Cancer Res Treat.  2004 Oct;36(5):275-286.

Transcription Factors in the Cellular Signaling Network as Prime Targets of Chemopreventive Phytochemicals

• Surh YJ

Abstract

Accumulating evidence from epidemiologic and laboratory studies support an inverse relationship between a regular consumption of fruits and vegetables and the risk of specific cancers. Numerous phytochemicals derived from edible plants have been reported to possess ability to interfere with a specific stage of carcinogenic process. Multiple mechanisms have been proposed to account for the anti-carcinogenic actions of dietary constituents, but more attention has recently focussed on intracellular signaling cascades as common molecular targets of a wide variety of chemopreventive phytochemicals.

Keyword

Chemoprevention; Phytochemicals; Signal transduction; Cell signaling network; NF- kappa B; AP-1; Mitogen-activated protein kinases; Nrf2; Keap1

MeSH Terms

Chemoprevention
Fruit
Mitogen-Activated Protein Kinases
Phytochemicals*
Plants, Edible
Signal Transduction
Transcription Factor AP-1
Transcription Factors*
Vegetables
Mitogen-Activated Protein Kinases
Transcription Factor AP-1
Transcription Factors

Figure

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  Fig. 1 Dietary phytochemicals blocking or suppressing multi-stage carciniogenesis. Certain chemopreventive phytochemicals in the diet inhibit metabolic activation of the procarcinogens to ultimate electrophilic species and/or their covalent interaction with target cell DNA, thereby blocking the initiation (blocking agents). Alternatively dietary blocking agents can stimulate the detoxification of the pro- or ultimate carcinogens. Others suppress the later steps (promotion and progression) of multi-stage carcinogenesis (suppressing agents). Some phytochemicals can act as both blocking and suppressing agents. Adapted from reference 1.

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  Fig. 2 The schematic representation of signalling cascades and activation of NF-κB and AP-1. The activation of NF-κB and AP-1 begin, in general, with stimulation of specific receptors at the cell surface and recruitment of adaptor proteins, which targets the external signal for specific transduction pathways controlled by various kinases. The NF-κB pathway converges upon a high-molecular mass multiprotein complex named IκB kinase (IKK) signalsome, which, in turn, promotes the phosphorylation of IκB. This targets IκB for ubiquitination and subsequent degradation by 26S proteasome. As a consequence, NF-κB is released and rapidly translocated to the nucleus where it binds to specific promoter regions of various genes. The IKK signalsome is activated by the NF-κB inducing kinase (NIK), which lies immediately upstream of IKKs in the NF-κB activation cascades. NIK mutants block TNF-α-and IL-1-induced NF-κB activation. Although the upstream signalling pathways that regulate NIK has not been clarified yet, there is some evidence to support the involvement of MAPK/ERK kinase kinase-1 (MEKK1), a kinase upstream of ERK as well as p38 MAPK. Many studies have revealed a close association between the MAPK activity and phosphorylation and degradation of IκB protein, which facilitates nuclear translocation and subsequent DNA binding of NF-κB in various cell systems. Recent reports showed that the NF-κB activation is also regulated by the Akt signaling pathway (46,47). Phosphatidylinositol 3-kinase (PI3-K) activates Akt/protein kinase B via phosphorylation by 3-phosphoinositide-dependent protein kinase-1 (PDK1). Genistein specifically inhibits Akt activity and Akt-mediated NF-κB activation. Epiegallocatechin gallate (EGCG) can block the activities of PI3-K and Akt. There is crosstalk between the Akt and NF-κB signaling pathways- Akt phosphorylation leads to activation of NF-κB by stimulating IκB kinase (IKK) activity. IKK is also a target of chemopreventive phytochemicals, including curcumin, resveratrol and EGCG (see text for details). AP-1 heterodimers are constitutively localized within the nucleus, and transactivation of AP-1 is achieved through phosphorylation of its activation domain by MAPK. The MAPK family proteins have been shown to play a key role in regulating AP-1 activation in various types of cells in culture. AP-1 represents a heterogenous set of dimeric proteins consisting of members of Jun, Fos, and ATF families. AP-1 mRNA expression is dependent on MAPKs: ERK1/2 phosphorylates Elk-1, JNK phosphorylates ATF-2 and c-Jun in c-fos and c-jun induction, respectively, while p38 phosphorylates both Elk-1 and ATF-2. c-Fos and c-Jun form heterodimers and bind to AP-1 response element in the promoter of target genes. The target molecule/event blocked/inactivated by chemopreventive phytochemicals is marked with a symbol (⦸).

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  Fig. 3 Proposed pathways for CBP- and kinase-dependent transcriptional activation of NF-κB and AP-1. A number of extracellular stimuli triggered activation of specific kinases that are involved in transcriptional activation of NF-κB and AP-1. IKK phosphorylates IκB, which results in degradation of IκB by the proteasomes, releasing p65, which then translocates to the nucleus. Some kinases phosphorylate p65, the functionally active NF-κB transactivation subunit. Phosphorylated p65 enhances NF-κB-dependent transcription, probably by influencing the binding affinities of p65 to coactivators such as p300/CBP (CREB-binding protein) or transcriptional initiation complex. CBP is a general coactivator protein that bridges DNA-bound transcription factors to the basal transcription machinery. In addition, CBP, having intrinsic acetyltransferase activity, acetylates p50 (NF-κBκDNA binding subunit). This causes increased affinity of NF-κB to DNA. On the other hand, JNK-induced phosphorylation of c-Jun, a component of the AP-1 complex, facilitates its interaction with p300/CBP. Abbreviations: GSK-3β,..glycogen synthase kinase-3; IκB, Inhibitor of NF-κB; MSK1, mitogen- and stress-activated protein kinase 1; P, phosphate; PKAc, catalytic subunit of protein kinase A; PKB/AKT, protein kinase B; JNK, c-Jun-N-terminal kinase; AP-1, activator protein-1.

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  Fig. 4 Signaling pathways involved in the Antioxidant responsive element (ARE)-mediated transcriptional response through the activation of Nrf2. A protein designated as Kelch-like-ECH-associated protein 1 (Keap1) has been shown to be a cytoplasmic repressor of Nrf2 and thus inhibits its ability to translocate to nucleus and transactivate the ARE. These two proteins interact with each other through the double glycine-rich domains of Keap1 and a hydrophilic region in the Neh2 domain of Nrf2. Keap1 contains many cysteine residues. Oxidants or phase II enzyme inducers can cause oxidation or covalent modification of these cysteine residues. As a result, Nrf2 is released from the repressor (Keap1). Dissociation of the Keap1-Nrf2 complex is also assumed to be facilitated through the phosphorylation of Nrf2 by upstream kinase(s) such as MAPK, PKC, or PI3-K. PI3-K is also considered to phosphorylate CCAAT/enhancer binding protein-β (C/EBP-β), inducing its translocation to the nucleus and binding to the CCAAT sequence of C/EBP-β response element within the xenobiotic response element (XRE), in conjunction with Nrf2 binding to ARE. Active C/EBPβ may compete with C/EBP-α for the C/EBP binding site. After nuclear translocation, Nrf2 associates with a small Maf protein, forming a heterodimer that binds to ARE, stimulating the ARE-driven expression of gene that encode phase-II detoxifying or antioxidant enzymes, such as glutathione S-transferase alpha2 (GSTA2), NAD(P)H:quinone oxidoreductase (NQO1), γ-glutamate cysteine ligase (γ -GCLC and γ -GCLM), and heme oxygenase-1 (HO-1). Nrf2 can also dimerize with c-Jun, ATF-4, PMF and PPAR-gamma, but the physiologic significance of Nrf2 dimerization with such different bZIP proteins remains to be clarified. Curcumin and caffeic acid phenethyl ester (CAPE) disrupt the Nrf2-Keap1 complex, leading to increased Nrf2 binding to ARE. Sulforaphane directly interacts with Keap1 by covalent binding to its thiol groups. 6-(Methylsulfinyl)hexyl isothiocyanate (6-HITC) - a sulforaphane analog derived from Japanese horseradish (wasabi)-stimulates nuclear translocation of Nrf2.

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