle regulatory proteins, including cyclins A and D1 and cyclin-dependent kinase (CDK)6 and p53-independent induction of p21WAF1. Cell cycle arrest was also associated with the accumulation of hypophosphorylated Rb and p27KIP1. Resveratrol inhibited mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK)1 > extracellular signal-regulated protein kinase (ERK)1/2 signalling, downregulated c-Jun, and suppressed activating protein (AP)-1 DNA-binding and promoter activity. In addition, the inhibition of MEK1 > ERK1/2 signalling appears to be independent of retinoblastoma protein (pRb) hypophosphorylation in A431 cells, as PD098059 did not suppress pRb phosphorylation. Our results demonstrate that resveratrol affects multiple cellular targets in A431 cells, and that the downregulation of both AP-1 and pRb contributes to its antiproliferative activity in these cells.

Fra-1 a target for cancer prevention or intervention

The transcription factor activator protein-1 (AP-1) has been implicated as a driver of carcinogenesis since its original characterization. Oncogenic transcription factors like AP-1 are becoming new targets for cancer intervention. Inhibitors of AP-1 have been shown to block tumor promotion, transformation, progression and invasion. The Fos related antigen-1 (Fra-1) is activated in multiple cancers and gene ablation can suppress the invasive phenotypes of many tumor cell lines. This review focuses on the regulation of fosl1 expression, stabilization and activation of the Fra-1 polypeptide and on Fra-1-mediated tumorigenesis.

Heparin (GAG-hed) inhibits LCR activity of human papillomavirus type 18 by decreasing AP1 binding

BACKGROUND

High risk HPVs are causative agents of anogenital cancers. Viral E6 and E7 genes are continuously expressed and are largely responsible for the oncogenic activity of these viruses. Transcription of the E6 and E7 genes is controlled by the viral Long Control Region (LCR), plus several cellular transcription factors including AP1 and the viral protein E2. Within the LCR, the binding and activity of the transcription factor AP1 represents a key regulatory event in maintaining E6/E7 gene expression and uncontrolled cell proliferation. Glycosaminoglycans (GAGs), such as heparin, can inhibit tumour growth; they have also shown antiviral effects and inhibition of AP1 transcriptional activity. The purpose of this study was to test the heparinoid GAG-hed, as a possible antiviral and antitumoral agent in an HPV18 positive HeLa cell line.

METHODS

Using in vivo and in vitro approaches we tested GAG-hed effects on HeLa tumour cell growth, cell proliferation and on the expression of HPV18 E6/E7 oncogenes. GAG-hed effects on AP1 binding to HPV18-LCR-DNA were tested by EMSA.

RESULTS

We were able to record the antitumoral effect of GAG-hed in vivo by using as a model tumours induced by injection of HeLa cells into athymic female mice. The antiviral effect of GAG-hed resulted in the inhibition of LCR activity and, consequently, the inhibition of E6 and E7 transcription. A specific diminishing of cell proliferation rates was observed in HeLa but not in HPV-free colorectal adenocarcinoma cells. Treated HeLa cells did not undergo apoptosis but the percentage of cells in G2/M phase of the cell cycle was increased. We also detected that GAG-hed prevents the binding of the transcription factor AP1 to the LCR.

CONCLUSION

Direct interaction of GAG-hed with the components of the AP1 complex and subsequent interference with its ability to correctly bind specific sites within the viral LCR may contribute to the inhibition of E6/E7 transcription and cell proliferation. Our data suggest that GAG-hed could have antitumoral and antiviral activity mainly by inhibiting AP1 binding to the HPV18-LCR.

Palytoxin: exploiting a novel skin tumor promoter to explore signal transduction and carcinogenesis

Palytoxin is a novel skin tumor promoter, which has been used to help probe the role of different types of signaling mechanisms in carcinogenesis. The multistage mouse skin model indicates that tumor promotion is an early, prolonged, and reversible phase of carcinogenesis. Understanding the molecular mechanisms underlying tumor promotion is therefore important for developing strategies to prevent and treat cancer. Naturally occurring tumor promoters that bind to specific cellular receptors have proven to be useful tools for investigating important biochemical events in multistage carcinogenesis. For example, the identification of protein kinase C as the receptor for the prototypical skin tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) (also called phorbol 12-myristate 13-acetate, PMA) provided key evidence that tumor promotion involves the aberrant modulation of signaling cascades that govern cell fate and function. The subsequent discovery that palytoxin, a marine toxin isolated from zoanthids (genus Palythoa), is a potent skin tumor promoter yet does not activate protein kinase C indicated that investigating palytoxin action could help reveal new aspects of tumor promotion. Interestingly, the putative receptor for palytoxin is the Na(+),K(+)-ATPase. This review focuses on palytoxin-stimulated signaling and how palytoxin has been used to investigate alternate biochemical mechanisms by which important targets in carcinogenesis can be modulated.

S100A8 and S100A9 in inflammation and cancer

Calprotectin (S100A8/A9), a heterodimer of the two calcium-binding proteins S100A8 and S100A9, was originally discovered as immunogenic protein expressed and secreted by neutrophils. Subsequently, it has emerged as important pro-inflammatory mediator in acute and chronic inflammation. More recently, increased S100A8 and S100A9 levels were also detected in various human cancers, presenting abundant expression in neoplastic tumor cells as well as infiltrating immune cells. Although, many possible functions have been proposed for S100A8/A9, its biological role still remains to be defined. Altogether, its expression and potential cytokine-like function in inflammation and in cancer suggests that S100A8/A9 may play a key role in inflammation-associated cancer.

c-Jun activation is associated with proliferation and angiogenesis in invasive breast cancer

c-Jun is a component of the transcription factor activator protein 1 (AP-1), which binds and activates transcription at TRE/AP-1 elements. Extra- or intracellular signals, including growth factors, transforming oncoproteins, and UV irradiation, stimulate phosphorylation of c-Jun at serine 63/73 and activate c-Jun-dependent transcription. Therefore, activated c-Jun potentially plays an important role in carcinogenesis and cancer progression. To evaluate expression patterns of activated c-Jun in breast cancer in relation to angiogenesis and proliferation, we performed immunohistochemistry on 103 cases of invasive breast cancer with an antibody recognizing phosphorylated c-Jun at serine 73. Activated c-Jun showed a predominantly nuclear expression at the invasive front in 38% of invasive breast cancer cases. Furthermore, expression of activated c-Jun was seen in mitotic cells of the invasive front in 50% of cases. Occasionally, fibroblasts, endothelial cells, and benign breast cells showed nuclear expression. Activated nuclear c-Jun expression showed positive correlations with expression of hyperphosphorylated pRb, vascular endothelial growth factor, and with microvessel density. Mitotic c-Jun expression was associated with pRb and microvessel density. Stromal c-Jun expression showed positive relations with microvessel density. In survival analysis, no significant relation was found with activated c-Jun expression and survival, although a trend with poor survival was found for mitotic cells overexpressing activated c-Jun (P = .09). Our results show that activated c-Jun is predominantly expressed at the invasive front in breast cancer and is associated with proliferation and angiogenesis. Earlier studies have established a functional, in vitro link between activated c-Jun and tumor angiogenesis. Our present results in breast cancer patients confirm this relation in vivo for the first time. Therefore, c-Jun/AP-1 targeting may provide new ways to block tumor angiogenesis.

Squamous cell carcinoma growth in mice and in culture is regulated by c-Jun and its control of matrix metalloproteinase-2 and -9 expression

Squamous cell carcinoma (SCC) is an invasive malignancy of epidermal keratinocytes. Surgical excision is currently the main treatment; however, this can cause scarring and disfigurement. There is accordingly, an acute need for alternative strategies to treat SCC. The transcription factor c-Jun is expressed in human SCC and another common form of invasive skin cancer, basal cell carcinoma together with the mitogenic marker-proliferating cell nuclear antigen. Here, we have employed DNAzymes (catalytic DNA molecules) targeting c-Jun (Dz13) to inhibit c-Jun expression in SCC cells. Dz13 inhibits SCC proliferation and suppresses solid SCC tumor growth and tumor angiogenesis in severe combined immunodeficient mice. We further demonstrate that Dz13 inhibits c-Jun, together with matrix metalloproteinase (MMP)-2 and MMP-9 expression in the tumors, consistent with DNAzyme inhibition of MMP-2 and MMP-9 gelatinolytic activity by zymography. Dz13 also suppressed the expression of vascular endothelial growth factor and fibroblast growth factor-2 in the tumors. These findings demonstrate that c-Jun regulates SCC growth and suggest that DNAzymes targeting this transcription factor may potentially be useful as inhibitors of cutaneous carcinoma.

Oxidants and signaling by mitogen-activated protein kinases in lung epithelium

Oxidants in cigarette smoke and generated from asbestos fibers activate mitogen-activated protein kinase (MAPK) signaling cascades in lung epithelial cells in vitro and in vivo. These signaling pathways lead to the enhanced ability of Jun and Fos family members (i.e., components of the activator protein [AP]-1 transcription factor) to activate transcription of a number of AP-1-dependent target genes involved in cell proliferation or death, differentiation, and inflammation. Research by the Basbaum laboratory has been critical in showing that mucin transcription in response to cigarette smoke and gram-positive bacteria is mediated through activation of the epidermal growth factor receptor and MAPK cascades. Work from our laboratories supports the concept that MAPK signaling and AP-1 transactivation by cigarette smoke and asbestos may synergize in lung epithelial cell injury, compensatory proliferation of lung epithelial cells, and carcinogenesis, supporting a mechanistic framework for the striking increases in lung cancer incidence in asbestos workers who smoke. Targeting of MAPKs and inter-related signaling cascades may be critical to the prevention of lung cancers and control of mucin overproduction in a number of lung diseases including asthma, cystic fibrosis, chronic bronchitis, and chronic obstructive pulmonary disease.

A novel aspartic proteinase-like gene expressed in stratified epithelia and squamous cell carcinoma of the skin

Homeostasis of stratified epithelia, such as the epidermis of the skin, is a sophisticated process that represents a tightly controlled balance between proliferation and differentiation. Alterations of this balance are associated with common human diseases including cancer. Here, we report the cloning of a novel cDNA sequence, from mouse back skin, that is induced by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) and codes for a hitherto unknown aspartic proteinase-like protein (Taps). Taps represents a potential AP-1 target gene because TPA-induced expression in epidermal keratinocytes critically depends on c-Fos, and co-treatment with dexamethasone, a potent inhibitor of AP-1-mediated gene regulation, resulted in impaired activation of Taps expression. Taps mRNA and protein are restricted to stratified epithelia in mouse embryos and adult tissues, implicating a crucial role for this aspartic proteinase-like gene in differentiation and homeostasis of multilayered epithelia. During chemically induced carcinogenesis, transient elevation of Taps mRNA and protein levels was detected in benign skin tumors. However, its expression is negatively associated with dedifferentiation and malignant progression in squamous cell carcinomas of the skin. Similar expression was observed in squamous skin tumors of patients, suggesting that detection of Taps levels represents a novel strategy to discriminate the progression state of squamous skin cancers.

Delayed wound healing and epidermal hyperproliferation in mice lacking JunB in the skin

The cutaneous response to injury and stress comprises a temporary change in the balance between epidermal proliferation and differentiation as well as an activation of the immune system. Soluble factors play an important role in the regulation of these complex processes by coordinating the intercellular communication between keratinocytes, fibroblasts, and inflammatory cells. In this study, we demonstrate that JunB, a member of the activator protein-1 transcription factor family, is an important regulator of cytokine expression and thus critically involved in the cutaneous response to injury and stress. Mice lacking JunB in the skin develop normally, indicating that JunB is neither required for cutaneous organogenesis, nor homeostasis. However, upon wounding and treatment with the phorbol ester 12-O-decanoyl-phorbol-13-acetate, JunB-deficiency in the skin likewise resulted in pronounced epidermal hyperproliferation, disturbed differentiation, and prolonged inflammation. Furthermore, delayed tissue remodelling was observed during wound healing. These phenotypic skin abnormalities were associated with JunB-dependent alterations in expression levels and kinetics of important mediators of wound repair, such as granulocyte macrophage colony-stimulating factor, growth-regulated protein-1, macrophage inflammatory protein-2, and lipocalin-2 in both the dermal and epidermal compartment of the skin, and a reduced ability of wound contraction of mutant dermal fibroblasts in vitro.

Cyanidin-3-glucoside, a natural product derived from blackberry, exhibits chemopreventive and chemotherapeutic activity

Epidemiological data suggest that consumption of fruits and vegetables has been associated with a lower incidence of cancer. Cyanidin-3-glucoside (C3G), a compound found in blackberry and other food products, was shown to possess chemopreventive and chemotherapeutic activity in the present study. In cultured JB6 cells, C3G was able to scavenge ultraviolet B-induced *OH and O2-* radicals. In vivo studies indicated that C3G treatment decreased the number of non-malignant and malignant skin tumors per mouse induced by 12-O-tetradecanolyphorbol-13-acetate (TPA) in 7,12-dimethylbenz[a]anthracene-initiated mouse skin. Pretreatment of JB6 cells with C3G inhibited UVB- and TPA-induced transactivation of NF-kappaB and AP-1 and expression of cyclooxygenase-2 and tumor necrosis factor-alpha. These inhibitory effects appear to be mediated through the inhibition of MAPK activity. C3G also blocked TPA-induced neoplastic transformation in JB6 cells. In addition, C3G inhibited proliferation of a human lung carcinoma cell line, A549. Animal studies showed that C3G reduced the size of A549 tumor xenograft growth and significantly inhibited metastasis in nude mice. Mechanistic studies indicated that C3G inhibited migration and invasion of A549 tumor cells. These finding demonstrate for the first time that a purified compound of anthocyanin inhibits tumor promoter-induced carcinogenesis and tumor metastasis in vivo.

Prognostic role of c-Jun activation in patients with areca quid chewing-related oral squamous cell carcinomas in Taiwan

BACKGROUND

Overexpression or activation of c-Jun has been implicated in the pathogenesis of several types of cancer. Treatment of oral cells with areca nut extract in culture can increase c-Jun proto-oncogene mRNA levels. The purpose of this study was to investigate the possible role of c-Jun activation in the pathogenesis and prognosis of areca quid chewing-related oral cancer in Taiwan.

METHODS

We immunohistochemically examined c-Jun protein activation in human oral squamous cell carcinomas (SCC) using an anti-phosphospecific c-Jun (pc-Jun) antibody on paraffin-embedded sections.

RESULTS

Positive pc-Jun staining was observed in 42 of 70 (60%) cases of oral SCC. No significant correlation was found between pc-Jun expression and patients' age, gender, oral habit, cancer location, clinical stage, tumor size and lymph node status. Kaplan-Meier curves showed that oral SCC patients with positive pc-Jun staining or positive lymph node metastasis had significantly shorter overall survival (p < 0.018 and 0.001, respectively, by log-rank test).

CONCLUSION

These results indicate that c-Jun activation may play an important role in the carcinogenesis of oral SCC. Positive pc-Jun staining may serve as an adjuvant marker of worse prognosis in patients with oral SCC in Taiwan.

The AP-1 transcription factor regulates postnatal mammary gland development

The AP-1 transcription factor is activated by multiple growth factors that are critical regulators of breast cell proliferation. We previously demonstrated that AP-1 blockade inhibits breast cancer cell growth in vitro. Yet a specific role of AP-1 in normal mammary gland development has not been studied. Using a bi-transgenic mouse expressing an inducible AP-1 inhibitor (Tam67), we found that the AP-1 factor regulates postnatal proliferation of mammary epithelial cells. Mammary epithelial proliferation was significantly reduced after AP-1 blockade in adult, prepubertal, pubertal, and hormone-stimulated mammary glands. In pubertal mice, mammary cell proliferation was greatly reduced, and the cells that did proliferate failed to express Tam67. We also observed structural changes such as suppressed branching and budding, reduced gland tree size, and less fat pad occupancy in developing mammary glands after AP-1 blockade. We further demonstrated that Tam67 suppressed the expression of AP-1-dependent genes (TIMP-1, vimentin, Fra-1, and fibronectin) and the AP-1-dependent growth regulatory genes (cyclin D1 and c-myc) in AP-1-blocked mammary glands. We therefore conclude that AP-1 factor is a pivotal regulator of postnatal mammary gland growth and development.

AP-1 differentially expressed proteins Krp1 and fibronectin cooperatively enhance Rho-ROCK-independent mesenchymal invasion by altering the function, localization, and activity of nondifferentially expressed proteins

The transcription factor AP-1, which is composed of Fos and Jun family proteins, plays an essential role in tumor cell invasion by altering gene expression. We report here that Krp1, the AP-1 up-regulated protein that has a role in pseudopodial elongation in v-Fos-transformed rat fibroblast cells, forms a novel interaction with the nondifferentially expressed actin binding protein Lasp-1. Krp1 and Lasp-1 colocalize with actin at the tips of pseudopodia, and this localization is maintained by continued AP-1 mediated down-regulation of fibronectin that in turn suppresses integrin and Rho-ROCK signaling and allows pseudopodial protrusion and mesenchyme-like invasion. Mutation analysis of Lasp-1 demonstrates that its SH3 domain is necessary for pseudopodial extension and invasion. The results support the concept of an AP-1-regulated multigenic invasion program in which proteins encoded by differentially expressed genes direct the function, localization, and activity of proteins that are not differentially expressed to enhance the invasiveness of cells.

Kaposi's sarcoma-associated herpesvirus induction of AP-1 and interleukin 6 during primary infection mediated by multiple mitogen-activated protein kinase pathways

Kaposi's sarcoma is an angioproliferative disseminated tumor of endothelial cells linked to infection with Kaposi's sarcoma-associated herpesvirus (KSHV). AP-1 transcription factors are involved in diverse biological processes, including infection and replication of viruses, cell growth, oncogenesis, angiogenesis, and invasion of cancer cells. Here we show that KSHV activates AP-1 during primary infection. The activation of AP-1 at the early stage of KSHV infection is mainly mediated by virus entry events. Concurrently, KSHV infection strongly activates MEK, JNK, and to a lesser extent, p38 mitogen-activated protein kinase (MAPK) pathways. Specific inhibitors or dominant negative constructs of MEK and JNK completely abolish AP-1 activation by KSHV, while those of p38 reduce it by half. Furthermore, individual MAPK pathways differentially regulate KSHV activation of AP-1 components. KSHV activation of AP-1 leads to the transcriptional induction of interleukin 6 (IL-6), which is inhibited by inhibitors or dominant negative constructs of MAPK pathways. Together, these results demonstrate that KSHV induces AP-1 and IL-6 during primary infection by modulating multiple MAPK pathways. Because of the diverse roles of IL-6, AP-1, and MAPK pathways in viral infection and tumor induction and promotion, these results have important implications in the pathogenesis of KSHV-induced malignancies.

Role for Akt/Protein Kinase B and Activator Protein-1 in Cellular Proliferation Induced by the Human T-cell Leukemia Virus Type 1 Tax Oncoprotein

Human T-cell leukemia virus type 1 is an oncogenic retrovirus etiologically causal of adult T-cell leukemia. The virus encodes a Tax oncoprotein, which functions in transcriptional regulation, cell cycle control, and transformation. Because adult T-cell leukemia is a highly virulent cancer that is resistant to numerous chemotherapeutic treatments, to understand better this disease it is important to comprehend how human T-cell leukemia virus type 1 promotes cellular growth and survival. Most of the existing data point to Tax activation of NF-kappaB as important for cellular proliferation and transformation. We show here that Tax, in the absence of NF-kappaB signaling, can activate activator protein-1 to promote cellular proliferation and survival. Tax is shown to activate activator protein-1 through the phosphatidylinositol 3-kinase/Akt pathway.

Invasion is a genetic program regulated by transcription factors

The invasive and metastatic behaviour of tumours impacts crucially on the clinical management of cancer. Accordingly, it is important to understand the regulation of tumour cell invasiveness. Genetic analysis of worms, Drosophila and mice has provided evidence that invasion is a genetic pathway regulated by transcription factors that are often implicated in tumour cell invasion. Recent evidence has revealed much concerning the role of one particular transcription factor, AP1, which is involved in the regulation of a multigenic invasion program in which upregulated and downregulated genes function as invasion effectors and suppressors, respectively. Differentially expressed genes cooperatively enhance pseudopod elongation during the mesenchymal mode of invasion by altering the function, localisation and activity of non-differentially expressed proteins.

Long-term treatment of photoaged human skin with topical retinoic acid improves epidermal cell atypia and thickens the collagen band in papillary dermis

BACKGROUND

Risk of photocarcinogenesis and the relevance of collagen in wrinkle effacement are two issues related to prolonged use of retinoic acid (RA) that have not been fully addressed.

OBJECTIVE

Our purpose was to investigate the degree of epidermal cellular atypia and the thickness of papillary dermal collagen in photoaging after long-term use of RA.

METHODS

Thirty-four subjects with photoaged skin were treated daily with 0.05% RA for at least 6 months. Epidermal cellular atypia was graded by means of a semiquantitative scale. Thickness of collagen band was measured by using image-analysis software.

RESULTS

Compared with pretreatment findings, melanocytic and keratinocytic atypia was significantly reduced and the collagen band thickness doubled.

LIMITATIONS

This was an open-label study.

CONCLUSION

Improvement in epidermal cellular atypia is consistent with the ability of RA to act as a chemopreventive agent in epithelial carcinogenesis. Prolonged use also significantly increased collagen matrix deposition in dermal repair zones, which most likely contributes to wrinkle effacement by RA.

Regulation of Nrf2, NF-kappaB, and AP-1 signaling pathways by chemopreventive agents

The inhibition of carcinogenesis by chemopreventive agents has been demonstrated in many tumorigenesis animal models. The chemopreventive mechanisms of those phytochemicals have been investigated extensively, though mostly in in vitro cell culture systems. The cellular signaling cascades mediated by transcription factors, including nuclear factor E2-related factor 2 (Nrf2), nuclear factor-kappaB (NF-kappaB), and activator protein-1 (AP-1), have been shown to play pivotal roles in tumor initiation, promotion, and progression processes. Thus, as demonstrated by previous substantive mechanistic studies, they appear to be ideal targets for cancer chemoprevention. In this review, we discuss the current progress and future challenges on our understanding of the molecular mechanisms in cancer chemoprevention by phytochemicals, focusing on the regulation of Nrf2, NF-kappaB, and AP-1 signaling pathways.

The role of c-Jun in the AP-1 activation induced by naturally occurring isothiocyanates

Despite strong evidence that isothiocyanates (ITCs) inhibit cancer development, there are also reports that some of them induce or promote carcinogenesis. The molecular basis of the latter is largely unknown. We report here that all three ITCs that caused urinary bladder cancer in rats, including allyl ITC, benzyl ITC, and phenethyl ITC, increased the transactivation of activator protein 1 (AP-1) and AP-1 DNA binding in human bladder cancer UM-UC-3 cells. Amongst all Fos and Jun family members examined, only were the levels of c-Jun and Fra-2 consistently elevated by the ITCs. However, whereas c-Jun was identified as the predominant component in the AP-1 DNA binding complex, Fra-2 was not detected, suggesting that c-Jun may be mainly responsible for ITC-induced AP-1 activation. c-Jun was also induced by the ITCs in other bladder cancer cell lines (both human and rat) and by their N-acetylcysteine derivatives--their main urinary metabolites. c-Jun induction by the ITCs appears to involve both transcriptional activation and protein phosphorylation; the latter resulted from activation of c-Jun N-terminal kinase by the ITCs. Because c-Jun has been implicated in cancer development, including human bladder cancer, our data suggest that c-Jun activation may play an important role in ITC-induced bladder carcinogenesis.

Altered regulation of c-jun and its involvement in anchorage-independent growth of human lung cancers

The c-jun oncogene is frequently overexpressed in non-small-cell lung cancers (NSCLC), but its functional involvement in lung cancer development has not been clearly elucidated. In this study, we found that among the immediate-early serum responsible genes, exemplified by c-jun, c-fos and c-myc, induction of c-jun in a human bronchial epithelial cell line, BEAS-2B, was dependent on anchorage, in contrast to clear induction of c-fos and c-myc under both anchorage-dependent and -independent conditions. In fact, forced expression of c-jun in BEAS-2B cells significantly increased cell viability and colony formation in soft agar. Furthermore, we also found that such anchorage-dependent regulation of c-jun was lost in a significant fraction of human lung cancer cell lines. Interestingly, suppressed anchorage-independent but not anchorage-dependent growth was noted by constitutive expression of a dominant-negative c-jun mutant in a lung cancer cell line showing dysregulated and sustained c-jun expression in the absence of anchorage. These findings suggest that dysregulated c-jun expression may be involved in the acquisition of anchorage independence in the process of human lung carcinogenesis.

A high-throughput fluorescence-anisotropy screen that identifies small molecule inhibitors of the DNA binding of B-ZIP transcription factors

We have developed a high-throughput fluorescence anisotropy screen, using a 384-well format, to identify small molecules that disrupt the DNA binding of B-ZIP proteins. Binding of a B-ZIP dimer to fluorescently labeled DNA can be monitored by fluorescence anisotropy. We screened the National Cancer Institute diversity set of 1990 compounds to identify small molecules that disrupt the B-ZIP|DNA complex of CREB, C/EBPbeta, VBP, and AP-1 (FOS|JUND) bound to their cognate DNA sequence. We identified 21 compounds that inhibited the DNA binding of at least one B-ZIP protein, and 12 representative compounds were grouped depending on whether they displaced ethidium bromide from DNA. Of the 6 compounds that did not displace ethidium bromide, 2 also inhibited B-ZIP binding to DNA in a secondary electrophoretic mobility shift assay screen with some specificity. Thermal stability monitored by circular dichroism spectroscopy demonstrated that both compounds bound the basic region of the B-ZIP motif. NSC13778 preferentially binds C/EBPalpha 1000-fold better than it binds C/EBPbeta. Chimeric proteins combining C/EBPalpha and C/EBPbeta mapped the binding of NSC13778 to three amino acids immediately N terminal of the leucine zipper of C/EBPalpha. These experiments suggest that the DNA binding of B-ZIP transcription factors is a potential target for clinical intervention.

Inhibition of p38 mitogen-activated protein kinase and phosphatidylinositol 3-kinase decreases UVB-induced activator protein-1 and cyclooxygenase-2 in a SKH-1 hairless mouse model

Activation of activator protein-1 (AP-1) and increased expression of cyclooxygenase-2 (COX-2) have been clearly shown to play a functional role in UVB-induced skin tumor promotion. In this study, we examined UVB-induced signal transduction pathways in SKH-1 mouse epidermis leading to increases in COX-2 expression and AP-1 activity. We observed rapid increases in p38 mitogen-activated protein kinase (MAPK) signaling through activation of p38 MAPK and its downstream target, MAPK activated protein kinase-2. UVB also increased phosphatidylinositol 3-kinase (PI3K) signaling as observed through increases in AKT and GSK-3beta phosphorylation. Activation of the p38 MAPK and PI3K pathways results in the phosphorylation of cyclic AMP-responsive element binding protein, which was also observed in UVB-irradiated SKH-1 mice. Topical treatment with SB202190 (a specific inhibitor of p38 MAPK) or LY294002 (a specific inhibitor of PI3K) significantly decreased UVB-induced AP-1 activation by 84% and 68%, respectively, as well as COX-2 expression. Our data show that in mouse epidermis, UVB activation of the p38 MAPK and PI3K pathways leads to AP-1 activation and COX-2 expression.

Differential inhibition of UVB-induced AP-1 and NF-kappaB transactivation by components of the jun bZIP domain

Potential targets for chemoprevention of nonmelanoma skin cancer include UV-induced nuclear factor kappaB (NF-kappaB) and activator protein-1 (AP-1) activation in keratinocytes. Inhibition of both these ultraviolet light B (UVB)-induced transcription factors has been shown with the dominant-negative c-jun mutant, TAM67; however, its mechanism of action has not yet been determined. Here we demonstrated that transient transfection of a mouse keratinocyte cell line (308) with a dominant-negative phosphorylation mutant of c-Jun before exposure to 250 J/m(2) UVB inhibits transactivation mediated by both AP-1 and NF-kappaB transcription factors to levels below those of UVB exposed controls. Through the utilization of immunoprecipitation techniques, protein-protein interactions between NF-kappaB family members IkappaBalpha, IkappaBbeta, p50, and p65 (Rel-A) were identified with an Xpress tagged dominant-negative c-Jun (TAM67) protein. Expression of the leucine zipper domain of the TAM67 protein inhibited UVB-induced NF-kappaB transactivation but not AP-1 transactivation. Expression of the bZIP domain of the TAM67 protein was able to inhibit transactivation mediated by both transcription factors. These data demonstrate that TAM67 is able to inhibit two significant UVB-induced molecular targets AP-1 and NF-kappaB, and that the inhibition of these two transcription factor families is potentially due to protein-protein interactions between different regions of the dominant-negative c-Jun protein.

Epidermal expression of the translation inhibitor programmed cell death 4 suppresses tumorigenesis

Programmed cell death 4 (Pdcd4) is a novel repressor of in vitro transformation. Pdcd4 directly inhibits the helicase activity of eukaryotic translation initiation factor 4A, a component of the translation initiation complex. To ascertain whether Pdcd4 suppresses tumor development in vivo, we have generated transgenic mice that overexpress Pdcd4 in the epidermis (K14-Pdcd4). K14-regulated Pdcd4 expression caused a neonatal short-hair phenotype due to early catagen entry compared with matched wild-type siblings. In response to the 7,12-dimethylbenz(a)anthracene (DMBA)/12-O-tetradecanoylphorbol-13-acetate (TPA) mouse skin carcinogenesis protocol, K14-Pdcd4 mice showed significant reductions in papilloma formation, carcinoma incidence, and papilloma-to-carcinoma conversion frequency compared with wild-type mice. The translational efficiency of an mRNA engineered to form a structured 5' untranslated region (UTR) was attenuated in primary keratinocytes when Pdcd4 was overexpressed. Pdcd4 inhibited by 46% TPA-induced activator protein-1 (AP-1)-dependent transcription, an event required for tumorigenesis. CDK4 and ornithine decarboxylase (ODC) are candidates for Pdcd4-regulated translation as their mRNAs contain 5'structured UTRs. In K14-Pdcd4 primary keratinocytes expressing activated Ha-Ras to mimic DMBA-initiated epidermis, ODC and CDK4 protein levels were decreased by 40% and 46%, respectively. Expression of a protein encoded by 5' unstructured mRNA showed no change. These results extend to an in vivo model the observations that Pdcd4 inhibits both translation initiation and AP-1 activation while decreasing benign tumor development and malignant progression. The K14-Pdcd4 mice seem to validate translation initiation as a novel target for cancer prevention.

The tumor suppressor p16(INK4a) prevents cell transformation through inhibition of c-Jun phosphorylation and AP-1 activity

Inactivation of the p16(INK4a) tumor suppressor protein is critical for the development of human cancers, including human melanoma. However, the molecular basis of the protein's inhibitory effect on cancer development is not clear. Here we investigated a possible mechanism for p16(INK4a) inhibition of neoplastic transformation and UV-induced skin cancer. We show that p16(INK4a) suppresses the activity of c-Jun N-terminal kinases (JNKs) and that it binds to the glycine-rich loop of the N-terminal domain of JNK3. Although p16(INK4a) does not affect the phosphorylation of JNKs, its interaction with JNK inhibits c-Jun phosphorylation induced by UV exposure. This, in turn, interferes with cell transformation promoted by the H-Ras-JNK-c-Jun-AP-1 signaling axis.

Production of chemokine CXCL1/KC by okadaic acid through the nuclear factor-kappaB pathway

The murine chemokine CXCL1/KC is known as a chemoattractant for neutrophil infiltration and as a promoter of tumor growth. To determine its relevance in tumorigenesis, we first asked whether okadaic acid (OKA), a natural tumor promoter and a potent protein phosphatase 1 and 2A inhibitor, stimulates KC expression and if it does, through what pathway, in a promotable mouse epidermal-like JB6 cell line commonly used for studying molecules related to tumor promotion. We found that OKA stimulated the de novo synthesis of KC mRNA and protein in a dose- and time-dependent manner. To determine the mechanism by which OKA stimulated the expression of KC at the transcriptional level, transient transfection assays using serially deleted sections of KC promoter fused to luciferase reporter gene were performed. These studies showed that transactivation of KC promoter by OKA specifically involved the region between -104 and -59 containing the two nuclear factor-kappaB (NF-kappaB) response elements (kappaB1 and kappaB2). Further analyses using the mutated NF-kappaB response elements kappaB1 and kappaB2 indicated that both regions were required for optimum transactivation of KC by OKA with the former NF-kappaB response element playing a more significant role in regulating KC expression. Gel-shift and supershift analyses demonstrated the involvement of three NF-kappaB subunits, p65, p50 and c-Rel, with p65 as the major subunit in the NF-kappaB dimer complex. Additionally, immunohistochemistry and western blot analyses confirmed the presence of p65 in the nucleus with its transactivation domain phosphorylated at serine 536. In summary, this is the first report to show that the tumor promoter OKA can stimulate the de novo synthesis and secretion of KC, and that this stimulation is mediated through the NF-kappaB pathway in JB6 cells.

c-Fos-dependent induction of the small ras-related GTPase Rab11a in skin carcinogenesis

Malignant transformation of mouse skin by tumor promoters and chemical carcinogens, such as the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), is a multistage process leading to the formation of squamous cell carcinomas. It has been shown that mice lacking the AP-1 family member c-Fos exhibit an impaired transition from benign to malignant skin tumors. Here, we demonstrate enhanced expression of the small Ras-related GTPase Rab11a after short-term TPA treatment of mouse back skin. Expression of Rab11a in vivo and in vitro critically depended on c-Fos, because TPA application to the back skin of c-Fos-deficient mice and to mouse embryonic fibroblasts did not induce Rab11a mRNA or protein expression. Moreover, dexamethasone, which is a potent inhibitor of AP-1-mediated transactivation that exhibits anti-inflammatory and anti-tumor promoting activities, inhibited TPA-induced expression of Rab11a. Within the Rab11a gene promoter, we identified a functional AP-1 binding element that exhibited elevated c-Fos binding activity after TPA treatment of keratinocytes. Enhanced expression was not restricted to chemically induced mouse skin tumors but was also found in tumor specimens derived from patients with epithelial skin tumors. These data identify Rab11a as a novel, tumor-associated c-Fos/AP-1 target and may point to an as yet unrecognized function of Rab11a in the development of skin cancer.

Transcriptional activities of retinoic acid receptors

Vitamin A derivatives plays a crucial role in embryonic development, as demonstrated by the teratogenic effect of either an excess or a deficiency in vitamin A. Retinoid effects extend however beyond embryonic development, and tissue homeostasis, lipid metabolism, cellular differentiation and proliferation are in part controlled through the retinoid signaling pathway. Retinoids are also therapeutically effective in the treatment of skin diseases (acne, psoriasis and photoaging) and of some cancers. Most of these effects are the consequences of retinoic acid receptors activation, which triggers transcriptional events leading either to transcriptional activation or repression of retinoid-controlled genes. Synthetic molecules are able to mimic part of the biological effects of the natural retinoic acid receptors, all-trans retinoic acid. Therefore, retinoic acid receptors are considered as highly valuable therapeutic targets and limiting unwanted secondary effects due to retinoid treatment requires a molecular knowledge of retinoic acid receptors biology. In this review, we will examine experimental evidence which provide a molecular basis for the pleiotropic effects of retinoids, and emphasize the crucial roles of coregulators of retinoic acid receptors, providing a conceptual framework to identify novel therapeutic targets.

Blackberry extracts inhibit activating protein 1 activation and cell transformation by perturbing the mitogenic signaling pathway

Blackberries are natural rich sources of bioflavonoids and phenolic compounds that are commonly known as potential chemopreventive agents. Here, we investigated the effects of fresh blackberry extracts on proliferation of cancer cells and neoplastic transformation induced by 12-O-tetradecanoylphorbol-13-acetate (TPA), as well as the underlying mechanisms of signal transduction pathways. Using electron spin resonance, we found that blackberry extract is an effective scavenger of free radicals, including hydroxyl and superoxide radicals. Blackberry extract inhibited the proliferation of a human lung cancer cell line, A549. Pretreatment of A549 cells with blackberry extract resulted in an inhibition of 8-hydroxy-2'-deoxyguanosine (8-OHdG) formation induced by ultraviolet B (UVB) irradiation. Blackberry extract decreased TPA-induced neoplastic transformation of JB6 P+ cells. Pretreatment of JB6 cells with blackberry extract resulted in the inhibition of both UVB- and TPA-induced AP-1 transactivation. Furthermore, blackberry extract also blocked UVB- or TPA-induced phosphorylation of ERKs and JNKs, but not p38 kinase. Overall, these results indicated that an extract from fresh blackberry may inhibit tumor promoter-induced carcinogenesis and associated cell signaling, and suggest that the chemopreventive effects of fresh blackberry may be through its antioxidant properties by blocking reactive oxygen species-mediated AP-1 and mitogen-activated protein kinase activation.

Alcohol, vitamin A, and cancer

Chronic and excessive alcohol intake is associated with an increased risk of a variety of cancers (e.g., oral cavity, larynx, esophagus, liver, lung, colorectal, and breast). Retinoids (vitamin A and its derivatives) are known to exert profound effects on cellular growth, cellular differentiation, and apoptosis, thereby controlling carcinogenesis. Lower hepatic vitamin A levels have been well documented in alcoholics. Substantial research has been done, investigating the mechanisms by which excessive alcohol interferes with retinoid metabolism. More specifically, (1) alcohol acts as a competitive inhibitor of vitamin A oxidation to retinoic acid involving alcohol dehydrogenases and acetaldehyde dehydrogenases; (2) alcohol-induced cytochrome P450 enzymes (CYP), particularly CYP2E1, enhance catabolism of vitamin A and retinoic acid; and (3) alcohol alters retinoid homeostasis by increasing vitamin A mobilization from liver to extrahepatic tissues. As a consequence, long-term and excessive alcohol intake results in impaired status of retinoic acid, the most active derivative of vitamin A and a ligand for both retinoic acid receptors and retinoid X receptors. Moreover, this alcohol-impaired retinoic acid homeostasis interferes with (1) retinoic acid signaling (e.g., down-regulates retinoid target gene expression) and (2) retinoic acid "cross-talk" with the mitogen-activated protein kinase [(MAPK), including Jun N-terminal kinase, extracellular signal-regulated kinase, and p38 kinase] signaling pathway. In addition, restoration of retinoic acid homeostasis by retinoic acid supplementation restored the normal status of both retinoid and MAPK signaling, thereby maintaining normal cell proliferation and apoptosis in alcohol-fed animals. These observations would have implications for the prevention of alcohol-promoted liver (and peripheral tissue) carcinogenesis. However, a better understanding of the alcohol-retinoid interaction and the molecular mechanisms involved is needed before retinoids can be pursued in the prevention of alcohol-related carcinogenesis in human beings, particularly regarding the detrimental effects of polar metabolites of vitamin A.

Notch signaling in the integrated control of keratinocyte growth/differentiation and tumor suppression

Oncogenesis is closely linked to abnormalities in cell differentiation. Notch signaling provides an important form of intercellular communication involved in cell fate determination, stem cell potential and differentiation. Here we review the role of this pathway in the integrated growth/differentiation control of the keratinocyte cell type, and the maintenance of normal skin homeostasis. In parallel with the pro-differentiation function of Notch1 in keratinocytes, we discuss recent evidence pointing to a tumor suppressor function of this gene in both mouse skin and human cervical carcinogenesis. The possibility that Notch signaling elicits signals with a duality of growth positive and negative function will be discussed.

Regulation of Ultraviolet B Radiation-Mediated Activation of AP1 Signaling by Retinoids in Primary Keratinocytes

The main cause of skin cancer and photo-aging is chronic exposure to ultraviolet B (UVB) radiation. Such damage can be ameliorated by retinoid treatment. UVB-radiation-induced skin carcinogenesis is associated with the induction of activator protein 1 (AP1) signaling and factors, namely FOS and JUN family members. We investigated the effects of several retinoids, all-trans-retinoic acid (tRA), 9-cis-retinoic acid (cRA), and N-(4-hydroxyphenyl)-retinamide (HPR), on UVB-induced damage in primary mouse keratinocytes. In addition, the interplay between UVB radiation, retinoid receptors, and AP1 signaling was assessed using Western blot analysis and ribonuclease protection and gene reporter assays. Exposure of keratinocytes to UVB radiation caused a down-regulation of the retinoid receptor protein levels in a proteasome-mediated manner. In contrast, FOS and JUN proteins were transiently induced shortly after exposure to UVB radiation. Retinoid treatment caused a dose-dependent reduction in the levels of retinoid receptor proteins. When irradiated cells were treated with retinoids, no significant effects on AP1 protein expression were noted. Interestingly, pretreatments with tRA and cRA, but not HPR, suppressed UVB-radiation-induced AP1 activity by more than 50%, whereas post-treatment failed to produce similar effects. Our findings indicate that the inhibition of AP1 activity by retinoids explains, at least in part, the chemopreventive potential of retinoids in UV-radiation-associated epidermal damage.