Activation of AP-1 by okadaic acid in mouse keratinocytes associated with hyperphosphorylation of c-jun

Okadaic acid activates Wnt/β-catenin-signaling in human HepaRG cells

The lipophilic phycotoxin okadaic acid (OA) occurs in the fatty tissue and hepatopancreas of filter-feeding shellfish. The compound provokes the diarrhetic shellfish poisoning (DSP) syndrome after intake of seafood contaminated with high levels of the DSP toxin. In animal experiments, long-term exposure to OA is associated with an elevated risk for tumor formation in different organs including the liver. Although OA is a known inhibitor of the serine/threonine protein phosphatase 2A, the mechanisms behind OA-induced carcinogenesis are not fully understood. Here, we investigated the influence of OA on the β-catenin-dependent Wnt-signaling pathway, addressing a major oncogenic pathway relevant for tumor development. We analyzed OA-mediated effects on β-catenin and its biological function, cellular localization, post-translational modifications, and target gene expression in human HepaRG hepatocarcinoma cells treated with non-cytotoxic concentrations up to 50 nM. We detected concentration- and time-dependent effects of OA on the phosphorylation state, cellular redistribution as well as on the amount of transcriptionally active β-catenin. These findings were confirmed by quantitative live-cell imaging of U2OS cells stably expressing a green fluorescent chromobody which specifically recognize hypophosphorylated β-catenin. Finally, we demonstrated that nuclear translocation of β-catenin mediated by non-cytotoxic OA concentrations results in an upregulation of Wnt-target genes. In conclusion, our results show a significant induction of the canonical Wnt/β-catenin-signaling pathway by OA in human liver cells. Our data contribute to a better understanding of the molecular mechanisms underlying OA-induced carcinogenesis.

Okadaic acid: more than a diarrheic toxin

Okadaic acid (OA) is one of the most frequent and worldwide distributed marine toxins. It is easily accumulated by shellfish, mainly bivalve mollusks and fish, and, subsequently, can be consumed by humans causing alimentary intoxications. OA is the main representative diarrheic shellfish poisoning (DSP) toxin and its ingestion induces gastrointestinal symptoms, although it is not considered lethal. At the molecular level, OA is a specific inhibitor of several types of serine/threonine protein phosphatases and a tumor promoter in animal carcinogenesis experiments. In the last few decades, the potential toxic effects of OA, beyond its role as a DSP toxin, have been investigated in a number of studies. Alterations in DNA and cellular components, as well as effects on immune and nervous system, and even on embryonic development, have been increasingly reported. In this manuscript, results from all these studies are compiled and reviewed to clarify the role of this toxin not only as a DSP inductor but also as cause of alterations at the cellular and molecular levels, and to highlight the relevance of biomonitoring its effects on human health. Despite further investigations are required to elucidate OA mechanisms of action, toxicokinetics, and harmful effects, there are enough evidences illustrating its toxicity, not related to DSP induction, and, consequently, supporting a revision of the current regulation on OA levels in food.

A preliminary operational classification system for nonmutagenic modes of action for carcinogenesis

This article proposes a system of categories for nonmutagenic modes of action for carcinogenesis. The classification is of modes of action rather than individual carcinogens, because the same compound can affect carcinogenesis in more than one way. Basically, we categorize modes of action as: (1) co-initiation (facilitating the original mutagenic changes in stem and progenitor cells that start the cancer process) (e.g. induction of activating enzymes for other carcinogens); (2) promotion (enhancing the relative growth vs differentiation/death of initiated clones (e.g. inhibition of growth-suppressing cell-cell communication); (3) progression (enhancing the growth, malignancy, or spread of already developed tumors) (e.g. suppression of immune surveillance, hormonally mediated growth stimulation for tumors with appropriate receptors by estrogens); and (4) multiphase (e.g., "epigenetic" silencing of tumor suppressor genes). A priori, agents that act at relatively early stages in the process are expected to manifest greater relative susceptibility in early life, whereas agents that act via later stage modes will tend to show greater susceptibility for exposures later in life.

Retinoic acid enhances differentiation of v-myb-transformed monoblasts induced by okadaic acid

Differentiation of various leukemic cells can be induced by liganded retinoic acid receptors and protein phosphatase inhibitors. In this study, we explored the effects of okadaic acid (OA), the phosphatase inhibitor, and retinoic acid (RA) in v-myb-transformed monoblasts BM2. OA induced differentiation of BM2 monoblasts into macrophage-like cells, as documented by analyses of cell morphology, cell cycle, phagocytic activity, non-specific esterase activity, production of reactive oxygen species and expression of vimentin and Mo-1. In contrast to many other leukemic cell lines, BM2 cells do not respond to retinoic acid. However, once exposed to OA and RA simultaneously, BM2 cells differentiate along monocyte/macrophage pathway more efficiently. We conclude that RA enhances differentiation of v-myb-transformed monoblasts induced by protein phosphorylation.

Interleukin-6 increases rat metalloproteinase-13 gene expression through Janus kinase-2-mediated inhibition of serine/threonine phosphatase-2A

Interleukin-6 (IL-6) increases metalloproteinase-13 (MMP-13) gene expression by increasing phosphorylated c-Jun and by inhibiting serine/threonine phosphatase-2A (PP2A) activity. We investigated the mechanisms by which IL-6 induces c-Jun phosphorylation and PP2A inactivation in Rat-1 fibroblasts. We show that IL-6 increased MMP-13 mRNA, phosphorylated c-Jun, and activator protein 1 (AP1) binding activity without increasing c-Jun-N-terminal kinase (JNK) activity. These effects did not seem to be mediated by ERK, p38 MAP kinase, phosphatidylinositol-3-kinase, calmoduline-dependent protein kinase, protein kinase C (PKC) or protein kinase A since inhibition with specific inhibitors did not abrogate these effects. IL-6 increases PP2A catalytic subunit tyrosine phosphorylation. Inhibition of the tyrosine kinase Jak2, with the specific inhibitor AG490, abrogated this effect. Likewise, this Jak2 inhibitor blocked the effects of IL-6 on c-Jun phosphorylation, AP1 binding activity and metalloproteinase-13 gene expression. We conclude that IL-6 increases MMP-13 gene expression by activation of Jak2, resulting in tyrosine phosphorylation of the catalytic subunit of PP2A, which in turn decreases PP2A activity and prolongs c-Jun phosphorylation.

Identification of a functional peroxisome proliferator-activated receptor response element in the rat catalase promoter

Peroxisomal proliferator-activated receptor (PPAR)gamma has been shown to decrease the inflammatory response via transrepression of proinflammatory transcription factors. However, the identity of PPARgamma responsive genes that decrease the inflammatory response has remained elusive. Because generation of the reactive oxygen species hydrogen peroxide (H(2)O(2)) plays a role in the inflammatory process and activation of proinflammatory transcription factors, we wanted to determine whether the antioxidant enzyme catalase might be a PPARgamma target gene. We identified a putative PPAR response element (PPRE) containing the canonical direct repeat 1 motif, AGGTGA-A-AGTTGA, in the rat catalase promoter. In vitro translated PPARgamma and retinoic X receptor-alpha proteins were able to bind to the catalase PPRE. Promoter deletion analysis revealed that the PPRE was functional, and a heterologous promoter construct containing a multimerized catalase PPRE demonstrated that the PPRE was necessary and sufficient for PPARgamma-mediated activation. Treatment of microvascular endothelial cells with PPARgamma ligands led to increases in catalase mRNA and activity. These results demonstrate that PPARgamma can alter catalase expression; this occurs via a PPRE in the rat catalase promoter. Thus, in addition to transrepression of proinflammatory transcription factors, PPARgamma may also be modulating catalase expression, and hence down-regulating the inflammatory response via scavenging of reactive oxygen species.

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

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

Cellular N-Ras promotes cell survival by downregulation of Jun N-terminal protein kinase and p38

Cellular N-Ras provides a steady-state antiapoptotic signal, at least partially through the regulation of phosphorylated Akt and Bad levels. Fibroblasts lacking c-N-Ras expression are highly sensitive to the induction of apoptosis by a variety of agents. Reduction of pBad and pAkt levels using a phosphatidylinositol 3-kinase inhibitor was not sufficient to sensitize the control cell population to the high level of apoptosis observed in the N-Ras knockout cell lines, suggesting that c-N-Ras provides at least one other antiapoptotic signal. Stimulation of the control cells with apoptotic agents results in a transient increase in Jun N-terminal protein kinase (JNK)/p38 activity that decreased to baseline levels during the time course of the experiments. In all cases, however, sustained JNK/p38 activity was observed in cells lacking c-N-Ras expression. This correlated with sustained levels of phosphorylated MKK4 and MKK3/6, upstream activators of JNK and p38, respectively. Mimicking the sustained activation of JNK in the control cells did result in increasing their sensitivity to apoptotic agents, suggesting that prolonged JNK activity is a proapoptotic event. We also examined the potential downstream c-N-Ras targets that might be involved in regulating the duration of the JNK/p38 signal. Only the RalGDS 37G-N-Ras protein protected the N-Ras knockout cells from apoptosis and restored transient rather than sustained JNK activation. These data suggest that cellular N-Ras provides an antiapoptotic signal through at least two distinct mechanisms, one which regulates steady-state pBad and pAkt levels and one which regulates the duration of JNK/p38 activity following an apoptotic challenge.

Suppression of okadaic acid-induced apoptosis by overexpression of calpastatin in human UV(r)-1 cells

Proteolytic systems have various involvements in apoptotic pathways. To understand the role of calpain in apoptosis, calpastatin, a specific inhibitor of calpain, was overexpressed in human UV(r)-1 fibroblasts by transfection of its cDNA. The elevated expression of calpastatin resulted in decreased survival in the presence of okadaic acid (OA) but in no apparent alteration in the sensitivity toward other drugs such as 5-fluorouracil, mitomycin C and methotrexate. After treatment with OA, a typical apoptotic DNA ladder was observed in control vector-transfected cells but not in calpastatin-transfected cells. This indicates that OA-induced apoptosis was suppressed by overexpression of calpastatin. Further immunoblot analysis showed that the OA-induced hyperphosphorylation of c-Jun was inhibited in calpastatin-transfected cells. This might be involved in the resistance to OA-induced cell death in calpastatin-overproducing cells.

GDNF-induced neurite formation was stimulated by protein kinase inhibitors and suppressed by Ras inhibitors

The effects of various inhibitors on the glial cell line-derived neurotrophic factor (GDNF)-induced neurite formation in TGW human neuroblastoma cells were investigated. Treatment of cells with Ser/Thr protein kinase inhibitors such as staurosporine, H-7, H-8 and HA-1004, induced neurite formation without GDNF. On the other hand, tyrosine kinase inhibitors such as erbstatin, genistein and herbimycin A did not produce neurites per se, but effectively enhanced the GDNF-induced neurite formation. A phosphatase inhibitor, okadaic acid, and Ras inhibitors such as oxanosine, damnacanthal and conophylline strongly suppressed the effect of GDNF. These results suggest that a tyrosine protein kinase has a suppressive role in the neurite formation induced by GDNF and that Ras is necessary for the signaling initiated by GDNF.