JNK activation is required for JB6 cell transformation induced by tumor necrosis factor-alpha but not by 12-O-tetradecanoylphorbol-13-acetate

Involvement of c-jun NH(2)-terminal kinases in resveratrol-induced activation of p53 and apoptosis

Resveratrol, a constituent of grapes and other foods, is one of the most promising agents for cancer prevention. In a previous study, we showed that the antitumor activity of resveratrol occurs through extracellular signal-regulated protein kinases (ERKs) and p38 kinase-mediated p53 activation. In this study, we also determined that c-jun NH(2)-terminal kinases (JNKs) are involved in resveratrol-induced p53 activation and induction of apoptosis. In the JB6 mouse epidermal cell line, resveratrol activated JNKs dose-dependently within a dose range of 10-40 microM, the same dosage responsible for the inhibition of tumor promoter-induced cell transformation. Stable expression of a dominant negative mutant of JNK1 or disruption of the Jnk1 or Jnk2 gene markedly inhibited resveratrol-induced p53-dependent transcription activity and induction of apoptosis. Furthermore, resveratrol-activated JNKs were shown to phosphorylate p53 in vitro, but this activity was repressed in the cells expressing a dominant negative mutant of JNK1 or in Jnk1 or Jnk2 knockout (Jnk1(-/-) or Jnk2(-/-)) cells. These data suggested that JNKs act as mediators of resveratrol-induced activation of p53 and apoptosis, which may occur partially through p53 phosphorylation.

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.

Role of MAP kinases in UVB-induced phosphorylation of p53 at serine 20

Phosphorylation of the p53 tumor suppressor protein is one of the key regulatory steps in its activation process. Serine 20 phosphorylation of p53 has been shown to be required for the activation of p53 following UV radiation, but the signaling pathway mediating UV-induced phosphorylation is unknown. Here, we determined the role of MAP kinases in UVB-induced phosphorylation and found that JNKs are directly involved in the phosphorylation of p53 at serine 20. In a mouse JB6 epidermal cell line, dominant negative JNK1 abrogated UVB-induced phosphorylation of p53 at serine 20, whereas dominant negative p38 kinase or its inhibitor, SB202190, partially attenuated the phosphorylation. In contrast, dominant negative ERK2 or the MEK1 inhibitor, PD98059, had no effect on p53 phosphorylation at serine 20. Importantly, UVB-activated or active recombinant JNK1/2, or the p38 kinase downstream target, MAPKAPK-2, but not ERKs or p38 kinase, phosphorylated p53 at serine 20 in vitro. Furthermore, phosphorylation of p53 at serine 20 by UVB-activated JNKs and UVB-induced p53-dependent transcriptional activity were suppressed in Jnk1 or Jnk2 knockout (Jnk1(-/-) or Jnk2(-/-)) cells. Additionally, Jnk1(-/-), Jnk2(-/-), and p53-deficient (p53(-/-)) cells, as well as re-introduction of a p53 mutant with substitution of serine 20 to alanine into p53(-/-) cells, were defective for UVB-induced apoptosis. These findings strongly suggest that JNKs are the major direct signaling mediators of UVB-induced p53 phosphorylation at serine 20.

Cannabinoid-receptor-independent cell signalling by N-acylethanolamines

Anandamide and other polyunsaturated N-acylethanolamines (NAEs) exert biological activity by binding to cannabinoid receptors. These receptors are linked to G(i/o) proteins and their activation leads to extracellular-signal-regulated protein kinase (ERK) and c-Jun N-terminal kinase (JNK) mitogen-activated protein kinase (MAP kinase) activation, inhibition of cAMP-dependent signalling and complex changes in the expression of various genes. Saturated and monounsaturated NAEs cannot bind to cannabinoid receptors and may thus mediate cell signalling through other targets. Here we report that both saturated/monounsaturated NAEs and anandamide (20:4(n-6) NAE) stimulate cannabinoid-receptor-independent ERK phosphorylation and activator protein-1 (AP-1)-dependent transcriptional activity in mouse epidermal JB6 cells. Using a clone of JB6 P(+) cells with an AP-1 collagen-luciferase reporter construct, we found that 16:0, 18:1(n-9), 18:1(n-7), 18:2(n-6) and 20:4(n-6) NAEs stimulated AP-1-dependent transcriptional activity up to 2-fold, with maximal stimulation at approx. 10-15 microM. Higher NAE concentrations had toxic effects mediated by alterations in mitochondrial energy metabolism. The AP-1 stimulation appeared to be mediated by ERK but not JNK or p38 signalling pathways, because all NAEs stimulated ERK1/ERK2 phosphorylation without having any effect on JNK or p38 kinases. Also, overexpression of dominant negative ERK1/ERK2 kinases completely abolished NAE-induced AP-1 activation. In contrast with 18:1(n-9) NAE and anandamide, the cannabinoid receptor agonist WIN 55,212-2 did not stimulate AP-1 activity and inhibited ERK phosphorylation. The NAE-mediated effects were not attenuated by pertussis toxin and appeared to be NAE-specific, as a close structural analogue, oleyl alcohol, failed to induce ERK phosphorylation. The data support our hypothesis that the major saturated and monounsaturated NAEs are signalling molecules acting through intracellular targets without participation of cannabinoid receptors.

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.

Omega 3 but not omega 6 fatty acids inhibit AP-1 activity and cell transformation in JB6 cells

Epidemiological and animal-based investigations have indicated that the development of skin cancer is in part associated with poor dietary practices. Lipid content and subsequently the derived fatty acid composition of the diet are believed to play a major role in the development of tumorigenesis. Omega 3 (omega3) fatty acids, including docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), can effectively reduce the risk of skin cancer whereas omega 6 (omega6) fatty acids such as arachidonic acid (AA) reportedly promote risk. To investigate the effects of fatty acids on tumorigenesis, we performed experiments to examine the effects of the omega3 fatty acids EPA and DHA and of the omega6 fatty acid AA on phorbol 12-tetradecanoate 13-acetate (TPA)-induced or epidermal growth factor (EGF)-induced transcription activator protein 1 (AP-1) transactivation and on the subsequent cellular transformation in a mouse epidermal JB6 cell model. DHA treatment resulted in marked inhibition of TPA- and EGF-induced cell transformation by inhibiting AP-1 transactivation. EPA treatment also inhibited TPA-induced AP-1 transactivation and cell transformation but had no effect on EGF-induced transformation. AA treatment had no effect on either TPA- or EGF-induced AP-1 transactivation or transformation, but did abrogate the inhibitory effects of DHA on TPA- or EGF-induced AP-1 transactivation and cell transformation in a dose-dependent manner. The results of this study demonstrate that the inhibitory effects of omega3 fatty acids on tumorigenesis are more significant for DHA than for EPA and are related to an inhibition of AP-1. Similarly, because AA abrogates the beneficial effects of DHA, the dietary ratio of omega6 to omega3 fatty acids may be a significant factor in mediating tumor development.

Mitogen- and stress-activated protein kinase 1 mediates activation of Akt by ultraviolet B irradiation

In this study, we investigated the mechanism by which UVB irradiation activates Akt (also known as protein kinase B (PKB)) in mouse epidermal JB6 cells. Treatment with a phosphatidylinositol 3-kinase inhibitor, LY 294002, or expression of a dominant negative mutant of p85 (regulatory component of phosphatidylinositol 3-kinase) inhibited UVB-induced Akt activation. Interestingly, Akt activation by UVB was attenuated by treatment with PD 98059, a specific mitogen-activated protein kinase/extracellular signal-regulated protein kinase (Erk) kinase 1 inhibitor, or SB 202190, a specific p38 kinase inhibitor. Furthermore, the expression of a dominant negative mutant of Erk2 or p38 kinase, but not that of c-Jun N-terminal kinase 1 (JNK1), blocked UVB-induced Akt activation. The expression of a dominant negative mutant of p85 or treatment with LY 294002 also inhibited UVB-induced Erk phosphorylation. The UVB-activated mitogen-activated protein kinase members, which were immunoprecipitated from cells exposed to UVB, did not phosphorylate Akt. Instead, Akt was phosphorylated at both threonine 308 and serine 473 and activated by UVB-activated mitogen- and stress-activated protein kinase 1 (Msk1). The expression of a Msk1 C-terminal kinase-dead mutant inhibited UVB-induced phosphorylation and activation of Akt. These data thus suggested that UVB-induced Akt activation was mediated through Msk1, which is a downstream kinase of the Erk and p38 kinase signaling pathways.

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.

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.

ERKs and p38 kinase phosphorylate p53 protein at serine 15 in response to UV radiation

Phosphorylation of the p53 tumor suppressor protein is likely to play an important role in regulating its activity. Serine 15 phosphorylation of p53 leads to a stabilization of p53 by reducing its interaction with murine double minute 2, a negative regulatory partner. Recently, p53 was reported to be activated and phosphorylated at serine 15 following UV radiation. However, the signaling pathway that mediates UV-induced phosphorylation is less well characterized. Here, we provide evidence that UVB-induced phosphorylation of p53 at serine 15 is mediated directly by ERKs and p38 kinase. We find that in a mouse JB6 epidermal cell line, ERKs and p38 kinase form a complex with p53 following UVB radiation. Inhibition of ERKs or p38 kinase activity by the use of a dominant negative mutant of ERK2 or p38 kinase or their respective specific inhibitor, PD98059 or SB202190, results in abrogation of UVB-induced phosphorylation of p53 at serine 15. Strikingly, incubation of UVB-activated ERKs or p38 kinase immunoprecipitated complex with exogenous p53 shows serine 15 phosphorylation of both exogenous and co-precipitated endogenous p53 protein. Additionally, active recombinant ERK1/2 and p38 kinase but not JNKs are also able to phosphorylate p53 at serine 15 in vitro. Furthermore, pretreatment of cells with PD98059 or SB202190 blocks p53-dependent transcription activity but increases the level of p53 co-precipitated murine double minute. These results strongly suggest that both ERKs and p38 kinase have a direct role in UVB-induced phosphorylation of p53 at serine 15 in vivo.

Resveratrol suppresses TNF-induced activation of nuclear transcription factors NF-kappa B, activator protein-1, and apoptosis: potential role of reactive oxygen intermediates and lipid peroxidation

Resveratrol (trans-3,4',5-trihydroxystilbene), a polyphenolic phytoalexin found in grapes, fruits, and root extracts of the weed Polygonum cuspidatum, exhibits anti-inflammatory, cell growth-modulatory, and anticarcinogenic effects. How this chemical produces these effects is not known, but it may work by suppressing NF-kappaB, a nuclear transcription factor that regulates the expression of various genes involved in inflammation, cytoprotection, and carcinogenesis. In this study, we investigated the effect of resveratrol on NF-kappaB activation induced by various inflammatory agents. Resveratrol blocked TNF-induced activation of NF-kappaB in a dose- and time-dependent manner. Resveratrol also suppressed TNF-induced phosphorylation and nuclear translocation of the p65 subunit of NF-kappaB, and NF-kappaB-dependent reporter gene transcription. Suppression of TNF-induced NF-kappaB activation by resveratrol was not restricted to myeloid cells (U-937); it was also observed in lymphoid (Jurkat) and epithelial (HeLa and H4) cells. Resveratrol also blocked NF-kappaB activation induced by PMA, LPS, H2O2, okadaic acid, and ceramide. The suppression of NF-kappaB coincided with suppression of AP-1. Resveratrol also inhibited the TNF-induced activation of mitogen-activated protein kinase kinase and c-Jun N-terminal kinase and abrogated TNF-induced cytotoxicity and caspase activation. Both reactive oxygen intermediate generation and lipid peroxidation induced by TNF were suppressed by resveratrol. Resveratrol's anticarcinogenic, anti-inflammatory, and growth-modulatory effects may thus be partially ascribed to the inhibition of activation of NF-kappaB and AP-1 and the associated kinases.

Activator protein 1 (AP-1)- and nuclear factor kappaB (NF-kappaB)-dependent transcriptional events in carcinogenesis

Generation of reactive oxygen species (ROS) during metabolic conversion of molecular oxygen imposes a constant threat to aerobic organisms. Other than the cytotoxic effects, many ROS and oxidants are also potent tumor promoters linking oxidative stress to carcinogenesis. Clonal variants of mouse epidermal JB6 cells originally identified for their differential susceptibility to tumor promoters also show differential reduction-oxidation (redox) responses providing a unique model to study oxidative events in tumor promotion. AP-1 and NF-kappaB, inducible by tumor promoters or oxidative stimuli, show differential protein levels or activation in response to tumor promoters in JB6 cells. We further demonstrated that AP-1 and NF-kappaB are both required for maintaining the transformed phenotypes where inhibition of either activity suppresses transformation response in JB6 cells as well as human keratinocytes and transgenic mouse. NF-kappaB proteins or extracellular signal-regulated kinase (ERK) but not AP-1 proteins are shown to be sufficient for conversion from transformation-resistant to transformation-susceptible phenotype. Insofar as oxidative events regulate AP-1 and NF-kappaB transactivation, these oxidative events can be important molecular targets for cancer prevention.

Effects of food factors on signal transduction pathways

Consumption of plant-derived foods, especially fruits and vegetables, has been linked to decreased risk of cancer. Laboratory studies with animals and cells in culture have shown cancer preventive activity of chemicals isolated from soy, tea, rice and many green, yellow and orange fruits and vegetables. Using cell culture, transgenic mice and knockout mice models to examine the anti-cancer effects of these dietary factors at the molecular level, we found that (11) (-)-epigallocatechin gallate (EGCG), the major active polyphenol in green tea, and theaflavins, the major active components in black tea, inhibit epidermal growth factor (EGF)- or 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced JB6 cell transformation. At the same dose range that inhibited cell transformation, EGCG and theaflavins inhibited activator protein-1 (AP-1) activation. These compounds also inhibited ultraviolet B (UVB)-induced AP-1 and nuclear factor kappa B (NFkappaB)-dependent transcriptional activation; (2) resveratrol, found at high levels in grapes, inhibited cell transformation through the induction of apoptosis, mediated through JNK and p53-dependent pathways; (3) inositol hexaphosphate (InsP6), an active compound from rice and other grains, inhibited TPA- or EGF-induced transformation and signal transduction through its effects on phosphatidylinositol-3 kinase (PI-3) kinase; (4) phenethyl isothiocyanate (PEITC), which occurs as a conjugate in certain cruciferous vegetables, inhibited cell transformation corresponding with the induction of apoptosis. An elevation of p53 is required for PEITC-induced apoptosis. Our studies indicated that the chemopreventive effect of these food factors may be mediated by their effects on different signal transduction pathways; (5) retinoids (vitamin A and its metabolites) inhibited tumor promoter-induced cell transformation and tumor promotion in transgenic mice through the inhibition of AP-1 action but not through the activation of retinoic acid response element (RARE).

Effect of ionizing radiation on the differentiation of ROS 17/2.8 osteoblasts through free radicals

Although the acceleration of bone regeneration by radiation has been reported, the mechanisms of action of radiation on bone are unclear. The present results indicate that ionizing radiation-stimulated differentiation could result from the generation of reactive oxygen species during radiation exposure. The free radical release is considered as the most important mechanism of bone effect by radiation treatment. In addition, we report that radiation induced transient activation of c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) activation and the transcription factor, AP-1. The JNK and AP-1 activation is mediated with radiation-released free radicals in ROS 17/2.8 osteoblasts. These results indicate that ionizing radiation at a single dose of up to 5 Gray stimulates differentiation of ROS 17/2.8 osteoblasts via free radial release which may affect JNK/SAPK and AP-1 activities.