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

Flavanones structure-related inhibition on TPA-induced tumor promotion through suppression of extracellular signal-regulated protein kinases: involvement of prostaglandin E2 in anti-promotive process

Biological functions of flavanones have been studied extensively, however, the structure-related activities of flavanones on 12-o-tetradecanoylphorbol 13-acetate (TPA)-induced promotive effects are still unclear. In this study, flavanone, 2'-OH flavanone, 4'-OH flavanone, 6-OH flavanone showed the most significant dose-dependent inhibition on TPA-induced proliferative effects among eight tested flavanones in NIH3T3 cells. TPA-induced mitogen activated protein kinases (MAPK) phosphorylation, ornithine decarboxylase (ODC), c-Jun, and cyclooxygenase 2 (COX-2) protein expressions in a time-dependent manner, and the maximal inductive time point is at 1 h for MAPK phosphorylation and 6 h for others. Flavanone, 2'-OH flavanone, 4'-OH flavanone, 6-OH flavanone showed the dose-dependent inhibition on TPA-stimulated MAPK phosphorylation, COX-2, ODC, c-Jun protein expressions. Induction of, prostaglandin E(2) (PGE(2)) production was detected in TPA-treated NIH3T3 cells, and flavanone, 2'-OH flavanone, 4'-OH flavanone, 6-OH flavanone inhibited significantly PGE(2) production induced by TPA. Addition of PGE(2) reverses the inhibitory activities of flavanone, 2'-OH flavanone, 4'-OH flavanone, 6-OH flavanone on TPA-induced proliferation. And, PD98059, a specific inhibitor of ERKs, inhibited TPA-induced MAPK phosphorylation, accompanied by decreasing COX-2, c-Jun, and ODC protein expression, and showed dose-dependent inhibition on TPA-induced proliferation in cells. These results demonstrated that PGE(2) is an important mediator in TPA-induced proliferation, and MAPK phosphorylation was located at the upstream of COX-2, c-Jun, and ODC gene expressions in TPA-induced responses. Furthermore, flavanone, 2'-OH flavanone, 4'-OH flavanone, 6-OH flavanone (100 microM) suppressed TPA-induced colony formation associated with blocking MAPK phosphorylation, ODC, c-Jun, and COX-2 proteins expression. And, 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay showed that flavanone, 2'-OH flavanone, 4'-OH flavanone, 6-OH flavanone did not perform potent anti-radical activities among these eight tested compounds. In conclusion, this study provided molecular evidences to demonstrate that flavanone, 2'-OH flavanone, 4'-OH flavanone, 6-OH flavanone were potent inhibitors on TPA-induced responses without notable cytotoxicity through suppression of PGE(2) production; and anti-radical activity of flavanones was not correlated with preventing the occurrence of tumor promotion. We proposed that blocking TPA-induced intracellular signaling responses might be involved in the anti-promotive mechanism of flavanones.

AP-1, NF-kappa-B, and ERK activation thresholds for promotion of neoplastic transformation in the mouse epidermal JB6 model

The promotion-sensitive mouse epidermal JB6 cells (clone 41) have been used to identify the tumor-promoting activity of various compounds. Because treatment by tumor promoters [12-O-tetradecanoylphorbol-13-acetate (TPA), epidermal growth factor (EGF), or tumor necrosis factor alpha (TNF-alpha)] transforms clone 41 cells to anchorage-independent and tumorigenic phenotypes, they are considered to be undergoing late-stage tumor promotion. Here we address the question of how much activation of transformation-relevant transcription factors [activator protein-1 (AP-1), ternary complex factors (TCFs), or nuclear factor kappa-B (NF-kappa-B)] is required for transformation response and how much tumor promoter produces significant risk of transformation. Stable transfectants harboring a reporter construct with an AP-1 response element, serum-response element (SRE), or NF-kappa-B response element were established. We examined the relationship between concentration of tumor promoters, key signaling events, and activation of the transcription factors. A concentration of > 0.2 nM TPA or 0.12 ng/mL (0.02 nM) EGF produced a significant increase in transformation response as well as in extracellular signal-regulated protein kinase (ERK), SRE, or AP-1 activation. Treatment with > 0.4 U/mL (2.35 pM) TNF-alpha increased NF-kappa-B activity and transformation response in a dose-dependent manner. However, transformation response decreased at > 33 U/mL TNF-alpha due to a cytotoxic response. These findings suggest that the signaling pathway leading to the activation of ERK, TCF, and AP-1 proteins constitutes a major factor determining the risk of tumor promotion by TPA or EGF. Cell toxicity in addition to NF-kappa-B activation should be considered in predicting TNF-alpha-induced transformation response.

Novel protein kinase C isoforms and mitogen-activated kinase kinase mediate phorbol ester-induced osteopontin expression

BACKGROUND AND AIMS

The expression of osteopontin (OPN), a protein postulated to play a role in tumorigenesis, is induced by the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA) in vivo and in the in vitro initiation-promotion skin carcinogenesis model (JB6 cells). Although TPA-induced OPN expression in JB6 cells has been suggested to involve protein kinase C (PKC), the PKC isoforms and the downstream pathway mediating OPN expression have not been extensively studied.

METHODS

Using the JB6 cell model, we determined the involvement of PKC isoforms, mitogen-activated protein kinase kinase (MAPK kinase/MEK) and MAPK in TPA-induced OPN expression using inhibitors specific to PKC isoforms and MEK and performing Northern blot analyses. Western blot analyses of cells treated with specific inhibitors were also performed to determine whether PKC isoforms or MEK were involved in activation of MAPK.

KEY RESULTS

TPA increased the steady-state level of OPN mRNA as early as 2-4h and this expression persisted for at least 4 days. TPA induction of OPN expression in JB6 cells is mediated through PKC epsilon and PKC delta, which also mediated the phosphorylation of MAPK. Additionally, inhibition of MEK activity, which activates MAPK, attenuated TPA-induced OPN expression. These findings suggest that activation of MAPK is important in mediating OPN expression.

CONCLUSION

TPA-induced steady-state OPN mRNA expression in mouse JB6 cells involves the activation of MAPK mediated through PKC epsilon and/or PKC delta.

Tumour necrosis factor-alpha mediates tumour promotion via a PKC alpha- and AP-1-dependent pathway

Tumour necrosis factor-alpha (TNF-alpha) deficient mice (TNF-alpha(-/-) mice) are resistant to skin carcinogenesis. Cellular signalling via the transcription factor complex AP-1 is thought to play a key role in tumour promotion. The induction of a specific subset of AP-1 responsive genes thought to be important for tumour development, namely GM-CSF, MMP-9 and MMP-3, was suppressed in TNF-alpha(-/-) compared to wild-type mouse skin in response to the tumour promotor TPA. The differential induction of these genes correlated with a temporal shift in AP-1 activation and c-Jun expression in TNF-alpha(-/-) compared to wild-type epidermis. The major receptor for TPA-induced signalling in basal keratinocytes, PKC alpha, was also differentially regulated in wild-type compared with TNF-alpha(-/-) epidermis. A marked delay in TPA-induced intracellular translocation and downregulation of PKC alpha was observed in TNF-alpha(-/-) epidermis, which correlated with the deregulated TPA-induced AP-1 activation and c-Jun expression. The frequency of DNA adduct formation and c-Ha-ras mutations was the same in wild-type and TNF-alpha(-/-) epidermis after DMBA treatment, suggesting that TNF-alpha was not involved in tumour initiation. These data suggest that the pro-inflammatory cytokine TNF-alpha is a critical mediator of tumour promotion, acting via a PKC alpha- and AP-1-dependent pathway. This may be one mechanism by which chronic inflammation increases susceptibility to cancer.

JDP2, a repressor of AP-1, recruits a histone deacetylase 3 complex to inhibit the retinoic acid-induced differentiation of F9 cells

Up-regulation of the c-jun gene is a critical event in the retinoic acid (RA)-mediated differentiation of embryonal carcinoma F9 cells. Activating transcription factor 2 (ATF-2) and p300 cooperate in the activation of transcription of the c-jun gene during the differentiation of F9 cells. We show here that the overexpression of Jun dimerization protein 2 (JDP2), a repressor of AP-1, inhibits the transactivation of the c-jun gene by ATF-2 and p300 by recruitment of the histone deacetylase 3 (HDAC3) complex, thereby repressing the RA-induced transcription of the c-jun gene and inhibiting the RA-mediated differentiation of F9 cells. Moreover, chromatin immunoprecipitation assays showed that the JDP2/HDAC3 complex, which binds to the differentiation response element within the c-jun promoter in undifferentiated F9 cells, was replaced by the p300 complex in response to RA, with an accompanying change in the histone acetylation status of the chromatin, the initiation of transcription of the c-jun gene, and the subsequent differentiation of F9 cells. These results suggest that JDP2 may be a key factor that controls the commitment of F9 cells to differentiation and shed new light on the mechanism by which an AP-1 repressor functions.

Downregulated AP-1 activity is associated with inhibition of Protein-Kinase-C-dependent CD44 and ezrin localisation and upregulation of PKC theta in A431 cells

Progression to an invasive, metastatic tumour requires the coordinated expression and function of a number of gene products, as well as their regulation in the context of invasion. The transcription factor AP-1 regulates expression of many of those genes necessary for implementation of the invasion programme. Two such gene products, CD44 and ezrin, are both upregulated in fibroblasts transformed by v-fos and are commonly implicated in cell motility and invasion. Here we report that CD44 and ezrin colocalise to membrane ruffles and microvilli of A431 cells after treatment with EGF. However, A431 cells expressing dominant-negative c-Jun (TAM67), and which as a consequence fail to invade in response to EGF, also fail to correctly localise CD44 and ezrin. CD44 and ezrin are both substrates for Protein Kinase C, and we show that their EGF-dependent colocalisation requires Protein Kinase C activity. Associated with TAM67 expression and disrupted CD44 and ezrin colocalisation is the increased expression and activation of the novel PKC theta isoform. Expression of PKC theta in A431 cells results in the inhibition of cell motility and disrupted localisation of CD44 and ezrin. We propose that AP-1 regulates the integrity of Protein Kinase C signalling and identifies PKC theta as a potential suppressor of the invasion programme.

Retinoic acid receptors inhibit AP1 activation by regulating extracellular signal-regulated kinase and CBP recruitment to an AP1-responsive promoter

Retinoids exhibit antineoplastic activities that may be linked to retinoid receptor-mediated transrepression of activating protein 1 (AP1), a heterodimeric transcription factor composed of fos- and jun-related proteins. Here we show that transcriptional activation of an AP1-regulated gene through the mitogen-activated protein kinase (MAPK)-extracellular signal-regulated kinase (ERK) pathway (MAPK(ERK)) is characterized, in intact cells, by a switch from a fra2-junD dimer to a junD-fosB dimer loading on its promoter and by simultaneous recruitment of ERKs, CREB-binding protein (CBP), and RNA polymerase II. All-trans-retinoic acid (atRA) receptor (RAR) was tethered constitutively to the AP1 promoter. AP1 transrepression by retinoic acid was concomitant to glycogen synthase kinase 3 activation, negative regulation of junD hyperphosphorylation, and to decreased RNA polymerase II recruitment. Under these conditions, fra1 loading to the AP1 response element was strongly increased. Importantly, CBP and ERKs were excluded from the promoter in the presence of atRA. AP1 transrepression by retinoids was RAR and ligand dependent, but none of the functions required for RAR-mediated transactivation was necessary for AP1 transrepression. These results indicate that transrepressive effects of retinoids are mediated through a mechanism unrelated to transcriptional activation, involving the RAR-dependent control of transcription factors and cofactor assembly on AP1-regulated promoters.

Ultraviolet B (UVB) induction of the c-fos promoter is mediated by phospho-cAMP response element binding protein (CREB) binding to CRE and c-fos activator protein 1 site (FAP1) cis elements

The ultraviolet B (UVB) portion (280-320 nm) of the ultraviolet spectrum has been shown to contribute to the development of non-melanoma skin cancer in humans. Research in the human keratinocyte cell line, HaCaT, revealed that UVB irradiation caused the upregulation of the transcription factor activator protein-1 (AP-1). The AP-1 complex formed in UVB-irradiated HaCaT cells is specifically composed of c-fos and Jun D. c-Fos expression was induced in a manner that correlated with the UVB-induced activation of AP-1. To investigate how c-fos expression is regulated by UVB irradiation, the role of each of four cis elements within the c-fos promoter was evaluated. Clustered point mutations at the sis inducible element (SIE), serum response element (SRE), c-fos AP-1 site (FAP1), or cyclic AMP response elements (CRE) significantly inhibited UVB induction of the c-fos promoter. This indicated that all four cis elements are required for maximum promoter activity. The CRE and FAP1 elements were the two most active cis elements that mediate the UVB transactivation of c-fos. Homodimers of phosphorylated cAMP response element binding protein (CREB) were induced by UVB irradiation to bind to each of these elements. Therefore, CREB may function as an important regulatory protein in the UVB-induced expression of c-fos.

STAT3 activation is required for interleukin-6 induced transformation in tumor-promotion sensitive mouse skin epithelial cells

STAT3, a member of signal transducers and activators of transcription (STATs) originally discovered as mediators in cytokine signaling pathways, plays an active role in oncogenesis. However, the function of STAT3 in signaling multistage carcinogenesis, especially in transformation of tumor-promotion sensitive epithelial cells has not been elucidated. The present study demonstrates that STAT3 is activated in interleukin-6 induced transformation in mouse skin epithelial cells. DNA binding and transcriptional activities of STAT3 were significantly increased by interleukin-6. This induced anchorage-independent transformation in tumor-promotion sensitive JB6 mouse skin P+ cells but not in the resistant variant P- cells. Two forms of dominant negative STAT3 (mutant of transcriptional domain, mF, or DNA-binding domain, mD) were stably transfected into P+ cells. Activation of STAT3 was abolished and importantly, interleukin-6 induced anchorage-independent growth was absent in both mutant STAT3 transfectants. To determine the genes targeted by STAT3, three matrix metalloproteinase proteins linked with carcinogenesis of epithelial cells were analysed. Both basal and interleukin-6 induced expression of collagenase I and stromelysin I, but not gelatinase A, were inhibited in the mutant STAT3 transfectants. Furthermore, transfection of a wild type STAT3 restored STAT3 transactivation and response to interleukin-6 induced transformation in mutant STAT3 transfectants, which up-regulated collagenase I and stromelysin I as well. Together, these results provide the first evidence that STAT3 activation is required in the progression of multistage carcinogenesis of mouse skin epithelial cells, and matrix metalloproteinases are actively involved in STAT3-mediated cell transformation.

Protection against human papillomavirus type 16-E7 oncogene-induced tumorigenesis by in vivo expression of dominant-negative c-jun

Expression of the human papillomavirus (HPV) type 16 E6 and E7 gene products is a risk factor for human cervical carcinogenesis as well as skin and oral carcinogenesis. Expression of the HPV-16 E7 gene in mouse skin induces hyperplasia and enhances tumor promotion. Expression of dominant-negative c-jun (TAM67) in the mouse skin protects mice from 7,12-dimethylbenz[a]anthracene (DMBA)/12-O-tetradecanoylphorbol-13-acetate (TPA)-induced papillomagenesis without blocking mitogen-induced hyperproliferation. To determine the role of activator protein-1 (AP-1) in HPV-induced cancer, we crossed HPV-16 E7 mice with TAM67 mice and analyzed the effects of DMBA/TPA on tumor promotion. We showed that expression of TAM67 protected mice from HPV-16 E7-enhanced tumorigenesis, suggesting AP-1 as a target for prevention of HPV-induced cancer.

Manganese superoxide dismutase deficiency enhances cell turnover via tumor promoter-induced alterations in AP-1 and p53-mediated pathways in a skin cancer model

Previous studies in our laboratories demonstrated that overexpression of manganese superoxide dismutase (MnSOD) suppressed both the incidence and multiplicity of papillomas in a DMBA/TPA multi-stage skin carcinogenesis model. The activity of activator protein-1 (AP-1), which is associated with tumor promotion, was reduced in MnSOD transgenic mice overexpressing MnSOD in the skin, suggesting that MnSOD may reduce tumor incidence by suppressing AP-1 activation. In the present study, we report that reduction of MnSOD by heterozygous knockout of the MnSOD gene (Sod2 -/+, MnSOD KO) increased the levels of oxidative damage proteins and the activity of AP-1 following TPA treatment. RNA levels of ornithine decarboxylase (ODC) were also increased, suggesting an increase in cell proliferation in the KO mice. Histological examination confirmed that the number of proliferating cells in DMBA/TPA-treated mouse skin were higher in the KO mice. Interestingly, histological examination also demonstrated greater numbers of apoptotic cells in the KO mice after DMBA/TPA treatment. Evidence of apoptosis, including DNA fragmentation, cytochrome c release from mitochondria, and caspase 3 activation were also observed by biochemical assays of the skin tissues. Apoptosis was associated with an increase in nuclear levels of p53 as determined by Western analysis. Quantitative immunogold ultrastructural analysis confirmed that p53 immunoreactive protein levels were increased to a greater level in the nuclei of epidermal cells from MnSOD KO mice compared to epidermal nuclei from wild type mice similarly treated. Moreover, p53 levels further increased in the mitochondria of DMBA/TPA treated mice, and this increase was much greater in the MnSOD KO than in the wild type mice, suggesting a link between MnSOD deficiency and mitochondrial-mediated apoptosis. Pathological examination reveals no difference in the incidence and frequency of papillomas comparing the KO mice and their wild type littermates. Taken together, these results suggest that: (1) MnSOD deficiency enhanced TPA-induced oxidative stress and AP-1 and p53 levels, consistent with the increase in both proliferation and apoptosis events in the MnSOD KO mice, and (2) increased apoptosis may negate increased proliferation in the MnSOD deficient mice during an early stage of tumor development.

AP-1 as a regulator of cell life and death

The transcription factor AP-1 (activator protein-1) is involved in cellular proliferation, transformation and death. Using mice and cells lacking AP-1 components, the target-genes and molecular mechanisms mediating these processes were recently identified. Interestingly, the growth-promoting activity of c-Jun is mediated by repression of tumour suppressors, as well as upregulation of positive cell cycle regulators. Mostly, c-Jun is a positive regulator of cell proliferation, whereas JunB has the converse effect. The intricate relationships between the different Jun proteins, their activities and the mechanisms that mediate them will be discussed.

The role of AP-1, NF-kappaB and ROS/NOS in skin carcinogenesis: the JB6 model is predictive

Generation of reactive oxygen species (ROS) stimulates transcription by activating transcription factors activator protein 1 (AP-1) and nuclear factor kappaB (NF-KB). The mouse epidermal JB6 cells constitute a model system that has significantly contributed to the understanding of these events. Clonal variants of JB6 cells are differentially responsive to transformation induced by tumor promoters such as phorbol esters (TPA), epidermal growth factor (EGF) and tumor necrosis factor alpha (TNF-alpha), as well as oxidative stress. TPA and EGF, acting through the MAP kinase pathway, activate AP-1 and subsequently NF-kappaB proteins and downstream transcription processes that are involved in the transformation response in transformation-sensitive (P+) JB6 cells. The effect of TNF-alpha is primarily on the NF-kappaB pathway. ROS and other free radicals can activate AP-1 and NF-KB transcription coordinately. In JB6 cells, both ERK/Fra-1 and NF-kappaB activity is essential for the transformation response. Inhibition of NF-kappaB and AP-1 activity abrogates transformation in JB6 cells as well as in transgenic mice and human keratinocytes. A similar effect is seen with antioxidants, which inhibit NF-kappaB and AP-1 activity as well as transformation in JB6 cells. The JB6 model is therefore valuable for monitoring early events in oxidative stress related signaling leading to carcinogenesis, and for identifying molecular targets for cancer chemoprevention.

Acetaldehyde activates Jun/AP-1 expression and DNA binding activity in human oral keratinocytes

Oral cancer is a significant health problem, particularly among individuals that ingest alcohol in combination with the use of tobacco products. The enhanced development of tobacco-initiated oral cancers by ethanol suggests that ethanol or one of its metabolites may act as a type of tumor promoter. Nevertheless, the mechanisms underlying the ability of ethanol to enhance oral carcinogenesis remain unclear. We hypothesize that acetaldehyde, the first metabolite of ethanol, may activate the expression and/or activity of Jun/AP-1 in oral keratinocytes analogous to the phorbol ester TPA and other tumor promoters in epidermal keratinocytes. To test this hypothesis, we treated HPV immortalized, non-tumorigenic human oral keratinocytes with acetaldehyde at various concentrations and for various times and measured several parameters of Jun/AP-1expression and function. Our results indicated that c-Jun mRNA and protein levels increased in the acetaldehyde treated cells compared to untreated control cells. Moreover, Jun/AP-1 DNA binding activity was rapidly activated by acetaldehyde in a dose-dependent fashion. The increases in Jun protein and AP-1 DNA binding activity were accompanied by increased transactivation of an AP-1 responsive reporter construct as well as increased transcript levels of a candidate AP-1 responsive gene, stromelysin 3. The levels of acetaldehyde employed were minimally toxic to the cells as determined by MTT assays. Thus, acetaldehyde was found to activate the expression and activity of an oncogenic transcription factor in HPV-initiated cells. Taken together, these results suggest that acetaldehyde may participate, at least in part, in the promotion stage of oral carcinogenesis.

AP-1 transrepressing retinoic acid does not deplete coactivators or AP-1 monomers but may target specific Jun or Fos containing dimers

Retinoic acid (RA) inhibits tumor promotion in many models in vivo and in vitro, among them mouse epidermal JB6 cells. RA treatment suppresses 12-O-tetradecanoylphorbol-13-acetate (TPA) induced AP-1 activity, an activity that is required for transformation of JB6 P+ cells. The molecular mechanism of AP-1 transrepression by retinoids is unclear, especially as related to inhibition of transformation. Overexpression of AP-1 components did not rescue TPA induced AP-1 activation nor did a GST pull down experiment implicate direct binding, thus rendering unlikely both a Jun/Fos-RA-RAR direct interaction and a Jun/Fos sequestration mechanism. Overexpression of p300, SRC-1 or pCAF did not abrogate AP-1 suppression by RA, thus arguing against coactivator competition. Overexpression of the corepressor silencing mediator for retinoic acid and thyroid hormone receptors (SMRT) suppressed AP-1 activity. However, SMRT but not RA inhibited cJun transactivation, suggesting SMRT does not mediate RA transrepression. RA treatment also did not block TPA induced ERK phosphorylation, Jun/Fos family protein expression except for cFos, or DNA binding of the AP-1 complex. The transcriptional activities of full-length JunB and full-length Fra-1, but not the transactivation domain fusions, were increased by TPA treatment and suppressed by RA. Since these full-length fusions have bzip domains, the results suggest that JunB and/or Fra-1-containing dimers may constitute one target of RA for transrepression of AP-1.

Repression of activator protein-1-mediated transcriptional activation by the Notch-1 intracellular domain

Developmental decisions that control cell fate are commonly regulated by the Notch signaling pathway. Activation of transmembrane Notch receptors results in proteolytic liberation of the intracellular domain of Notch, which translocates into the nucleus, binds a repressor (C promoter binding factor 1/RBP-Jkappa, Su(H), and Lag-1 (CSL)), and induces target genes. We found that the intracellular domain of human Notch-1 (NIC-1) represses activator protein-1 (AP-1)-mediated transactivation. Because numerous genes that control immune and inflammatory responses are AP-1-dependent and Notch regulates immune cell function, we investigated the underlying molecular mechanisms. Repression of AP-1 by NIC-1 did not represent a general inhibitory effect on transcription because nuclear factor kappaB-dependent transcription and transcription driven by a constitutive promoter and enhancer were not affected by NIC-1. The physiological relevance of the repression was supported by the facts that repression was apparent in multiple cell lines, endogenous AP-1 target genes were repressed, and similar concentrations of NIC-1 were required for CSL-dependent activation and AP-1 repression. The RBP-Jkappa-associated molecule domain of NIC-1 that mediates CSL binding and distinct sequences necessary for transactivation were required for repression. However, there was not a strict correlation between the sequence requirements for CSL-dependent activation and AP-1 repression. Repression correlated with predominant nuclear localization of NIC-1 and was not accompanied by disruption of c-Jun amino-terminal kinase-dependent signaling events required for AP-1 activation or by defective AP-1 DNA binding activity. These results provide evidence for negative cross-talk between Notch and AP-1, which may have important consequences for controlling diverse biological processes.

Chemoprevention of human skin cancer

The incidence of skin cancer has been rising in recent years with significant effects on public health. Primary prevention has proven inadequate in impacting the incidence of skin cancer, thus stimulating the development of chemopreventive strategies. The majority of skin cancer chemoprevention studies focus on occurrence of new nonmelanoma skin cancers (NMSC) in individuals with a previous NMSC, or on reduction in the number of premalignant skin lesions such as actinic keratoses (AK). Dysplastic nevi, a likely precursor of melanoma, are also potential targets for chemoprevention strategies. Premalignant lesions are especially attractive as endpoints since they are more common than frank cancer, resulting in reduced sample size, length, and cost of clinical trials. Development of new agents that affect the pathogenesis of skin cancer will be discussed, from elucidation of molecular targets to implementation of trials designed to determine the effects of chemopreventive interventions on human skin cancer.

Retinoic acid inhibits hepatic Jun N-terminal kinase-dependent signaling pathway in ethanol-fed rats

Retinoic acid (RA) supplementation suppresses ethanol-enhanced hepatocyte hyperproliferation in rats; however, little is known about the mechanism(s). Here, we investigated whether RA affects the protein kinase signaling pathways in the liver tissues of rats fed with a high dose of ethanol for a prolonged period of time (6 months). Results show that there were greater levels of phosphorylated Jun N-terminal kinase (JNK) and phosphorylated c-Jun protein, but not total JNK protein, in livers of ethanol-fed rats vs those of controls. Moreover, ethanol feeding to rats increased the levels of phosphorylated mitogen-activated protein kinase kinase-4 (MKK-4) and decreased the levels of mitogen-activated kinase phosphatase-1 (MKP-1) in liver tissue. However, hepatic levels of phosphorylated-p38 protein and total-p38 protein were not altered by the ethanol treatment. In contrast, all-trans-RA supplementation at two doses in ethanol-fed rats greatly attenuated the ethanol-induced hepatic phosphorylation of MKK-4, phosphorylated-JNK and c-Jun proteins. The level of MKP-1 was increased in ethanol-fed rats supplemented with all-trans-RA. Further, ethanol-induced hepatocyte hyperproliferation, measured by immunostaining for proliferating cell nuclear antigen, were markedly decreased by all-trans-RA supplementation. Interestingly, hepatic apoptosis in the liver of ethanol-fed rats after 6 months of treatment decreased significantly. This decrease of hepatic apoptosis in ethanol-fed rats was prevented by all-trans-RA supplementation in a dose-dependent manner. The results from these studies indicate that restoration of RA homeostasis is critical for the regulation of JNK-dependent signaling pathway and apoptosis in the liver of ethanol-fed rats.

Differential regulation of activator protein 1 activity by glycyrrhizin

Glycyrrhizin, a major component of Glycyrrhiza uralensis (licorice) root, is a saponin and exhibits a number of pharmacological effects, including anti-inflammation, anti-ulcer, anti-allergy, and anticarcinogenesis. Activator protein I (AP-1), a nuclear transcription factor, consists of Jun/Fos heterodimers or Jun/Jun homodimers, and blocking of tumor promoter-induced AP-1 activity could inhibit induced cellular transformation. In order to elucidate the molecular mechanism of glycyrrhizin-induced anticarcinogenesis, effect of glycyrrhizin on the AP-1 activity in untreated and tumor promoter-12-O-tetradecanoylphorbol-13-acetate (TPA)-treated conditions was analyzed in this study. Glycyrrhizin induced the AP-1/TATA reporter activity in a dose-dependent fashion, which was judged by chloramphenicol acetyltransferase assay and electrophoretic mobility-shift assay. Similar results were observed in HepG2 and Vero cells, suggested that glycyrrhizin effect was cell type-independent. In addition, the cis element responsible for glycyrrhizin activity was AP-1 responsive element. Further analysis indicated that glycyrrhizin exhibited a different regulation on the AP-1 activity in untreated and TPA-treated cells. Glycyrrhizin induced the AP-1 activity in untreated cells, while it inhibited the TPA-induced AP-1 activation in TPA-treated cells. These results provide insight into the biological actions of glycyrrhizin and the molecular basis for the development of new chemoprotective agents for cancer.

Transactivation of Fra-1 and consequent activation of AP-1 occur extracellular signal-regulated kinase dependently

Mitogen-activated protein (MAP) kinase, extracellular-signal-regulated kinases (ERKs) play an important role in activating AP-1-dependent transcription. Studies using the JB6 mouse epidermal model and a transgenic mouse model have established a requirement for AP-1-dependent transcription in tumor promotion. Tumor promoters such as 12-O-tetradecanoylphorbol-13-acetate (TPA) and epidermal growth factor induce activator protein 1 (AP-1) activity and neoplastic transformation in JB6 transformation-sensitive (P(+)) cells, but not in transformation-resistant (P(-)) variants. The resistance in one of the P(-) variants can be attributed to the low levels of the MAP kinases, ERKs 1 and 2, and consequent nonresponsiveness to AP-1 activation. The resistant variant is not deficient in c-fos transcription. The purpose of these studies was to define the targets of activated ERK that lead to AP-1 transactivation. The results establish that the transactivation domain of Fra-1 can be activated, that activation of Fra-1 is ERK dependent, and that a putative ERK phosphorylation site must be intact for activation to occur. Fra-1 was activated by TPA in ERK-sufficient P(+) cells but not in ERK-deficient P(-) cells. A similar activation pattern was seen for c-Fos but not for Fra-2. Gel shift analysis identified Fra-1 as distinguishing mitogen-activated (P(+)) from nonactivated (P(-)) AP-1 complexes. A second AP-1-nonresponsive P(-) variant that underexpresses Fra-1 gained AP-1 response upon introduction of a Fra-1 expression construct. These observations suggest that ERK-dependent activation of Fra-1 is required for AP-1 transactivation in JB6 cells.

Transactivation of the epidermal growth factor receptor is involved in 12-O-tetradecanoylphorbol-13-acetate-induced signal transduction

The mechanism of 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced tumor promotion is still not well understood even though it is thought to be related to the protein kinase C/mitogen-activated protein kinase/AP-1 pathway. Recently, TPA was also found to induce epidermal growth factor receptor (EGFR) activity. Here, we investigated whether the EGFR is a necessary component for TPA-induced signal transduction associated with tumor promotion. We demonstrated that potent inhibitors of the EGFR, PD153035 and AG1478, blocked TPA-induced phosphorylation of extracellular signal-regulated kinases (ERKs), AP-1 activity, and cell transformation. Egfr gene deficiency blocked TPA-induced ERK activity and AP-1 binding activity. The blocking of the ectodomain of the EGFR by a monoclonal antibody depressed TPA-induced ERK activity and AP-1 DNA binding activity. The use of a neutralizing antibody for heparin-binding EGF, one of the ligands of EGFR, blocked TPA-induced phosphorylation of ERKs. BB-94, a potent inhibitor of matrix metalloproteinases, which are activators of ectodomain shedding of EGFR ligands, also blocked TPA-induced ERK activity, AP-1 DNA binding, and cell transformation but had no effect on EGF-induced signal transduction. Anti-EGFR, anti-heparin-binding EGF, and BB-94 each blocked TPA-induced EGFR phosphorylation, but only anti-EGFR could block EGF-induced EGFR phosphorylation. Based on these results, we conclude that the EGFR is required for mediating TPA-induced signal transduction. EGFR transactivation induced by TPA is a mechanism by which the EGFR mediates TPA-induced tumor promotion-related signal transduction.

Overexpression of bone morphogenetic protein-6 (BMP-6) in murine epidermis suppresses skin tumor formation by induction of apoptosis and downregulation of fos/jun family members

Bone morphogenetic protein-6 (BMP-6) is a member of the transforming growth factor-beta superfamily. In murine skin, BMP-6 is highly expressed in postmitotic keratinocytes from day 15.5 p.c. till day 6 p.p. Expression in adult skin remains at very low levels, but pathological conditions such as wounding induce the expression of BMP-6. We demonstrate that tumor promotion by TPA (12-O-tetradecanoylphorbol-13-acetate) also induces expression of BMP-6 in suprabasal keratinocytes. This induction is due to post-transcriptional regulation since the level of BMP-6 mRNA remained unchanged. We performed two-stage skin carcinogenesis experiments with transgenic mice epidermally overexpressing BMP-6. These mice display augmented mitotic indices in normal and TPA-treated epidermis when compared to controls. Despite this hyperproliferation, BMP-6 transgenics showed a delayed development and strong suppression of benign and malignant skin tumor formation. In order to resolve this paradox we determined apoptotic frequencies as well as the expression of constituents of AP-1 (activator protein-1) which is essential for tumor promotion. A higher rate of apoptotic keratinocytes was detectable in transgenic mice versus controls and a downregulation of mRNA for jun/fos family members in transgenic skin after TPA-treatment. Thus expression of BMP-6 augments apoptosis and downregulates the transcription of AP-1 family members thereby establishing tumor resistance.

JAC, a direct target of oncogenic transcription factor Jun, is involved in cell transformation and tumorigenesis

Using subtractive hybridization techniques, we have isolated a gene termed JAC that is strongly and specifically activated in avian fibroblasts transformed by the v-jun oncogene of avian sarcoma virus 17 (ASV17), but not in cells transformed by other oncogenic agents. Furthermore, JAC is highly expressed in cell lines derived from jun-induced avian fibrosarcomas. Kinetic analysis using a doxycycline-controlled conditional cell transformation system showed that expression of the 0.8-kb JAC mRNA is induced rapidly upon activation of the oncogenic v-jun allele. Nucleotide sequence analysis and transcriptional mapping revealed that the JAC gene contains two exons, with the longest ORF confined to exon 2. The deduced 68-amino acid chicken JAC protein is rich in cysteine residues and displays 37% sequence identity to mammalian high-sulfur keratin-associated proteins. The promoter region of JAC contains a consensus (5'-TGACTCA-3') and a nonconsensus (5'-TGAGTAA-3') AP-1 binding site in tandem, which are both specifically bound by the Gag-Jun hybrid protein encoded by ASV17. Mutational analysis revealed that the two AP-1 sites confer strong transcriptional activation by Gag-Jun in a synergistic manner. Ectopic expression of JAC in avian fibroblasts leads to anchorage-independent growth, strongly suggesting that deregulation of JAC is an essential event in jun-induced cell transformation and tumorigenesis.

TOJ3, a target of the v-Jun transcription factor, encodes a protein with transforming activity related to human microspherule protein 1 (MCRS1)

Using the established quail cell line Q/d3 conditionally transformed by the v-jun oncogene, cDNA clones (TOJ2, TOJ3, TOJ5, TOJ6) were isolated by representational difference analysis (RDA) that correspond to genes which were induced immediately upon conditional activation of v-jun. One of these genes, TOJ3, is immediately and specifically activated after doxycycline-mediated v-jun induction, with kinetics similar to the induction of well characterized direct AP-1 target genes. TOJ3 is neither activated upon conditional activation of v-myc, nor in cells or cell lines non-conditionally transformed by oncogenes other than v-jun. Sequence analysis revealed that the TOJ3-specific cDNA encodes a 530-amino acid protein with significant sequence similarities to the murine or human microspherule protein 1 (MCRS1, MSP58), a nucleolar protein that directly interacts with the ICP22 regulatory protein from herpes simplex virus 1 or with p120, a proliferation-related protein expressed at high levels in most human malignant tumor cells. Similar to its mammalian counterparts, the TOJ3 protein contains a bipartite nuclear localization motif and a forkhead associated domain (FHA). Using polyclonal antibodies directed against a recombinant amino-terminal TOJ3 protein segment, the activation of TOJ3 in jun-transformed fibroblasts was also demonstrated at the protein level by specific detection of a polypeptide with an apparent M(r) of 65 000. Retroviral expression of the TOJ3 gene in quail or chicken embryo fibroblasts induces anchorage-independent growth, indicating that the immediate activation of TOJ3 in fibroblasts transformed by the v-jun oncogene contributes to cell transformation.

Sustained activation of mitogen-activated protein kinases and activator protein 1 by the hepatitis B virus X protein in mouse hepatocytes in vivo

Transcriptional activation of diverse cellular genes by the X protein (HBx) of hepatitis B virus (HBV) has been suggested as one of the mechanisms for HBV-associated hepatocellular carcinoma. However, such functions of HBx have been studied using transformed cells in culture and have not been examined in the normal adult hepatocytes, a natural host of HBV. Using an efficient hepatocyte-specific virus-based gene delivery system developed in our laboratory earlier, we studied the HBx action in vivo. We demonstrate that following virosome-mediated delivery of HBx DNA, a large population (>50%) of hepatocytes express the HBx protein in a dose-dependent manner, which induces a significant increase in the activity of extracellular signal-regulated kinases (ERKs) in the livers of HBx-transfected mice. Inhibition of HBx-induced ERK activation following intravenous administration of PD98059, a mitogen-activated protein kinase kinase kinase (MEK) inhibitor, confirmed the requirement for MEK in the activation of ERKs by HBx. Induction of ERK activity by HBx was sustained for up to 30 days. Interestingly, sustained activation of c-Jun N-terminal kinases (JNKs) for up to 30 days was also noted. Such constitutive ERK and JNK activation as a consequence of continued HBx expression also led to sustained stimulation of further downstream events, such as increased levels of c-Jun and c-Fos proteins along with the persistent induction of activator protein 1 binding activity. Taken together, our data suggest a critical role of these molecules in HBx-mediated cell transformation.

The growth of malignant keratinocytes depends on signaling through the PGE(2) receptor EP1

Recent discoveries shed light on the importance of prostaglandin (PG) production in the development of skin cancer. Work by Fischer et al. demonstrates that skin tumor promotion caused by ultraviolet B radiation can be decreased by up to 89% by blocking cyclooxygenase-2 (COX-2) with the drug Celecoxib. A similar study showed that Celecoxib can decrease new tumor formation by 44% in mice that already have tumors. These studies demonstrate the importance of COX-2 and PGs in the development of squamous cell carcinoma. We have explored growth signaling in a model of skin tumor progression. Because changes in PG production have been implicated in skin carcinogenesis, we examined this pathway. We found that malignant cell lines secrete more prostaglandin E(2) (PGE(2)) than the parental cells. We observed increased expression of COX-1 and -2. We also found that these cells express the PGE(2) receptors EP1 and EP4. When the cells are grown in the presence of indomethacin, the growth rate of the malignant cells is decreased. This effect can be reversed by addition of PGE(2) or an EP1 agonist to the medium. Thus, we have shown that skin tumor cells depend in part on PGE(2) signaling through the EP1 prostanoid receptor for their in vitro growth.

Keratinocyte-specific onset of serine protease BSSP expression in experimental carcinogenesis

Malignant transformation of mouse skin by chemical carcinogens and tumor promoters, such as the phorbol ester 12-O-tetradecanoylphorbol-13-acetate, is a multistage process leading to the formation of squamous cell carcinomas. In an effort to identify target genes whose expression is associated with skin tumorigenesis we combined elements of suppression subtractive hybridization with differential screening to isolate genes that are differentially upregulated in mouse skin after short-term treatment with 12-O-tetradecanoylphorbol-13-acetate and that exhibit a high constitutive expression in squamous cell carcinomas. Here, we report the detailed analysis of one of these cDNAs encoding the serine protease BSSP in mouse skin. Phorbol ester application increases BSSP expression in keratinocytes of the epidermis and the hair follicle several-fold starting 4 h post- treatment. Transcriptional activation of BSSP by 12-O-tetradecanoylphorbol-13-acetate was found to be independent of c-Fos expression and resistant to downregulation by glucocorticoids. By monitoring BSSP expression throughout experimental skin carcinogenesis we found strong constitutive expression in hyperplastic epidermis as well as in proliferatively active keratinocytes of benign and malignant skin tumors. These results establish a novel link between expression of an as yet ill-defined serine protease and skin carcinogenesis.

Ten ERK-related proteins in three distinct classes associate with AP-1 proteins and/or AP-1 DNA

We have identified seven ERK-related proteins ("ERPs"), including ERK2, that are stably associated in vivo with AP-1 dimers composed of diverse Jun and Fos family proteins. These complexes have kinase activity. We designate them as "class I ERPs." We originally hypothesized that these ERPs associate with DNA along with AP-1 proteins. We devised a DNA affinity chromatography-based analytical assay for DNA binding, the "nucleotide affinity preincubation specificity test recognition" (NAPSTER) assay. In this assay, class I ERPs do not associate with AP-1 DNA. However, several new "class II" ERPs do associate with DNA. p41 and p44 are ERK1/2-related ERPs that lack kinase activity and associate along with AP-1 proteins with AP-1 DNA. Class I ERPs and their associated kinase activity thus appear to bind AP-1 dimers when they are not bound to DNA and then disengage and are replaced by class II ERPs to form higher order complexes when AP-1 dimers bind DNA. p97 is a class III ERP, related to ERK3, that associates with AP-1 DNA without AP-1 proteins. With the exception of ERK2, none of the 10 ERPs appear to be known mitogen-activated protein kinase superfamily members.

Loss of JunB activity enhances stromelysin 1 expression in a model of the epithelial-to-mesenchymal transition of mouse skin tumors

Chemical carcinogenesis in mouse skin has been useful in delineating the molecular events that underlie squamous cell carcinoma progression. A late event in this progression, the epithelial-to-mesenchymal transition (EMT), is characterized by the loss of epithelial markers and the presence of mesenchymal markers. One mesenchymal marker associated with this transition is the matrix metalloproteinase stromelysin 1 (Str-1). To examine the molecular mechanisms regulating the expression of Str-1 during the EMT, genetically related mouse skin tumor cell lines representing the epithelial (B9(SQ)) and mesenchymal (A5(SP)) phenotypes were studied. As expected, B9(SQ) cells did not make Str-1, while A5(SP) cells did. B9(SQ)-A5(SP) somatic hybrids did not make Str-1, suggesting that a critical regulatory factor was a B9(SQ)-specific repressor. Str-1 promoter analysis revealed that a canonical AP-1 site was sufficient to maintain differential reporter gene activity. This result correlated with the observed loss of binding of the transcriptionally inactive JunB-Fra-2 AP-1 complex from B9(SQ) cells, being replaced primarily by the more active JunD-Fra-2 complex in A5(SP) cells. The higher level of JunB binding to both DNA and Fra-2 correlated with its hyperphosphorylation by Jun N-terminal kinase, an activity that was significantly higher in B9(SQ) cells. In the somatic hybrids, JunB gene expression was highly upregulated, a condition that also was sufficient to repress the expression of the endogenous Str-1 gene in A5(SP) cells. These data suggested that alterations in JunB activity, by changes in either phosphorylation or gene expression, contributed to the phenotypic differences that occur in this model of the EMT.

Activation of the activator protein-1 by the peroxisome proliferator clofibric acid in rat H4IIEC3 hepatoma cells

Clofibric acid (CA), a potent peroxisome proliferator (PP), has been shown to cause tumor formation in rat liver. The precise mechanism of action of PPs remains largely unknown. However, it has been proposed that they act by increasing reactive oxygen species (ROS), leading to a cellular oxidative stress. In the present study, we have investigated the effect of CA on the activator protein-1 (AP-1) expression in PP-responsive H4IIEC3 rat hepatoma cells. Electrophoretic mobility shift assays demonstrated that AP-1 activation was induced in cells treated with CA for 24 h at all concentrations of the fibrate. This activation was prolonged up to 48 h. Using transfection experiments with H4IIEC3 cells, we found that CA induced the expression of a reporter gene driven by AP-1 and that of the glutathione S-transferase P target gene. By supershift experiments, jun and fos proteins were identified as components of the CA-activated AP-1 complexes. Western blot analyses revealed that the induction of the AP-1 activity was not dependent to an increase in the levels of jun and fos proteins. Cotreatment of H4IIEC3 cells with CA and the antioxidant N-acetylcysteine or calphostin C, a specific inhibitor of protein kinase C (PKC), blocked the AP-1 activation and the expression of the AP-1-driven luciferase reporter gene. These results demonstrate that CA activates AP-1 in H4IIEC3 cells and that this induction is mediated via ROS and PKC.

Inhibitory effect of glycolic acid on ultraviolet-induced skin tumorigenesis in SKH-1 hairless mice and its mechanism of action

Glycolic acid, an alpha-hydroxy acid derived from fruit and milk sugars, has been used commonly as a cosmetic ingredient since it was discovered to have photoprotective and anti-inflammatory effects and antioxidant effects on ultraviolet (UV)B-irradiated skin. Little is known, however, about the functional role of glycolic acid on UV-induced skin tumorigenesis. In the present study, we examined the effect of glycolic acid on UV (UVA + UVB)-induced skin tumorigenesis and assessed several significant contributing factors in SKH-1 hairless mice. Inbred hairless female mice (15 animals/group) were irradiated for 5 d/wk at a total dose of 74.85 J/cm(2) UVA and 2.44 J/cm(2) UVB for 22 wk. Glycolic acid was applied topically twice a week at a dose of 8 mg/cm(2) immediately after UV irradiation. Glycolic acid reduced UV-induced skin tumor development. The protective effect of glycolic acid was a 20% reduction of skin tumor incidence, a 55% reduction of tumor multiplicity (average number of tumors/mouse), and a 47% decrease in the number of large tumors (larger than 2 mm). Glycolic acid also delayed the first appearance of tumor formation by about 3 wk. The inhibitory effect of glycolic acid on UV-induced tumor development was accompanied by decreased expression of the following UV-induced cell-cycle regulatory proteins: proliferating cell nuclear antigen (PCNA), cyclin D1, cyclin E, and the associated subunits cyclin-dependent kinase 2 (cdk2) and cdk4. In addition, the expression of p38 kinase, jun N-terminal kinase (JNK), and mitogen-activated protein kinase kinase (MEK) also was lower in UV + glycolic acid-treated skin compared with expression in UV-irradiated skin. Moreover, transcription factors activator protein 1 (AP-1) and nuclear factor kappaB (NF-kappaB) activation was significantly lower in UV + glycolic acid-treated skin compared with activation in UV-irradiated skin. These results show that glycolic acid reduced UV-induced skin tumor development. The decreased expression of the cell-cycle regulatory proteins PCNA, cyclin D1, cyclin E, cdk2, and cdk4 and the signal mediators JNK, p38 kinase, and MEK may play a significant role in the inhibitory effect of glycolic acid on UV-induced skin tumor development. In addition, the inhibition of activation of transcription factors AP-1 and NF-kappaB could contribute significantly to the inhibitory effect of glycolic acid.

UV-A-induced AP-1 activation requires the Raf/ERK pathway in human NCTC 2544 keratinocytes

UV-A irradiation causes a dose-dependent activation of ERK in human NCTC 2544 keratinocytes. The specific inhibition of either ERK activity or Raf kinase activity impedes the activation of AP-1 DNA binding induced by UV-A. In addition, UV-A raises AP-1 promoter transcriptional activity, which is downregulated in NCTC 2544 cells expressing an inactive mutant of Raf-1. We found that singlet oxygen might be one of the mediators in both UV-A-induced AP-1 DNA binding and transcriptional activity. These results strongly suggest that UV-A-induced AP-1 activity requires the Raf-ERK pathway and imply a singlet oxygen effector.

Jun, the oncoprotein

Cellular Jun (c-Jun) and viral Jun (v-Jun) can induce oncogenic transformation. For this activity, c-Jun requires an upstream signal, delivered by the Jun N-terminal kinase (JNK). v-Jun does not interact with JNK; it is autonomous and constitutively active. v-Jun and c-Jun address overlapping but not identical sets of genes. Whether all genes essential for transformation reside within the overlap of the v-Jun and c-Jun target spectra remains to be determined. The search for transformation-relevant targets of Jun is moving into a new stage with the application of DNA microarrays technology. Genetic screens and functional tests remain a necessity for the identification of genes that control the oncogenic phenotype.

AP-1 in cell proliferation and survival

A plethora of physiological and pathological stimuli induce and activate a group of DNA binding proteins that form AP-1 dimers. These proteins include the Jun, Fos and ATF subgroups of transcription factors. Recent studies using cells and mice deficient in individual AP-1 proteins have begun to shed light on their physiological functions in the control of cell proliferation, neoplastic transformation and apoptosis. Above all such studies have identified some of the target genes that mediate the effects of AP-1 proteins on cell proliferation and death. There is evidence that AP-1 proteins, mostly those that belong to the Jun group, control cell life and death through their ability to regulate the expression and function of cell cycle regulators such as Cyclin D1, p53, p21(cip1/waf1), p19(ARF) and p16. Amongst the Jun proteins, c-Jun is unique in its ability to positively regulate cell proliferation through the repression of tumor suppressor gene expression and function, and induction of cyclin D1 transcription. These actions are antagonized by JunB, which upregulates tumor suppressor genes and represses cyclin D1. An especially important target for AP-1 effects on cell life and death is the tumor suppressor p53, whose expression as well as transcriptional activity, are modulated by AP-1 proteins.

Function and regulation of AP-1 subunits in skin physiology and pathology

The mouse skin has become the model of choice to study the regulation and function of AP-1 subunits in many physiological and pathological processes in vivo and in vitro. Genetically modified mice, in vitro reconstituted skin equivalents and epidermal cell lines were established, in which AP-1-regulated genetic programs of cell proliferation, differentiation and tumorigenesis can be analysed. Since the epidermis, as our interface with the environment, is subjected to radiation and injury, signal transduction pathways and critical AP-1 members regulating the mammalian stress response could be identified. Regulated expression of important components of the cytokine network, cell surface receptors and proteases, which orchestrate the process of wound healing has been found to rely on AP-1 activity. Here we review our current knowledge on the function of AP-1 subunits and AP-1 target genes in these fascinating fields of skin physiology and pathology.

AP-1 in mouse development and tumorigenesis

Genetically modified mice have provided important insights into the biological functions of the dimeric transcription factor complex AP-1. Extensive analyses of mice and cells with genetically modified Fos or Jun proteins provide novel insights into the physiological functions of AP-1 proteins. Using knock-out strategies it was found that some components, such as c-Fos, FosB and JunD are dispensable, whereas others, like c-Jun, JunB and Fra-1 are essential in embryonic development and/or in the adult organism. Besides the specific roles of AP-1 proteins in developmental processes, we are beginning to obtain a better molecular understanding of the cell-context dependent function of AP-1 in cell proliferation and apoptosis, in bone biology as well as in multistep tumorigenesis.

Okadaic acid induces transcription of junB through a CCAAT box and NF-Y

The shellfish toxin, okadaic acid (OA), is a potent tumor promoter that induces expression of the proto-oncogene junB in mouse keratinocyte 308 cells. Here we show, through deletion analysis of the junB promoter, that sequences near the TATA box conferred transcriptional induction by OA. Transient transfections of luciferase constructs bearing the junB promoter with single mutations in various cis elements demonstrated that a promoter containing a mutated CCAAT box could not be induced by OA. When this CCAAT box was inserted into a heterologous promoter construct, OA induction was dependent on an intact CCAAT box. Flanking cis elements located near the CCAAT box, although not required for OA inducibility, did play a role in the basal level of transcription. NF-Y was shown by EMSA to bind to the CCAAT box. OA induction from the junB CCAAT box was blocked by dominant negative NF-YA as well as the CCAAT box-dependent anticancer drug, ET-473. Expression of a lexA/NF-YA chimeric protein demonstrated that OA induction was dependent on the binding of NF-Y family members. These studies demonstrate that OA can mediate transcriptional activation of junB through the classical CCAAT box and that transcription factor NF-Y plays a functional role in the induction.

Dietary energy restriction inhibits ERK but not JNK or p38 activity in the epidermis of SENCAR mice

Ongoing studies in our laboratory have demonstrated that dietary energy restriction (DER) inhibited 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced AP-1 transcription factor binding to DNA in the epidermis of SENCAR mice. To dissect the specific signal transduction pathways through which DER inhibits the AP-1:DNA binding, we analyzed the activities of three major MAP kinases that lead to the induction of AP-1. The changes in ERK1 and ERK2 protein expression and phosphorylation were further characterized by western blot analysis. Female SENCAR mice were pre-fed ad libitum (AL) or 40% DER diet for 8-10 weeks. The kinase activities in mouse epidermis were determined by immune complex kinase assays at 0.5, 1, 4, or 6 h following treatment with 3.2 nmol TPA to the shaved dorsal backs. ERK activity at 1 h post-TPA treatment was nearly 5-fold (P< 0.005) above basal levels in AL mice while the increase was abolished in DER mice. The TPA-induced ERK activity in AL mice was accompanied by increased phosphorylation of ERK1 and ERK2 (P< 0.05), which was abrogated in DER mice. In addition, DER mice exhibited reduced expression of total ERK1 and ERK2 and higher proportions of ERK1 and ERK2 phosphorylation in comparison with AL mice (P<0.05). JNK activity was decreased at 1 and 6 h but increased at 4 h (P<0.05) post-TPA treatment. TPA did not change p38 kinase activity at the time points tested. Neither JNK nor p38 activity was altered by DER. Taken together, our results indicated for the first time that DER blocked the TPA stimulation of ERK activity and suggested that the inhibition of TPA-induced AP-1 activity by DER is likely through inhibition of ERK but not JNK or p38 kinase pathway.

2,4,4'-trichlorobiphenyl increases STAT5 transcriptional activity

The promoting effects of polychlorinated biphenyls (PCBs) have been studied extensively in a variety of two-stage carcinogenesis models. However, the molecular mechanisms responsible for the promotion effects of PCBs have not been elucidated. We measured the effect of PCBs on DNA-binding proteins involved in cell proliferation and transformation. Male Sprague-Dawley rats were injected intraperitoneally with mono-, di-, tri-, tetra-, or hexachlorobiphenyls (300 micromol/kg/d) each day for 4 d and killed 4 h after the last injection. To detect alterations in nuclear proteins that could explain the tumor-promoter activity of PCBs, liver nuclear extracts were analyzed by electrophoretic mobility shift assays. Electrophoretic mobility shift assay analysis of signal transducers and activators of transcription (STAT)-binding activity to a consensus gamma-interferon-activated sequence (GAS) element was compared in liver nuclear extracts from treated rats. STAT-binding activity was eightfold to tenfold higher in nuclear extracts from animals treated with 2,4,4'-trichloro- (PCB 28) and 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153). Analysis of the protein complex binding to the GAS element, with antibodies specific for STAT3, STAT5, and STAT6, indicated that the protein complex was made up of STAT5 and STAT6 proteins. HepG2 cells transiently transfected with a luciferase reporter gene construct containing many STAT5 binding sites were treated with PCB 28 and PCB 153. PCB 28 stimulated a greater than 25-fold increase in luciferase activity at the highest concentration tested, 1.0 microg/mL. However, enhanced luciferase activity did not occur with PCB 153 treatment. 4-Chlorobiphenyl (PCB 3), PCB 28, and PCB 153 treatment of Sprague-Dawley rats resulted in a large increase in protein binding to a consensus activated protein-1 (AP-1) element. However, 3,4-dichlorobiphenyl (PCB 12) and 3,3',4,4'-tetrachlorobiphenyl (PCB 77) treatments did not increase AP-1 transcription activity. Further analysis of the proteins binding to the AP-1 consensus sequence with antibodies specific for c-fos, junD, and junB indicated that the protein composition consists of junD proteins. These data showed functional differences between noncoplanar and coplanar PCBs with respect to STAT activation and AP-1-DNA binding.

Induction of activator protein-1 through reactive oxygen species by crystalline silica in JB6 cells

We reported previously that freshly fractured silica (FFSi) induces activator protein-1 (AP-1) activation through extracellular signal-regulated protein kinases (ERKs) and p38 kinase pathways. In the present study, the biologic activities of FFSi and aged silica (ASi) were compared by measuring their effects on the AP-1 activation and phosphorylation of ERKs and p38 kinase. The roles of reactive oxygen species (ROS) in this silica-induced AP-1 activation were also investigated. We found that FFSi-induced AP-1 activation was four times higher than that of ASi in JB6 cells. FFSi also caused greater phosphorylation of ERKs and p38 kinase than ASi. FFSi generated more ROS than ASi when incubated with the cells as measured by electron spin resonance (ESR). Studies using ROS-sensitive dyes and oxygen consumption support the conclusion that ROS are generated by silica-treated cells. N-Acetylcysteine (an antioxidant) and polyvinyl pyridine-N-oxide (an agent that binds to Si-OH groups on silica surfaces) decreased AP-1 activation and phosphorylation of ERKs and p38 kinase. Catalase inhibited phosphorylation of ERKs and p38 kinase, as well as AP-1 activation induced by FFSi, suggesting the involvement of H(2)O(2) in the mechanism of silica-induced AP-1 activation. Sodium formate (an ( small middle dot)OH scavenger) had no influence on silica-induced MAPKs or AP-1 activation. Superoxide dismutase enhanced both AP-1 and MAPKs activation, indicating that H(2)O(2), but not O(2), may play a critical role in silica-induced AP-1 activation. These studies indicate that freshly ground silica is more biologically active than aged silica and that ROS, in particular H(2)O(2), play a significant role in silica-induced AP-1 activation.

A novel transformation suppressor, Pdcd4, inhibits AP-1 transactivation but not NF-kappaB or ODC transactivation

Pdcd4 is a novel transformation suppressor that is highly expressed in promotion-resistant (P-) mouse epidermal JB6 cells but not in susceptible (P+) cells. Overexpression of pdcd4 cDNA in stably transfected P+ cells rendered cells resistant to tumor promoter-induced transformation, indicating that elevated expression of Pdcd4 protein is sufficient to suppress neoplastic transformation. To determine whether Pdcd4 suppresses neoplastic transformation through inhibiting known transformation required events, we examined the possibility that pdcd4 inhibited the activation of AP-1 or NF-kappaB dependent transcription or of ornithine decarboxylase (ODC) activity. Activation of AP-1-dependent transcriptional activity was inhibited by pdcd4 expression in a concentration dependent manner. In contrast, Pdcd4 slightly increased NF-kappaB-dependent transcription and did not alter ODC enzymatic activity. Previous studies suggested that activation of AP-1 was required for P+ cell transformation as well as for tumor promotion in vivo. These results indicate that Pdcd4 functions as a transformation suppressor, possibly through inhibiting AP-1 activation in combination with other factors such as enhancing NF-kappaB activation. Pdcd4 may thus constitute a useful molecular target for cancer prevention.

Current prospects for controlling cancer growth with non-cytotoxic agents--nutrients, phytochemicals, herbal extracts, and available drugs

In animal or cell culture studies, the growth and spread of cancer can be slowed by many nutrients, food factors, herbal extracts, and well-tolerated, available drugs that are still rarely used in the clinical management of cancer, in part because they seem unlikely to constitute definitive therapies in themselves. However, it is reasonable to expect that mechanistically complementary combinations of these measures could have a worthwhile impact on survival times and, when used as adjuvants, could improve the cure rates achievable with standard therapies. The therapeutic options available in this regard include measures that: down-regulate serum free IGF-I; suppress the synthesis of mevalonic acid and/or certain derivatives thereof; modulate arachidonate metabolism by inhibiting 5-lipoxygenase, 12-lipoxygenase, or COX-2; antagonize the activation of AP-1 transcription factors; promote the activation of PPAR-gamma transcription factors; and that suppress angiogenesis by additional mechanisms. Many of these measures appear suitable for use in cancer prevention.

12-O-Tetradecanoylphorbol-13-acetate Induces Apoptosis in Renal Epithelial Cells through a Growth Signal Conflict Which Is Prevented by Activated ras1

12-O-Tetradecanoylphorbol-13-acetate (TPA) induced apoptosis in the pig renal epithelial cell line LLC-PK1 after 24 h of treatment as assessed by caspase 3 activation. Cotreatment of the cells with bryostatin markedly reduced the apoptotic effects of TPA. Okadaic acid, another tumor promoter, also induced apoptosis. Expression of an activated ras gene prevented TPA-induced apoptosis, while a dominant negative ras retarded the process. Taken together, these results suggest that TPA-induced apoptosis in LLC-PK1 may be analogous to TPA-induced tumor promotion in the two-stage model of skin carcinogenesis. Mechanistically, TPA-induced apoptosis seemed to be the result of a conflict of the growth-promoting affects of serum and the growth-retarding effects of TPA. This was manifested by a pronounced hypophosphorylation of the retinoblastoma gene product, pRb, which was prevented by activated ras. Apoptosis and pRb hypophosphorylation were associated with a reduction in cyclin D1 levels, suggesting that the growth-retarding effects of TPA were produced by modulation of this cell cycle protein. Interestingly, the mechanism of protection by activated ras did not seem to result from downstream activation of phosphatidylinositol-3-kinase (PI3K) as has been implicated in other systems. Additional analysis revealed that TPA-induced apoptosis was associated with the downregulation of the anti-apoptotic proteins Bcl-x and Mcl-1 and dependent on the activity of the transcription factor Jun.

Multiple regulatory elements in the murine stromelysin-3 promoter. Evidence for direct control by CCAAT/enhancer-binding protein beta and thyroid and retinoid receptors

Stromelysin-3 (ST3) belongs to the matrix metalloproteinase (MMPs) family, a protease family involved in tissue remodeling. Although this family of enzymes is regulated by nuclear receptors, few hormone-responsive elements have been demonstrated in MMP promoters. In order to identify regulatory elements and/or factors that control the expression of the mouse st3 gene, we have analyzed genomic sequences encompassing 5 kilobase pairs of the ST3 promoter. Analysis of these sequences revealed several CCAAT/enhancer-binding proteins (C/EBP) and retinoic acid-responsive elements (RAREs), as well as one thyroid-responsive element. However, in contrast to most MMP promoters, no AP-1-binding sites were identified. Specific binding activities were demonstrated for all elements. Consistent with previous reports, retinoid X receptor is required for maximal binding to the ST3 RAREs and the TRE. The ST3-C/EBP element was shown to mediate dose-dependent promoter activation by C/EBPbeta. Among the RAREs, the proximal DR1-RARE was shown to be sufficient for ST3 promoter activation by ligand-bound retinoid receptors, whereas the two distal DR2-RAREs appear to be involved more in the control of base-line promoter activity. Accordingly, ST3 expression was induced by retinoic acid and was reduced in cells where specific retinoic acid receptors had been inactivated. The involvement of these conserved regulatory elements is discussed in the context of physiological or pathological situations associated with st3 expression. Our findings therefore assign to C/EBP, retinoids, and thyroid hormone important roles in the regulation of ST3 gene expression.

Matrix metalloproteinase 9 promoter activity is induced coincident with invasion during tumor progression

Matrix metalloproteinase 9 (MMP-9, also known as gelatinase B or 92-kd Type IV collagenase) is overexpressed in many human and murine cancers. We induced carcinomas in mice carrying a transgene that links the MMP-9 promoter to the reporter ss-galactosidase so that activation of the MMP-9 promoter would be indicated by ss-galactosidase. Mammary carcinomas were induced by mating the MMP-9 promoter reporter transgenic mice with mice carrying a transgene for murine mammary tumor virus promoter linked to polyoma middle T antigen, a transgene that leads to rapid development of mammary tumors in female mice. None of the hyperplastic mammary glands and none of the carcinomas in situ expressed ss-galactosidase. However, all invasive tumors had evidence of ss-galactosidase expression. In addition to the breast carcinomas, a malignant teratoma in a female and a papillary adenocarcinoma in the pelvic region of a male arose and were also ss-galactosidase positive. We also induced skin tumors in the mice with the MMP-9 reporter transgene with 7, 12-dimethylbenz[a]anthracene (DMBA) treatment followed by phorbol 12 myristate 13-acetate (TPA). None of the papillomas or in situ carcinomas showed any ss-galactosidase expression, but expression was seen in invasive carcinoma. Although normal skin epithelial cells did not express ss-galactosidase, we did find staining in a few cells at the duct of the sebaceous gland at the base of the hair follicles. The MMP-9 reporter transgene did not lead to expression in the alveolar macrophages, confirming that additional upstream sequences are required for expression in macrophages. These experiments have revealed that MMP-9 promoter activity is induced coincident with invasion during tumor progression. Furthermore, this indicates that the more proximal upstream elements of the promoter are sufficient for MMP-9 transcription during tumor progression.

c-Jun and JunB antagonistically control cytokine-regulated mesenchymal-epidermal interaction in skin

Interactions between mesenchymal and epithelial cells are responsible for organogenesis and tissue homeostasis. This mutual cross-talk involves cell surface proteins and soluble factors, which are mostly the result of regulated transcription. To elucidate dimer-specific functions of the AP-1 family of transcription factors, we reconstituted skin by combining primary human keratinocytes and mouse wild-type, c-jun(-/-), and junB(-/-) fibroblasts. We have discovered an antagonistic function of these AP-1 subunits in the fibroblast-mediated paracrine control of keratinocyte proliferation and differentiation, and traced this effect to the IL-1-dependent regulation of KGF and GM-CSF. These data suggest that the relative activation state of these AP-1 subunits in a non-cell-autonomous, transregulatory fashion directs regeneration of the epidermis and maintenance of tissue homeostasis in skin.

c-Jun activation-dependent tumorigenic transformation induced paradoxically by overexpression or block of S-adenosylmethionine decarboxylase

All mammalian cells absolutely require polyamines (putrescine, spermidine, and spermine) for growth. Here we show that the overexpression of cDNA for S-adenosylmethionine decarboxylase (AdoMetDC), the main regulatory enzyme in the biosynthesis of higher polyamines, induces transformation of rodent fibroblasts when expressed in the sense or the antisense orientation. Both transformants were able to induce invasive tumors in nude mice. Neither transformation was associated with activation of the mitogen-activated protein kinases Erk1 and Erk2. Instead, the AdoMet DC sense, but not antisense, transformants displayed constitutive activation of the c-Jun NH(2)-terminal kinase (JNK) pathway. However, both transformations converged on persistent phosphorylation of endogenous c-Jun at Ser73. The phenotype of the AdoMetDC sense transformants was reversed by expression of dominant-negative mutants of SEK1 (MKK4), JNK1, and c-Jun (TAM-67), which were also found to impair cytokinesis. Similarly, TAM-67 reverted the morphology of the AdoMetDC-antisense expressors. This report is the first demonstration of a protein whose overexpression or block of synthesis can induce cell transformation. In addition, we show that the polyamine biosynthetic enzymes require c-Jun activation for eliciting their biological effects.

Regulation of a multigenic invasion programme by the transcription factor, AP-1: re-expression of a down-regulated gene, TSC-36, inhibits invasion

The transcription factor AP-1 (activator protein-1) is required for transformation by many oncogenes, which function upstream of it in the growth factor-ras signal transduction pathway. Previously, we proposed that one role of AP-1 in transformation is to regulate the expression of a multigenic invasion programme. As a test of this proposal we sought to identify AP-1 regulated genes based upon their differential expression in 208F rat fibroblasts transformed by FBR-v-fos (FBR), and to determine if they functioned in the invasion programme. Subtracted cDNA libraries specific for up- or down-regulated genes in FBRs compared to 208Fs were constructed and analysed. Northern analysis revealed that the cDNAs in both libraries represented differentially expressed genes. Nucleic acid sequence analysis of randomly selected cDNA clones from each library coupled with searches of nucleic acid and amino acid sequence databases determined that many of the cDNAs represented proteins that function in various aspects of the invasion process. Functional analysis of one the down-regulated genes, TSC-36/follistatin-related protein (TSC-36/Frp), which has not previously been associated with invasion, demonstrated that its expression in FBRs inhibited in vitro invasion. These results support the proposal that AP-1 in transformed cells regulates a multigenic invasion programme.

Induced expression of dominant-negative c-jun downregulates NFkappaB and AP-1 target genes and suppresses tumor phenotype in human keratinocytes

Neoplastically transformed mouse and human keratinocytes elevate transactivation of both activator protein 1 (AP-1) and nuclear factor kappaB (NFkappaB) transcription factors. The present study addresses the question of whether elevated NFkappaB in addition to elevated AP-1-dependent gene expression is necessary for maintaining the tumor cell phenotype. When a tetracycline-regulatable dominant-negative c-jun (TAM67, having a truncated transactivation domain) was expressed in tumorigenic human keratinocytes, AP-1- and NFkappaB- but not p53-dependent reporter activity was inhibited by 40-60%. Tumor phenotype, as measured by anchorage-independent growth, was inhibited by 90%. Neither AP-1/NFkappaB activation nor expression of tumor phenotype was inhibited in TAM67-harboring keratinocytes under noninducing conditions. Electrophoretic mobility shift analysis showed that induction of TAM67 expression slightly increased AP-1- but reduced NFkappaB DNA-binding activity. Immunoprecipitation showed that TAM67 interacted in keratinocyte nuclei with NFkappaB p65, suggesting that inhibition of NFkappaB by TAM67 is mediated by direct protein-protein interactions, possibly producing decreased binding to DNA or inactivating p65. To analyze the putative effector genes that may be targeted by TAM67, expression of genes responsive to AP-1 or NFkappaB was measured by reverse transcriptase-polymerase chain reaction in TAM67 transfectants with or without TAM67 induction. Induction of TAM67 inhibited or reduced the expression of collagenase I, stromelysin I (AP-1 responsive), and interleukins 1 and 6 (NFkappaB responsive). These results indicate that genes controlled by NFkappaB and by AP-1 may be transformation-relevant targets of TAM67 and that TAM67 may inhibit NFkappaB activation through direct interaction with NFkappaB p65. Moreover, the findings provide proof for the principle of using inducible TAM67 as a gene therapy to suppress tumor phenotype in human carcinoma cells.

Analysis of intestinal adaptation gene expression by cDNA expression arrays

BACKGROUND

As a tool for determining gene expression on a genomic scale, cDNA microarrays are a promising new technology that can be applied to the study of complex physiologic processes. The objective of this study was to characterize the expression of individual genes and patterns of gene expression that might provide insight into the mechanism of intestinal adaptation after massive small bowel resection.

METHODS

Male ICR mice underwent a 50% proximal small bowel resection (SBR) or sham operation. After 3 days, the remnant ileum was harvested, weighed, and RNA extracted. Changes in gene expression were detected utilizing Clontech Atlas mouse cDNA expression arrays. Some of these changes were confirmed by reverse transcriptase-polymerase chain reactions (RT-PCR) and Northern blots.

RESULTS

Analysis of these cDNA arrays revealed changes in the expression of multiple genes, including those involved in cell cycle regulation, apoptosis, DNA synthesis, and transcriptional regulation. The patterns of expression were consistent with the increased cell proliferation and apoptosis observed during intestinal adaptation. A large number of genes not previously associated with intestinal adaptation were identified.

CONCLUSIONS

This technology may facilitate the elucidation of the intricate cellular mechanisms underlying intestinal adaptation.