Blocking activator protein-1 activity, but not activating retinoic acid response element, is required for the antitumor promotion effect of retinoic acid

Tamoxifen increases nuclear respiratory factor 1 transcription by activating estrogen receptor beta and AP-1 recruitment to adjacent promoter binding sites

Little is known about endogenous estrogen receptor β (ERβ) gene targets in human breast cancer. We reported that estradiol (E(2)) induces nuclear respiratory factor-1 (NRF-1) transcription through ERα in MCF-7 breast cancer cells. Here we report that 4-hydroxytamoxifen (4-OHT), with an EC(50) of ~1.7 nM, increases NRF-1 expression by recruiting ERβ, cJun, cFos, CBP, and RNA polymerase II to and dismissing NCoR from the NRF1 promoter. Promoter deletion and transient transfection studies showed that the estrogen response element (ERE) is essential and that an adjacent AP-1 site contributes to maximal 4-OHT-induced NRF-1 transcription. siRNA knockdown of ERβ revealed that ERβ inhibits basal NRF-1 expression and is required for 4-OHT-induced NRF-1 transcription. An AP-1 inhibitor blocked 4-OHT-induced NRF-1 expression. The 4-OHT-induced increase in NRF-1 resulted in increased transcription of NRF-1 target CAPNS1 but not CYC1, CYC2, or TFAM despite increased NRF-1 coactivator PGC-1α protein. The absence of TFAM induction corresponds to a lack of Akt-dependent phosphorylation of NRF-1 with 4-OHT treatment. Overexpression of NRF-1 inhibited 4-OHT-induced apoptosis and siRNA knockdown of NRF-1 increased apoptosis, indicating an antiapoptotic role for NRF-1. Overall, NRF-1 expression and activity is regulated by 4-OHT via endogenous ERβ in MCF-7 cells.

Retinoid pathway and cancer therapeutics

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

Transcriptional regulation of CXC-ELR chemokines KC and MIP-2 in mouse pancreatic acini

Neutrophils and their chemoattractants, the CXC-ELR chemokines keratinocyte cytokine (KC) and macrophage inflammatory protein-2 (MIP-2), play a critical role in pancreatitis. While acute pancreatitis is initiated in acinar cells, it is unclear if these are a source of CXC-ELR chemokines. KC and MIP-2 have NF-κB, activator protein-1 (AP-1) sites in their promoter regions. However, previous studies have shown increased basal and reduced caerulein-induced AP-1 activation in harvested pancreatic tissue in vitro, which limits interpreting the caerulein-induced response. Moreover, recent studies suggest that NF-κB silencing in acinar cells alone may not be sufficient to reduce inflammation in acute pancreatitis. Thus the aim of this study was to determine whether acinar cells are a source of KC and MIP-2 and to understand their transcriptional regulation. Primary overnight-cultured murine pancreatic acini were used after confirming their ability to replicate physiological and pathological acinar cell responses. Upstream signaling resulting in KC, MIP-2 upregulation was studied along with activation of the transcription factors NF-κB and AP-1. Cultured acini replicated critical responses to physiological and pathological caerulein concentrations. KC and MIP-2 mRNA levels increased in response to supramaximal but not to physiological caerulein doses. This upregulation was calcium and protein kinase C (PKC), but not cAMP, dependent. NF-κB inhibition completely prevented upregulation of KC but not MIP-2. Complete suppression of MIP-2 upregulation required dual inhibition of NF-κB and AP-1. Acinar cells are a likely source of KC and MIP-2 upregulation during pancreatitis. This upregulation is dependent on calcium and PKC. MIP-2 upregulation requires both NF-κB and AP-1 in these cells. Thus dual inhibition of NF-κB and AP-1 may be a more successful strategy to reduce inflammation in pancreatitis than targeting NF-κB alone.

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

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

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

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

Molecular imaging of in vivo gene expression

Molecular imaging provides spatial and temporal information on cellular changes that occur during development and in disease. MRI and optical imaging of reporter genes allows for the visualization of promoter activity, protein-protein interactions, protein stability and the tracking of individual proteins and cells. Reporter genes can be genetically encoded in transgenic animals or detected through the administration of an exogenous contrast agent. Advances in molecular imaging of reporter genes have led to the development of imaging probes that detect changes in endogenous cellular changes. The ability to use contrast agents coupled with functional information on cellular events will allow for sensitive assessment of individual patient therapies, leading to an accurately tailored treatment regimen.

The promise of natural products for blocking early events in skin carcinogenesis

This perspective on Stratton et al. (beginning on p. 160), Kowalczyk et al. (beginning on p. 170), and Katiyar et al. (beginning on p. 179) highlights the common theme of translational investigation of natural substances and their molecular effects and mechanisms in preventing skin squamous cell carcinoma, which has potentially severe clinical consequences. These studies comprise results of naturally occurring phytochemicals and green tea polyphenols in mouse models of UV-induced and chemically induced skin carcinogenesis and results of perillyl alcohol in a phase IIa clinical trial-all pointing to the great promise of this exciting approach for better understanding of and preventing skin cancer.

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

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

The role of retinoid deficiency and estrogens as cofactors in cervical cancer

Persistent infection with high-risk human papillomaviruses (HR-HPVs) is involved in cervical cancer (CC), a major cause of cancer mortality worldwide. Infection occurs primarily at the transformation zone (TZ), the most estrogen- and retinoid-sensitive region of the cervix. Development of CC affects a small percentage of HR-HPV-infected women and often takes decades after infection, suggesting that HR-HPV is a necessary but not sufficient cause of CC. Thus, other cofactors are necessary for progression from cervical HR-HPV infection to cancer such as long-term use of hormonal contraceptives, multiparity, smoking, as well as micronutrient depletion and in particular retinoid deficiency, which alters epithelial differentiation, cellular growth and apoptosis of malignant cells. Therefore, early detection of HR-HPV and management of precancerous lesions together with a profound understanding of additional risk factors could be a strategy to avoid this disease. In this review we focus on the synergic effect of estrogens, retinoid deficiency and HR-HPVs in the development of CC. These risk factors may act in concert to induce neoplastic transformation in squamous epithelium of the cervix, setting the stage for secondary genetic or epigenetic events leading to cervical cancer.

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

Background

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

Results

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

Conclusion

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

NDRG2 suppresses cell proliferation through down-regulation of AP-1 activity in human colon carcinoma cells

Recently, the anti-tumor activity of N-myc downstream-regulated gene 2 (NDRG2) was elucidated, but the molecular mechanism of how NDRG2 works as a tumor suppressor is not well known. To determine the function of NDRG2 as a tumor suppressor, we established stable cell lines expressing NDRG2 protein or its mutant forms, and studied their effects on tumor cell growth. Interestingly, constitutive expression of wild-type NDRG2 induced the growth retardation of SW620 colon carcinoma cells. Introduction of NDRG2 into SW620 cells induced the decrease of c-Jun phosphorylation at Ser63, followed by the attenuation of activator protein-1 (AP-1) function as a transcriptional activator. Subsequently, the down-regulation of cyclin D1, which is known as a major target for AP-1 transcription activator, resulted in cell cycle arrest at G1/S phase. Additionally, treatment of NDRG2-siRNA on NDRG2-expressing cells has induced the recovery of c-Jun phosphorylation and cyclin D1 expression. Cell proliferation of those cells was also increased compared with untreated cells. NDRG2 mutants of which the phosphorylation sites at C-terminal region were removed by deletion or site-directed mutagenesis have shown no effect on cyclin D1 expression and could not induce cell growth retardation. In conclusion, NDRG2 modulates intracellular signals to control cell cycle through the regulation of cyclin D1 expression via phosphorylation pathway.

Assessing siRNA pharmacodynamics in a luciferase-expressing mouse

A significant barrier to the successful general development of small-interfering RNA (siRNA) therapeutics is the ability to deliver them systemically to target organs and cell types. In this study, we have developed a mouse strain that will facilitate the evaluation of the efficacy of siRNA delivery strategies. This strain contains robust ubiquitous expression of firefly luciferase from germ line Cre-mediated recombination of the ROSA26-LSL-Luc allele. We show that luciferase is highly and uniformly expressed in all tissues examined. Using this mouse model, we describe a facile assay that enables the assessment of the pharmacodynamics of a systemically delivered siRNA formulation. These mice can also be used as universal donors, enabling the efficient and sensitive monitoring of cell trafficking or tissue transplantation. The primary advantage of this approach is that siRNA efficacy against a nonessential target can be easily evaluated in any tissue. This strain should generally enhance the ability to rapidly screen, compare and optimize various siRNA formulations for tissue-targeted or -enhanced systemic delivery in a preclinical development setting.

The resveratrol analogue 3,5,3',4',5'-pentahydroxy-trans-stilbene inhibits cell transformation via MEK

Resveratrol, present in grapes and red wine, is reported to be a natural chemopreventive agent against cancer. However, the concentrations required to exert these effects may be difficult to achieve by drinking only 1 or 2 glasses of red wine a day. Therefore, developing more potent, nontoxic analogues of resveratrol may provide a feasible means of achieving an effective physiologic concentration. Here we report that the resveratrol analogue, 3,5,3',4',5'-pentahydroxy-trans-stilbene (RSVL2), inhibits 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced neoplastic transformation in JB6 P+ mouse epidermal cells. Further, we identified MEK/ERK signaling as the direct molecular target for the anticancer effects of RSVL2 and demonstrated that RSVL2 inhibited MEK1, but not Raf1 or ERK2 kinase activity. RSVL2 also dose-dependently suppressed MEK1 kinase activity induced by TPA and the inhibition of H-Ras-induced cell transformation was much stronger for RSVL2 than for PD098059 or resveratrol. Both in vitro and ex vivo pull-down assays indicated that RSVL2, but not resveratrol, directly bound with GST-MEK1, but did not compete with ATP for binding. Docking data indicated that the low inhibitory activity of resveratrol might be due to the lack of the hydroxyl group at the meta position of the B ring, thereby preventing resveratrol from forming a hydrogen bond with the backbone amide group of Ser212, which is the key interaction for stabilizing the inactive conformation of the activation loop.

Partially saturated canthaxanthin purified from Aspergillus carbonarius induces apoptosis in prostrate cancer cell line

A mutant Aspergillus carbonarius selected for temperature tolerance after UV treatment, when grown in shake flasks, produced mycelia bearing yellow pigment. Since the mutant was affected in sterol biosynthetic pathway, the pigment was apparently produced to maintain membrane fluidity and rigidity for growth sustenance in low-pH culture broth. Nuclear magnetic resonance analyses characterizing the pigment as a partially saturated canthaxanthin, containing beta-ionone end rings, suggested its application as a retinoid. When tested for this property in retinoic acid receptor expressing prostate cancer cell line, LNCaP, the fungal partially saturated canthaxanthin induced apoptosis. Low apoptosis percentage in DU145 prostrate cancer cells that does not express functional retinoic acid receptor-beta (RAR-beta) suggested binding specificity of the partially saturated canthaxanthin for RAR-beta.

Cocoa procyanidins suppress transformation by inhibiting mitogen-activated protein kinase kinase

Cocoa was shown to inhibit chemically induced carcinogenesis in animals and exert antioxidant activity in humans. However, the molecular mechanisms of the chemopreventive potential of cocoa and its active ingredient(s) remain unknown. Here we report that cocoa procyanidins inhibit neoplastic cell transformation by suppressing the kinase activity of mitogen-activated protein kinase kinase (MEK). A cocoa procyanidin fraction (CPF) and procyanidin B2 at 5 mug/ml and 40 mum, respectively, inhibited 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced neoplastic transformation of JB6 P+ mouse epidermal (JB6 P+) cells by 47 and 93%, respectively. The TPA-induced promoter activity and expression of cyclooxygenase-2, which is involved in tumor promotion and inflammation, were dose-dependently inhibited by CPF or procyanidin B2. The activation of activator protein-1 and nuclear factor-kappaB induced by TPA was also attenuated by CPF or procyanidin B2. The TPA-induced phosphorylation of MEK, extracellular signal-regulated kinase, and p90 ribosomal s6 kinase was suppressed by CPF or procyanidin B2. In vitro and ex vivo kinase assay data demonstrated that CPF or procyanidin B2 inhibited the kinase activity of MEK1 and directly bound with MEK1. CPF or procyanidin B2 suppressed JB6 P+ cell transformation induced by epidermal growth factor or H-Ras, both of which are known to be involved in MEK/ERK signal activation. In contrast, theobromine (up to 80 mum) had no effect on TPA-induced transformation, cyclooxygenase-2 expression, the transactivation of activator protein-1 or nuclear factor-kappaB, or MEK. Notably, procyanidin B2 exerted stronger inhibitory effects compared with PD098059 (a well known pharmacological inhibitor of MEK) on MEK1 activity and neoplastic cell transformation.

Inhibitory effects of caffeine analogues on neoplastic transformation: structure-activity relationship

Some xanthine analogues, including 1,3,7-trimethylxanthine (caffeine) and 1,3-dimethylxanthine (theophylline), have been shown to exert anticancer activities in both cell culture and animal models. The present study focused on the relationship of structure and activity of 50 different caffeine analogues in preventing epidermal growth factor (EGF)-induced malignant transformation of mouse epidermal JB6 promotion-sensitive (P+) Cl41 (JB6 P+) cells. Results indicated that the inhibition of cell transformation by the 1,3,7-trialkylxanthines depends on the number of carbons at the alkyl groups R1 and R3, but not R7. Notably, 1-ethyl-3-hexylxanthine (xanthine 70) was the most effective compound for inhibiting EGF-induced neoplastic transformation among the 50 xanthine analogues tested. The 50% inhibition of cell transformation (ICT(50)) value for xanthine 70 was 48- or 75-fold less than the ICT(50) value of caffeine or theophylline, respectively. Further study revealed that xanthine 70 (5-40 muM) dose dependently inhibited EGF-induced transactivation of activator protein 1 (AP-1), whereas theophylline or caffeine (up to 500 muM) had no effect on AP-1 activity. In addition, xanthine 70 (10 muM) inhibited 12-O-tetradecanoylphorbol-13-acetate- or H-Ras-induced neoplastic transformation in JB6 P+ cells by 78.2 or 62.0%, respectively. Collectively, these results indicated that the number of carbons at R1 and R3 is important for the antitumor-promoting activity of the trialkylxanthines and xanthine 70 might be a promising anticancer agent.

Protective actions of Rubus coreanus ethanol extract on collagenous extracellular matrix in ultraviolet-B irradiation-induced human dermal fibroblasts

Solar ultraviolet (UV) irradiation leads to distinct changes in the skin connective tissues by degradation of collagen, which is a major structural component in the extracellular matrix. UV irradiation induces the production of matrix metalloproteinases (MMP) capable of attacking native fibrillar collagen and responsible for inhibiting the construction of collagenous extracellular matrix. In this study, we attempted to investigate the protective actions of Rubus coreanus ethanol extract (RCE) on the MMP production and the consequent procollagen/collagen degradation in UV-B-irradiated human dermal fibroblasts. The analytical data showed that Rubus coreanus ethanol extract was mostly comprised of cyanidin 3-rutinoside. Pre-treatment of fibroblasts with this extract inhibited UV-B-induced production of MMP-1, MMP-8 and MMP-13 in dose-dependent manners. In addition, Western blot analysis and immunocytochemical staining assay revealed that RCE markedly augmented the cellular levels of procollagen/collagen declined in UV-B-exposed dermal fibroblasts. These results demonstrate that RCE blocks UV-B-induced increase of the collagen degradation by inhibiting MMP production. Thus, RCE may act as an agent inhibiting excessive dermal collagen degradation leading to the skin photoaging.

Anti-oxidative and photo-protective effects of coumarins isolated from Fraxinus chinensis

Free radicals and reactive oxygen species (ROS), which are generated by UV irradiation, may cause serious injury to skin cell membranes, DNA and functional proteins. In addition, these agents stimulate the expressions of matrix metalloproteinases (MMPs), which can degrade most components of the extracellular matrix (ECM), including collagen. In order to develop new anti-photoaging agents, five major components from the extract of Fraxinus chinensis extract (FCE) were identified. Two of the major components of FCE were found to be esculin (11.2%) and esculetin (1.9%). FCE (IC50: 50.0 microg/mL 1, 1-diphenyl-2-picrylhydrazyl (DPPH); 19.8 microg/mL, superoxide anion radical) and esculetin (IC50: 2.1 microg/mL DPPH; 0.6 microg/mL, superoxide anion radical) showed strong antioxidative activities. Of the compounds tested, esculetin showed the strongest scavenging activity against DPPH radicals, followed by superoxide anions from the xanthine/xanthine oxidase system. The intracellular ROS scavenging activity showed that oxidation of 5-(6-)-chloromethyl-2', 7'-dichlorodihydrofluorescein diacetate (CM-H2DCFDA) was effectively inhibited by esculetin, with potent free radical scavenging activity was also shown in UVB-irradiated human dermal fibroblasts (HDFs). Moreover, treatment of UVA-irradiated HDFs with esculetin resulted in dose-dependent decreases in the expression levels of MMP-1 mRNA and protein. From these results, FCE and one of its components, esculetin, were predicted to be potentially useful as ingredients in cosmetics for protecting against photoaging.

(-)Epigallocatechin gallate hampers collagen destruction and collagenase activation in ultraviolet-B-irradiated human dermal fibroblasts: involvement of mitogen-activated protein kinase

Ultraviolet (UV) irradiation leads to distinct changes in skin connective tissues by degradation of collagen, which is a major structural component in the extracellular matrix most likely mediated by matrix metalloproteinases (MMP), collagenases. These changes in collagenous skin tissues have been suggested to be causes of the skin wrinkling observed in premature aging of the skin. This study mimicked the action of environmental ultraviolet on skin and investigated whether (-)epigallocatechin gallate (EGCG), a bioactive catechin component of green tea, mechanistically inhibited activation of MMP-1, MMP-8, and MMP-13 and destruction of collagen in UV-B irradiated human dermal fibroblasts by modulating cellular signaling pathways. Cell viability was moderately decreased by > or = 30% in human dermal fibroblasts treated with 100 mJ/cm2 UV-B, accompanying a substantial generation of reactive oxygen species evidenced by DCF staining. Western blot analysis and immunocytochemical staining revealed that EGCG markedly suppressed collagen degradation enhanced in UV-B-exposed human dermal fibroblast. Pre-treatment of fibroblasts with EGCG also inhibited UV-B-induced production of collagenases, MMP-1, MMP-8 and MMP-13, in a dose-dependent manner. In addition, EGCG rapidly and substantially hampered UV-B irradiation-induced activation of ASK-1 and phosphorylation of MAPK, JNK, p38 MAPK, and ERK1/2, in dermal fibroblasts. These results demonstrate that EGCG has abilities to hamper UV-B-induced collagenolytic MMP production via interfering with the MAPK-responsive pathways. Therefore, EGCG may be a potential agent for the prevention and treatment of skin photoaging.

Equol, a metabolite of the soybean isoflavone daidzein, inhibits neoplastic cell transformation by targeting the MEK/ERK/p90RSK/activator protein-1 pathway

Daidzein and genistein are isoflavones found in soybean. Genistein is known to exhibit anticarcinogenic activities and inhibit tyrosine kinase activity. However, the underlying molecular mechanisms of the chemopreventive activities of daidzein and its metabolite, equol, are not understood. Here we report that equol inhibits 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced neoplastic transformation of JB6 P+ mouse epidermal cells by targeting the MEK/ERK/p90RSK/activator protein-1 signaling pathway. TPA-induced neoplastic cell transformation was inhibited by equol, but not daidzein, at noncytotoxic concentrations in a dose-dependent manner. Equol dose-dependently attenuated TPA-induced activation of activator protein-1 and c-fos, whereas daidzein did not exert any effect when tested at the same concentrations. The TPA-induced phosphorylation of ERK1/2, p90RSK, and Elk, but not MEK or c-Jun N-terminal kinase, was inhibited by equol but not by daidzein. In vitro kinase assays revealed that equol greatly inhibited MEK1, but not Raf1, kinase activity, and an ex vivo kinase assay also demonstrated that equol suppressed TPA-induced MEK1 kinase activity in JB6 P+ cell lysates. Equol dose-dependently inhibited neoplastic transformation of JB6 P+ cells induced by epidermal growth factor or H-Ras. Both in vitro and ex vivo pull-down assays revealed that equol directly bound with glutathione S-transferase-MEK1 to inhibit MEK1 activity without competing with ATP. These results suggested that the antitumor-promoting effect of equol is due to the inhibition of cell transformation mainly by targeting a MEK signaling pathway. These findings are the first to reveal a molecular basis for the anticancer action of equol and may partially account for the reported chemopreventive effects of soybean.

RAR and RXR modulation in cancer and metabolic disease

Retinoic acid receptors (RARs) are ligand-controlled transcription factors that function as heterodimers with retinoid X receptors (RXRs) to regulate cell growth and survival. The success of RAR modulation in the treatment of acute promyelocytic leukaemia (APL) has stimulated considerable interest in the development of RAR and RXR modulators. This has been aided by recent advances in the understanding of the biological role of RARs and RXRs and in the design of selective receptor modulators that might overcome the limitations of current drugs. Here, we discuss the challenges and opportunities for therapeutic strategies based on RXR and RAR modulators, with a focus on cancer and metabolic diseases such as diabetes and obesity.

Retinoids and skin: microarrays shed new light on chemopreventive action of all-trans retinoic acid

Despite the use of retinoids in the clinic for many years, their mode of action in the prevention of skin cancer is still unclear. Recent microarray analyses of the chemopreventive effect of all-trans retinoic acid (ATRA), one of the primary naturally occurring biologically active retinoids, in the two-stage mouse skin chemical carcinogenesis model have provided novel insight into their action. Comparison of the gene expression profiles of control skin to skin subjected to the two-stage protocol for 3 wk, with or without ATRA, has shown that approximately half of the genes regulated by 12-o-tetradecanoylphorbol-13-acetate (TPA) are oppositely regulated when ATRA is coadministered with TPA. It was further shown the Raf/Mek/Erk branch of mitogen-activated protein (MAP) kinase pathway contains a disproportionate number of oppositely regulated genes, thereby implicating it as one of the key pathways involved in tumor promotion by TPA, that is blocked by ATRA. This result has pointed the way toward the detailed study of Raf/Mek/Erk pathway signaling in skin cancer development and its potential as a target pathway for chemoprevention by ATRA and other chemopreventive drugs.

Hint1 inhibits growth and activator protein-1 activity in human colon cancer cells

There is accumulating evidence that histidine triad (HIT) nucleotide-binding protein 1 (HINT1), a member of the evolutionary highly conserved HIT protein super family, is a novel tumor suppressor. However, the mechanism of action of HINT1 with respect to tumor suppression is not known. In the present study, we found that a series of human colon cancer cell lines displayed various levels of expression of HINT1, with a very low level in SW480 cells. This cell line also displayed partial methylation of the promoter region of the Hint1 gene, and treatment of these cells with 5-azadeoxycitidine increased expression of Hint1 mRNA and protein. Therefore, the decreased expression of HINT1 in SW480 cells seems to be due to epigenetic silencing. Increased expression of HINT1 in these cells, using a retrovirus vector (pLNCX2) that encodes either wild-type (WT) Hint1 or a point mutant (His(112)/Asn(112)) of Hint1, inhibited the proliferation of SW480 cells. Because of the important role of the activator protein-1 (AP-1) transcription factor in cancer cells, we examined possible effects of HINT1 on AP-1 transcription factor activity in SW480 cells transfected with an AP-1-luciferase reporter. We found that cotransfection with a pHA-Hint1 plasmid DNA significantly inhibited this activity. Studies with inhibitors indicated that AP-1 activity in SW480 cells requires the activity of c-Jun NH(2)-terminal kinase (JNK) 2 and not JNK1. Cotransfection with the Hint1 plasmid DNA also inhibited AP-1-luciferase reporter activity in WT mouse embryo fibroblast (MEF) studies, and studies with JNK1 deleted or JNK2 deleted MEFs confirmed the essential role for JNK2, but not JNK1, in mediating AP-1 activity. Recent studies indicate that the protein plenty of SH3 (POSH) provides a scaffold that enhances JNK activity. We found that cotransfection of a plasmid DNA encoding POSH stimulated the phosphorylation of c-Jun and also AP-1 reporter activity, and cotransfection with Hint1 inhibited both of these activities. Furthermore, coimmunoprecipitation studies provided evidence that HINT1 forms an in vivo complex with POSH and JNK. These results suggest that HINT1 inhibits AP-1 activity by binding to a POSH-JNK2 complex, thus inhibiting the phosphorylation of c-Jun. This effect could contribute to the tumor suppressor activity of HINT1.

Cancer modeling: modern imaging applications in the generation of novel animal model systems to study cancer progression and therapy

Cancer is the result of a series of genetic and epigenetic mutations that evolve over years even decades and lead to the transformed phenotype. Paradoxically, most methods developed to study these changes are static and do not provide insights on the dynamics of the sequela of steps involved in tumorigenesis. This major shortcoming now can be overcome with the application of reporter genes and imaging technologies, which are providing tools to examine specific molecular events and their role in the carcinogenic process in single cells. In the last decade reporter-based biosensors were created to study gene transcription, protein/protein interactions, sub-cellular trafficking and protease activities; this wealth of systems enable to monitor intracellular signaling pathways at several key check points specifically involved in cancer cell development. The challenge is now to extend cell-based models to the generation of reporter mice, where non-invasive in vivo imaging technologies allow to follow single molecular events. When combined with murine models of cancer, these technologies will give an unprecedented opportunity to spatio-temporally investigate the molecular events resulting in neoplasia. The aim of the present review is to highlight the major changes occurring in this rapidly evolving field and their potential for increasing our knowledge in cancer biology and for the research of novel and more efficacious therapies.

Malignant transformation of DMBA/TPA-induced papillomas and nevi in the skin of mice selectively lacking retinoid-X-receptor alpha in epidermal keratinocytes

Retinoid-X-receptor alpha (RXRalpha), a member of the nuclear receptor (NR) superfamily, is a ligand-dependent transcriptional regulatory factor. It plays a crucial role in NR signalling through heterodimerization with some 15 NRs. We investigated the role of RXRalpha and its partners on mouse skin tumor formation and malignant progression upon topical DMBA/TPA treatment. In mutants selectively ablated for RXRalpha in keratinocytes, epidermal tumors increased in size and number, and frequently progressed to carcinomas. As keratinocyte-selective peroxisome proliferator-activated receptor gamma (PPARgamma) ablation had similar effects, RXRalpha/PPARgamma heterodimers most probably mediate epidermal tumor suppression. Keratinocyte-selective RXRalpha-null and vitamin-D-receptor null mice also exhibited more numerous dermal melanocytic growths (nevi) than control mice, but only nevi from RXRalpha mutant mice progressed to invasive human-melanoma-like tumors. Distinct RXRalpha-mediated molecular events appear therefore to be involved, in keratinocytes, in cell-autonomous suppression of epidermal tumorigenesis and malignant progression, and in non-cell-autonomous suppression of nevi formation and progression. Our study emphasizes the crucial role of keratinocytes in chemically induced epidermal and melanocytic tumorigenesis, and raises the possibility that they could play a similar role in UV-induced tumorigenesis, notably in nevi formation and progression to melanoma.

Molecular and cellular targets

Carcinogenesis is a multistage process consisting of initiation, promotion, and progression stages and each stage may be a possible target for chemopreventive agents. A significant outcome of these investigations on the elucidation of molecular and cellular mechanisms is the explication of signal transduction pathways induced by tumor promoters in cancer development. The current belief today is that cancer may be prevented or treated by targeting specific cancer genes, signaling proteins, and transcription factors. The molecular mechanisms explaining how normal cells undergo neoplastic transformation induced by tumor promoters are rapidly being clarified. Accumulating research evidence suggests that many of dietary factors, including tea compounds, may be used alone or in combination with traditional chemotherapeutic agents to prevent or treat cancer. The potential advantage of many natural or dietary compounds seems to focus on their potent anticancer activity combined with low toxicity and very few adverse side effects. This review summarizes some of our recent work regarding the effects of the various tea components on signal transduction pathways involved in neoplastic cell transformation and carcinogenesis.

Chemoprevention of Skin Carcinogenesis by Phenylretinamides: Retinoid Receptor–Independent Tumor Suppression

Fenretinide [N-(4-hydroxyphenyl)retinamide or 4-HPR] is a synthetic retinoid analogue with antitumor and chemopreventive activities. N-(4-Methoxyphenyl)retinamide (4-MPR) is the most abundant metabolite of 4-HPR detected in human serum following 4-HPR therapy. We have shown in in vitro studies that 4-HPR and 4-MPR can act independent of the classic nuclear retinoid receptor pathway and that 4-HPR, but not 4-MPR, can also activate nuclear retinoid receptors. In this study, we have compared the chemopreventive effects of topically applied 4-HPR and 4-MPR with the primary biologically active retinoid, all-trans retinoic acid (ATRA), in vivo in the mouse skin two-stage chemical carcinogenesis model. All three retinoids suppressed tumor formation but the effect of 4-HPR and 4-MPR, and not of ATRA, was sustained after their discontinuation. The tumor-suppressive effects of 4-HPR and 4-MPR were quantitatively and qualitatively similar, suggesting that the two may be acting through the same retinoid receptor-independent mechanism(s). We further explored this effect in vitro by analyzing primary cultures of mouse keratinocytes treated with the same retinoids. All three could induce apoptosis with a 48-hour treatment and only ATRA and 4-HPR induced an accumulation of cells in the G1 phase of the cell cycle. This finding is consistent with our previous results showing that the effects of phenylretinamides on the cell cycle are retinoid receptor dependent whereas apoptosis induction is not. A microarray-based comparison of gene expression profiles for mouse skin treated with 12-O-tetradecanoylphorbol-13-acetate (TPA) alone and TPA + 4-HPR or TPA + 4-MPR reveals a high degree of coincidence between the genes regulated by the two phenylretinamides. We propose that 4-HPR may exert therapeutic and chemopreventive effects by acting primarily through a retinoid receptor-independent mechanism(s) and that 4-MPR may contribute to the therapeutic effect of 4-HPR by acting through the same retinoid receptor-independent mechanism(s).

Effects of polycyclic aromatic hydrocarbons (PAHs) on vascular endothelial growth factor induction through phosphatidylinositol 3-kinase/AP-1-dependent, HIF-1alpha-independent pathway

Previous studies have demonstrated that exposure to polycyclic aromatic hydrocarbons (PAHs) and its derivatives is associated with an increased risk of skin cancers, and the carcinogenic effect of PAHs is thought to involve both tumor initiation and promotion. Whereas PAH tumor initiation is well characterized, the mechanisms involved in the tumor promotion of PAHs remain elusive. In the present study, we investigated the effects of PAHs on vascular endothelial growth factor (VEGF) expression by comparison of its induction between the active metabolite and its parent compound (B[a]PDE versus B[a]P) or between active compound and its relatively inactive analog (5-MCDE versus CDE). We found that exposure of cells to (+/-)-anti-benzo-[a]pyrene-7,8-diol-9,10-epoxide (B[a]PDE) or (+/-)-anti-5-methylchrysene-1,2-diol-3,4-epoxide (5-MCDE) led to marked induction of VEGF in Cl41 cells, whereas benzo[a]pyrene (B[a]P) or chrysene-1,2-diol-3,4-epoxide (CDE) did not exhibit significant inductive effects. Exposure of cells to B[a]PDE and 5-MCDE did not induce HIF-1alpha activation, whereas AP-1 was significantly activated. Moreover, overexpression of TAM67 (a dominant-negative mutant c-Jun) dramatically blocked that VEGF induction. Electrophoretic mobility shift assay showed that AP-1 was only able to specifically recognize and bind to its AP-1 potential binding site within -1136 and -1115 of the VEGF promoter region. Site-directed mutation of this AP-1 binding site eliminated the VEGF transcriptional activity induced by B[a]PDE, suggesting that the AP-1 binding site between -1136 and -1115 in the VEGF promoter region is critical for VEGF induction by B[a]PDE. In addition, overexpression of Deltap85 (a dominant-negative mutant PI-3K) impaired B[a]PDE- and 5-MCDE-induced VEGF induction. Considering our previous findings that PI-3K is an upstream mediator for c-Jun/AP-1 activation, we conclude that the VEGF induction by B[a]PDE and 5-MCDE is through PI-3K/AP-1-dependent and HIF-1alpha-independent pathways. These findings may help us to understand the mechanisms involved in PAH carcinogenic effects.

Retinoic acid inhibits airway smooth muscle cell migration

Airway remodeling in chronic asthma is characterized by increased smooth muscle mass that is associated with the reduction of the bronchial lumen as well as airway hyperresponsiveness. The development of agents that inhibit smooth muscle growth is therefore of interest for therapy to prevent asthma-associated airway remodeling. All-trans retinoic acid (ATRA) suppresses growth of vascular smooth muscle cells (SMCs) from the systemic and pulmonary circulation. The present study investigated the effects of ATRA on human bronchial (airway) SMCs. Human bronchial SMCs were found to express mRNAs for retinoic acid receptor (RAR)-alpha, -beta, -gamma, and retinoid X receptor (RXR)-alpha, -beta, but not RXR-gamma. Although ATRA was not effective in inhibiting proliferation or in inducing apoptosis in airway SMCs, we found that ATRA (0.2-2 microM) inhibited the SMC migration in response to platelet-derived growth factor (PDGF), as determined in a modified Boyden chamber assay. Both RAR and RXR agonists also blocked PDGF-induced airway SMC migration. ATRA also inhibited PDGF-induced actin reorganization associated with migration. PDGF-induced actin reorganization and migration were blocked by inhibitors of phosphatidylinositol 3 kinase (PI3K) and Akt. However, migration was blocked by inhibitors of the MEK/ERK pathway, with no effect on cytoskeletal reorganization. ATRA suppressed PDGF-induced Akt activation without influencing ERK activation. RAR was found to form protein-protein interactions with the p85 PI3K subunit. These results suggest that retinoic acid inhibits airway SMC migration through the modulation of the PI3K/Akt pathway.

9-cis-Retinoic acid inhibition of lung carcinogenesis in the A/J mouse model is accompanied by increased expression of RAR-beta but no change in cyclooxygenase-2

9-cis-Retinoic acid (9cRA) binds both retinoic acid receptors (RARs) and retinoid X receptors (RXRs) and has been shown to be a potential chemopreventive agent both in lung cancer cell culture studies and in clinical trials studying former smokers. However, direct evidence of the efficacy of 9cRA against lung tumor development in vivo is lacking. In the present study, we determined whether treatment with 9cRA has the potential to inhibit lung carcinogenesis by upregulating RAR-beta and down-regulating COX-2 expression in the A/J mouse lung cancer model. A/J mice (n=14-15/group) were treated as follows: (1) Control (Sham treated); (2) NNK (100mg NNK/kg body weight); (3) NNK+9cRA (15mg/kg diet); and (4) NNK+celecoxib (a COX-2-specific inhibitor, 500mg/kg diet). Tumor incidence, tumor multiplicity, RAR-beta mRNA, COX-2 mRNA, and COX-2 protein levels in lung samples of mice were determined 4 months after carcinogen injection. The results showed that mice receiving 9cRA supplementation had significantly lower tumor multiplicity (48% reduction, P<0.05) and showed a trend toward lower tumor incidence (40% reduction, P=0.078), as compared with the mice given NNK alone. Although, celecoxib treatment resulted in greater declines in tumor incidence and tumor multiplicity (75 and 88%, respectively, P<0.05), the chemoprotective effects of celecoxib were accompanied by increased mortality while 9cRA treatment resulted in no weight-loss associated toxicity or mortality. Supplementation with 9cRA was effective in increasing RAR-beta mRNA, but this increase was not accompanied by decreased levels of COX-2 mRNA or protein. These results suggest that 9cRA supplementation may provide protection against lung carcinogenesis and this effect may be mediated in part by 9cRA induction of RAR-beta, but not inhibition of COX-2 transcription.

Receptor-selective retinoids inhibit the growth of normal and malignant breast cells by inducing G1 cell cycle blockade

Despite advances in treatment, breast cancer continues to be the second leading cause of cancer mortality in women. Statistics suggest that while focus on treatment should continue, chemopreventive approaches should also be pursued. Previous studies have demonstrated that naturally occurring retinoids such as 9-cis retinoic acid (9cRA) can prevent breast cancer in animal models. However, these studies have also shown that these compounds are too toxic for general use. Work from our laboratory showed that an RXR-selective retinoid LGD1069 prevented tumor development in animal models of cancer with reduced toxicity as compared to an RAR-selective retinoid TTNPB. In the present study, we investigated the mechanisms by which receptor-selective retinoids inhibit the growth of normal and malignant breast cells. Our results demonstrate that the synthetic retinoids tested are as effective as 9cRA in suppressing the growth of normal human mammary epithelial cells (HMECs) and estrogen receptor-positive (ER-positive) breast cancer cells. Although the receptor-selective retinoids induce minimal amounts of apoptosis in T47D breast cancer cells, the predominant factor that leads to growth arrest is G1 cell cycle blockade. Our data indicate that this blockade results from the downregulation of Cyclin D1 and Cyclin D3, which in turn causes Rb hypophosphorylation. Non-toxic retinoids that are potent inducers of cell cycle arrest may be particularly useful for the prevention of breast cancer.

Inhibition of epidermal growth factor-induced cell transformation by tannins

The mouse epidermal JB6 cell system is a well developed model for studying tumor promotion, and the JB6 Cl 41 promotion sensitive (P+) cell line, in which transformed colonies are induced by epidermal growth factor (EGF), was used to test the anti-tumor promoting effect of seven tannins and two triterpenoids. We found that six tannins, ellagitannins (compounds 1, 2, 3 and 4) and chromone gallates (compounds 6 and 7), significantly blocked EGF-induced cell transformation in a concentration-dependent manner. The inhibition of cell transformation by the tannins was not due to growth inhibition. The ellagitannins, but not the chromone gallates, significantly attenuated EGF-induced activator protein 1 (AP-1) activation, a transcription factor. Compounds 1 and 3, among the ellagitannins analysed, inhibited the EGF-induced phosphorylation of extracellular-signal regulated protein kinases and p38 kinases, which regulate AP-1 activation. On the other hand, compounds 3 and 4 suppressed EGF-induced phosphatidylinositol 3-kinase (PI3K) activation. In addition, all tannins that blocked cell transformation markedly inhibited EGF-induced activation of Akt, a downstream effector of PI3K. Because signal-transduction pathways, including AP-1 and PI3K pathways, have been focused as prime targets for chemopreventive phytochemicals, our results suggest that inhibition by tannins of EGF-induced neoplastic transformation in JB6 cells is related to blocking of Akt activation, and also attenuation of AP-1 activation for ellagitannins.

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.

Reporter mice and drug discovery and development

In vivo reporter gene and imaging technologies have the potential to contribute to the drug discovery pipeline in several areas. They provide systems that enable the study of the biochemical activity of a target in disease, and in response to a drug, to be monitored over periods of time, and offer more accurate methods of measuring pharmacodynamics and toxicity. Although reporter-gene technology is in its infancy, with further refinement reporter animals could become a valuable tool in the early stages of target and lead identification and preclinical drug development.

Retinoids arrest breast cancer cell proliferation: retinoic acid selectively reduces the duration of receptor tyrosine kinase signaling

Retinoic acid (RA) induces cell cycle arrest of hormone-dependent human breast cancer (HBC) cells. Previously, we demonstrated that RA-induced growth arrest of T-47D HBC cells required the activity of the RA-induced protein kinase, protein kinase Calpha (PKCalpha) [J. Cell Physiol. 172 (1997) 306]. Here, we demonstrate that RA treatment of T-47D cells interfered with growth factor signaling to downstream, cytoplasmic and nuclear targets. RA treatment did not inhibit epidermal growth factor (EGF) receptor activation but resulted in rapid inactivation. The lack of sustained EGFR activation was associated with transient rather than sustained association of the EGFR with the Shc adaptor proteins and activation of Erk 1/2 and with compromised induction of expression of immediate early response genes. Inhibiting the activity of PKCalpha, a retinoic acid-induced target gene, prevented the effects of RA on cell proliferation and EGF signaling. Constitutive expression of PKCalpha, in the absence of RA, decreased cell proliferation and decreased EGF signaling. RA treatment increased steady-state levels of the protein tyrosine phosphatase PTP-1C and all measured effects of RA on EGF receptor function were reversed by the tyrosine phosphate inhibitor orthovanadate. These results indicate that RA-induced target genes, particularly PKCalpha, prevent sustained growth factor signaling, uncoupling activated receptor tyrosine kinases and nuclear targets that are required for cell cycle progression.

Spying on cancer: molecular imaging in vivo with genetically encoded reporters

Genetically encoded imaging reporters introduced into cells and transgenic animals enable noninvasive, longitudinal studies of dynamic biological processes in vivo. The most common reporters include firefly luciferase (bioluminescence imaging), green fluorescence protein (fluorescence imaging), herpes simplex virus-1 thymidine kinase (positron emission tomography), and variants with enhanced spectral and kinetic properties. When cloned into promoter/enhancer sequences or engineered into fusion proteins, imaging reporters allow transcriptional regulation, signal transduction, protein-protein interactions, oncogenic transformation, cell trafficking, and targeted drug action to be spatiotemporally resolved in vivo. Spying on cancer with genetically encoded imaging reporters provides insight into cancer-specific molecular machinery within the context of the whole animal.

Regulation of matrix metalloproteinases: an overview

Matrix metalloproteinases (MMPs) are a major group of enzymes that regulate cell-matrix composition. MMP genes show a highly conserved modular structure. Ample evidence exists on the role of MMPs in normal and pathological processes, including embryogenesis, wound healing, inflammation, arthritis, cardiovascular diseases, pulmonary diseases and cancer. The expression patterns of MMPs have interesting implications for the use of MMP inhibitors as therapeutic agents. Insights might be gained as to the preference for a general MMP inhibitor as opposed to an inhibitor designed to be specific for certain MMP family members as it relates to a defined disease state, and may give clues to potential side effects. The signalling pathways that lead to induction of expression of MMPs are still incompletely understood, but certain patterns are beginning to emerge. Regarding inhibition of MMP expression at the level of kinase pathways, it is possible that selective chemical inhibitors for distinct signalling pathways (e.g. MAPK, PKC) will hopefully, soon be available for initial clinical trials. Overexpression of selective dual specificity MAPK phosphatases have been shown to prevent MMP promoter activation which could also be used as a novel strategy to prevent activation of AP-1 and ETS transcription factors and MMP promoters in vivo. Interactions between members of different transcription factors provide fine-tuning of the transcriptional regulation of MMP promoter activity. MMPs play a crucial role in tumor invasion. Although the expression of MMPs in malignancies has been studied widely, the specific role of distinct MMPs in the progression of cancer may be more complex than has been assumed. For example, it has recently been shown that MMP-3, MMP-7, MMP-9 and MMP-12 can generate angiostatin from plasminogen, indicating that their expression in peritumoral area may in fact serve to limit angiogenesis and thereby inhibit tumor growth and invasion. The recent view about the role of stromal cells in the progression of cancer cell growth and metastasis is particularly interesting, and additional studies about the regulation of MMP gene expression and activity in malignancies are needed to understand the role and regulation of MMPs in tumor cell invasion.

Regulation of matrix metalloproteinases: an overview

Matrix metalloproteinases (MMPs) are a major group of enzymes that regulate cell-matrix composition. MMP genes show a highly conserved modular structure. Ample evidence exists on the role of MMPs in normal and pathological processes, including embryogenesis, wound healing, inflammation, arthritis, cardiovascular diseases, pulmonary diseases and cancer. The expression patterns of MMPs have interesting implications for the use of MMP inhibitors as therapeutic agents. Insights might be gained as to the preference for a general MMP inhibitor as opposed to an inhibitor designed to be specific for certain MMP family members as it relates to a defined disease state, and may give clues to potential side effects. The signalling pathways that lead to induction of expression of MMPs are still incompletely understood, but certain patterns are beginning to emerge. Regarding inhibition of MMP expression at the level of kinase pathways, it is possible that selective chemical inhibitors for distinct signalling pathways (e.g. MAPK, PKC) will hopefully, soon be available for initial clinical trials. Overexpression of selective dual specificity MAPK phosphatases have been shown to prevent MMP promoter activation which could also be used as a novel strategy to prevent activation of AP-1 and ETS transcription factors and MMP promoters in vivo. Interactions between members of different transcription factors provide fine-tuning of the transcriptional regulation of MMP promoter activity. MMPs play a crucial role in tumor invasion. Although the expression of MMPs in malignancies has been studied widely, the specific role of distinct MMPs in the progression of cancer may be more complex than has been assumed. For example, it has recently been shown that MMP-3, MMP-7, MMP-9 and MMP-12 can generate angiostatin from plasminogen, indicating that their expression in peritumoral area may in fact serve to limit angiogenesis and thereby inhibit tumor growth and invasion. The recent view about the role of stromal cells in the progression of cancer cell growth and metastasis is particularly interesting, and additional studies about the regulation of MMP gene expression and activity in malignancies are needed to understand the role and regulation of MMPs in tumor cell invasion.

Ectopic expression of nonliganded retinoic acid receptor beta abrogates AP-1 activity by selective degradation of c-Jun in cervical carcinoma cells

Expression of the nuclear retinoic acid receptor beta2 (RARbeta2) gene is often disturbed in cervical carcinoma cells. One important mechanism by which RARbeta2 can exert growth inhibitory function is based on its ability to repress the AP-1 transcription factor in a ligand-dependent manner. Because less is known about the biological effects of RARbeta in the absence of ligand, the corresponding cDNA was stably introduced into HPV18-positive HeLa cervical carcinoma cells. In the present study we describe a novel mechanism by which AP-1 becomes inactivated. Constitutive expression of nonliganded RARbeta abrogated both AP-1 binding affinity and activity by a selective degradation of the c-Jun protein as major dimerization partner, without substitution by other members of the Jun family. Blockage of the proteasomal pathway completely rescued c-Jun and reconstituted the AP-1 function. Moreover, HeLa RARbeta2 clones treated either with tumor necrosis factor-alpha or transfected with a constitutive active upstream mitogen-activated protein kinase (MEKK1Delta) also resulted in c-Jun phosphorylation and restoration of AP-1 affinity and functionality similar to that found in nontransfected parental HeLa cells. These data revealed an important cross-talk between trans-repression of AP-1 and nonliganded RARbeta in human papillomavirus-positive cells. Because AP-1 activity was not irreversibly disturbed, but could be switched on through activation of the Jun N-terminal kinase pathway, a model for the transient activation of AP-1 even in the presence of RARbeta as repressor is suggested.

RARgamma acts as a tumor suppressor in mouse keratinocytes

All-trans retinoic acid (RA), the principle biologically active form of vitamin A, is essential for many developmental process as well as homeostasis in the adult. Many lines of evidence also suggest that RA, acting through the RA receptors (RARs), can also suppress growth of tumors of diverse origin. To assess directly the role of the RARs in a model of epidermal tumorigenesis, we investigated the incidence of tumor formation using keratinocytes lacking specific RAR types. Our data suggest that loss of RARgamma, but not RARalpha, predisposed keratinocytes to v-Ha-Ras-induced squamous cell carcinoma. We also found that ablation of RARgamma, but not RARalpha, abolished RA-induced cell cycle arrest and apoptosis in these keratinocytes. Reconstitution of receptor expression into RAR-null cells restored sensitivity to RA, and reversed the tumorigenic potential of receptor-deficient keratinocytes. These data strongly support a tumor suppressor effect for the RARs, in particular endogenous RARgamma, in murine keratinocytes.

Techniques: Reporter mice – a new way to look at drug action

During the past decade remarkable progress in molecular genetics and the possibility of manipulating cells so that the expression of genes can directly 'report' on drug activity has produced major changes in drug development strategies. The recent description and pharmacological validation of reporter mice for in vivo analysis of hormone receptor activity opens new horizons for drug discovery. These novel animal models, in association with in vivo imaging technologies, provide a global view of the target tissues of drug action following acute and repeated drug treatment, thus enabling the prediction of potential side-effects in the early phase of preclinical studies. It is anticipated that further improvements of transgene architecture will lead to models that combine pharmacokinetic, pharmacodynamic and toxicological studies in a single step, which should provide a tremendous saving in time and, paradoxically, the number of animals to be sacrificed in the development of novel pharmacologically active molecules.

A critical role of PI-3K/Akt/JNKs pathway in benzo[a]pyrene diol-epoxide (B[a]PDE)-induced AP-1 transactivation in mouse epidermal Cl41 cells

Mouse skin tumorigenicity studies indicate that benzo[a]pyrene-7,8-diol-9,10-epoxide (B[a]PDE) contributes to carcinogenesis as both a tumor initiator and promoter. However, the mechanisms that mediate B[a]PDE tumor promotion effects remain unclear. Our results demonstrated that in mouse epidermal Cl41 cells, B[a]PDE treatment resulted in marked activation of AP-1 and its upstream MAPKs, including ERKs, JNKs and p38K. B[a]PDE exposure also led to activation of phosphotidylinositol 3-kinase (PI-3K), Akt and p70 S6 kinase (p70S6k). B[a]PDE-induced AP-1 transactivation was inhibited by pretreatment of cells with PI-3K inhibitors, wortmannin or Ly294002. In contrast, inhibition of p70S6k with rapamycin did not show any inhibitory effects. An overexpression of dominant-negative mutant of PI-3K, Deltap85, impaired B[a]PDE-induced activation of PI-3K, Akt and AP-1 transactivation. Furthermore, an overexpression of dominant-negative Akt mutant, Akt-T308A/S473A, blocked B[a]PDE-induced activation of Akt, AP-1 and JNKs, while it did not affect the activation of p70S6k, ERKs and p38 kinase. These results demonstrated that B[a]PDE was able to induce AP-1 transactivation and this AP-1 induction was specific through PI-3K/Akt/JNKs-dependent and p70S6k-independent pathways. This study also indicated that Akt-T308A/S473A blocks B[a]PDE-induced AP-1 activation specific through impairing JNK pathway. These findings will help us to understand the signal transduction pathways involved in the carcinogenic effects of B[a]PDE.

Retinoid-dependent restriction of human immunodeficiency virus type 1 replication in monocytes/macrophages

Vitamin A deficiency has been correlated with increased severity of human immunodeficiency virus type 1 (HIV-1)-associated disease. Moreover, vitamin A supplementation can reduce AIDS-associated morbidity and mortality. Our group and others have shown that retinoids, the bioactive metabolites of vitamin A, repress HIV-1 replication in monocytic cell lines and primary macrophages by blocking long-terminal-repeat (LTR)-directed transcription. Based on these studies, we hypothesize that retinoids are natural repressors of HIV-1 in vivo. We show here that all-trans-retinoic acid (RA)-mediated repression of HIV-1 activation requires pretreatment for at least 12 h and is blocked by the protein synthesis inhibitors cycloheximide and puromycin. Studies of the kinetics of RA-mediated repression in U1 cells and primary monocyte-derived macrophages (MDMs) reveal that the repressive effects of RA on HIV-1 expression are long-lasting but reversible. We demonstrate that HIV-1 expression is activated when U1 cells or MDMs are cultured in retinoid-free synthetic medium and show that physiological concentrations of RA repress this activation. In addition, the synthetic pan-retinoic acid receptor antagonist BMS-204 493 activates HIV-1 replication in U1 cells in a dose-dependent manner, suggesting that RA-induced transactivation of cellular gene expression is required for HIV-1 repression. Together, these data support the hypothesis that retinoids present in tissue culture media in vitro and serum in vivo maintain HIV-1 in a transcriptionally repressed state in monocytes/macrophages.

PI-3K and Akt are mediators of AP-1 induction by 5-MCDE in mouse epidermal Cl41 cells

5-Methylchrysene has been found to be a complete carcinogen in laboratory animals. However, the tumor promotion effects of (+/-)-anti-5-methylchrysene-1,2-diol-3,4-epoxide (5-MCDE) remain unclear. In the present work, we found that 5-MCDE induced marked activator protein-1 (AP-1) activation in Cl41 cells. 5-MCDE also induced a marked activation of phosphatidylinositol 3-kinase (PI-3K). Inhibition of PI-3K impaired 5-MCDE-induced AP-1 transactivation, suggesting that PI-3K is an upstream kinase involved in AP-1 activation by 5-MCDE. Furthermore, we found that Akt is a PI-3K downstream mediator for 5-MCDE-induced AP-1 transactivation, whereas another PI-3K downstream kinase, p70(S6K), was not involved in AP-1 activation by 5-MCDE. Moreover, inhibition of Akt activation blocked 5-MCDE-induced activation of extracellular signal-regulated protein kinases (ERKs) and c-Jun NH(2)-terminal kinases (JNKs), whereas it did not affect p38K activation. Consistently, overexpression of a dominant-negative mutant of ERK2 or JNK1 blocked the AP-1 activation by 5-MCDE. These results demonstrate that 5-MCDE is able to induce AP-1 activation, and the AP-1 induction is specifically through a PI-3K/Akt-dependent and p70(S6K)-independent pathway.