Ultraviolet radiation (UVR) activates p38 MAP kinase and induces post-transcriptional stabilization of the C/EBPδ mRNA in G0 growth arrested mammary epithelial cells

The m6A reader IMP2 directs autoimmune inflammation through an IL-17- and TNFα-dependent C/EBP transcription factor axis

Excessive cytokine activity underlies many autoimmune conditions, particularly through the interleukin-17 (IL-17) and tumor necrosis factor-α (TNFα) signaling axis. Both cytokines activate nuclear factor κB, but appropriate induction of downstream effector genes requires coordinated activation of other transcription factors, notably, CCAAT/enhancer binding proteins (C/EBPs). Here, we demonstrate the unexpected involvement of a posttranscriptional "epitranscriptomic" mRNA modification [N6-methyladenosine (m6A)] in regulating C/EBPβ and C/EBPδ in response to IL-17A, as well as IL-17F and TNFα. Prompted by the observation that C/EBPβ/δ-encoding transcripts contain m6A consensus sites, we show that Cebpd and Cebpb mRNAs are subject to m6A modification. Induction of C/EBPs is enhanced by an m6A methylase "writer" and suppressed by a demethylase "eraser." The only m6A "reader" found to be involved in this pathway was IGF2BP2 (IMP2), and IMP2 occupancy of Cebpd and Cebpb mRNA was enhanced by m6A modification. IMP2 facilitated IL-17-mediated Cebpd mRNA stabilization and promoted translation of C/EBPβ/δ in response to IL-17A, IL-17F, and TNFα. RNA sequencing revealed transcriptome-wide IL-17-induced transcripts that are IMP2 influenced, and RNA immunoprecipitation sequencing identified the subset of mRNAs that are directly occupied by IMP2, which included Cebpb and Cebpd Lipocalin-2 (Lcn2), a hallmark of autoimmune kidney injury, was strongly dependent on IL-17, IMP2, and C/EBPβ/δ. Imp2-/- mice were resistant to autoantibody-induced glomerulonephritis (AGN), showing impaired renal expression of C/EBPs and Lcn2 Moreover, IMP2 deletion initiated only after AGN onset ameliorated disease. Thus, posttranscriptional regulation of C/EBPs through m6A/IMP2 represents a previously unidentified paradigm of cytokine-driven autoimmune inflammation.

Stability of blueberry anthocyanin, anthocyanidin and pyranoanthocyanidin pigments and their inhibitory effects and mechanisms in human cervical cancer HeLa cells

The aim of this study was to conduct a comparative analysis on the stability and anticancer activities of anthocyanin, anthocyanidin and pyranoanthocyanidin pigments derived from blueberries.

HuR silencing elicits oxidative stress and DNA damage and sensitizes human triple-negative breast cancer cells to radiotherapy

HuR is an mRNA-binding protein whose overexpression in cancer cells has been associated with poor prognosis and resistance to therapy. While reports on HuR overexpression contributing to chemoresistance exist, limited information is available on HuR and radioresistance especially in triple-negative breast cancer (TNBC).

In this study we investigated the role of HuR in radiation resistance in three TNBC (MDA-MB-231, MDA-MB-468 and Hs578t) cell lines. Endogenous HuR expression was higher in TNBC cells compared to normal cells. siRNA mediated knockdown of HuR (siHuR) markedly reduced HuR mRNA and protein levels compared to scrambled siRNA (siScr) treatment. Further, siHuR treatment sensitized TNBC cells to ionizing radiation at 2 Gy compared to siScr treatment as evidenced by the significant reduction in clonogenic cell survival from 59%, 49%, and 65% in siScr-treated cells to 40%, 33%, and 46% in siHuR-treated MDA-MB-231, MDA-MB-468 and Hs578t cells, respectively. Molecular studies showed increased ROS production and inhibition of thioredoxin reductase (TrxR) in HuR knockdown cells contributed to radiosensitization. Associated with increased ROS production was evidence of increased DNA damage, demonstrated by a significant increase (p < 0.05) in γ-H2AX foci that persisted for up to 24 h in siHuR plus radiation treated cells compared to control cells. Further, comet assay revealed that HuR-silenced cells had larger and longer-lasting tails than control cells, indicating higher levels of DNA damage. In conclusion, our studies demonstrate that HuR knockdown in TNBC cells elicits oxidative stress and DNA damage resulting in radiosensitization.

Bortezomib-induced miRNAs direct epigenetic silencing of locus genes and trigger apoptosis in leukemia

MicroRNAs (miRNAs) have been suggested to repress transcription via binding the 3′-untranslated regions of mRNAs. However, the involvement and details of miRNA-mediated epigenetic regulation, particularly in targeting genomic DNA and mediating epigenetic regulation, remain largely uninvestigated. In the present study, transcription factor CCAAT/enhancer binding protein delta (CEBPD) was responsive to the anticancer drug bortezomib, a clinical and highly selective drug for leukemia treatment, and contributed to bortezomib-induced cell death. Interestingly, following the identification of CEBPD-induced miRNAs, we found that miR-744, miR-3154 and miR-3162 could target CpG islands in the 5′-flanking region of the CEBPD gene. We previously demonstrated that the Yin Yang 1 (YY1)/polycomb group (PcG) protein/DNA methyltransferase (DNMT) complex is important for CCAAT/enhancer binding protein delta (CEBPD) gene inactivation; we further found that Argonaute 2 (Ago2) interacts with YY1 and binds to the CEBPD promoter. The miRNA/Ago2/YY1/PcG group protein/DNMT complex linked the inactivation of CEBPD and genes adjacent to its 5′-flanking region, including protein kinase DNA-activated catalytic polypeptide (PRKDC), minichromosome maintenance-deficient 4 (MCM4) and ubiquitin-conjugating enzyme E2 variant 2 (UBE2V2), upon bortezomib treatment. Moreover, we revealed that miRNA binding is necessary for YY1/PcG group protein/DNMT complex-mediated epigenetic gene silencing and is associated with bortezomib-induced methylation on genomic DNA. The present study successfully characterized the interactions of the miRNA/Ago2/YY1/PcG group protein/DNMT complex and provided new insights for miRNA-mediated epigenetic regulation in bortezomib-induced leukemic cell arrest and cell death.

Distinct cis elements in the 3' UTR of the C. elegans cebp-1 mRNA mediate its regulation in neuronal development

The 3' untranslated regions (3' UTRs) of mRNAs mediate post-transcriptional regulation of genes in many biological processes. Cis elements in 3' UTRs can interact with RNA-binding factors in sequence-specific or structure-dependent manners, enabling regulation of mRNA stability, translation, and localization. Caenorhabditis elegans CEBP-1 is a conserved transcription factor of the C/EBP family, and functions in diverse contexts, from neuronal development and axon regeneration to organismal growth. Previous studies revealed that the levels of cebp-1 mRNA in neurons depend on its 3' UTR and are also negatively regulated by the E3 ubiquitin ligase RPM-1. Here, by systematically dissecting cebp-1's 3' UTR, we test the roles of specific cis elements in cebp-1 expression and function in neurons. We present evidence for a putative stem-loop in the cebp-1 3' UTR that contributes to basal expression levels of mRNA and to negative regulation by rpm-1. Mutant animals lacking the endogenous cebp-1 3' UTR showed a noticeable increased expression of cebp-1 mRNA and enhanced the neuronal developmental phenotypes of rpm-1 mutants. Our data reveal multiple cis elements within cebp-1's 3' UTR that help to optimize CEBP-1 expression levels in neuronal development.

Up-regulation of Tight-Junction Proteins by p38 Mitogen-Activated Protein Kinase/p53 Inhibition Leads to a Reduction of Injury to the Intestinal Mucosal Barrier in Severe Acute Pancreatitis

OBJECTIVES

The aim of this study was to explore the role of the p38 mitogen-activated protein kinase (p38MAPK)/p53 signaling pathway in injury to the intestinal mucosal barrier during severe acute pancreatitis (SAP).

METHODS

Both sham operation and SAP groups had 3 subgroups analyzed 3, 6, or 12 hours after the SAP induction. The concentrations of amylase, endotoxin, diamine oxidase, tumor necrosis factor α, and phospho-p38MAPK, p53, and caspase-3 and the messenger RNA levels of zonula occludens protein-1 and occludin in the intestine were measured. Immunohistochemical staining was used to determine the expression of zonula occludens protein-1 and occludin. Pathological changes of the pancreas and intestine were also assessed. Then, rats were randomly assigned to 5 groups-sham operation group, SAP group, 3 groups treated with different concentrations of p38MAPK-inhibitor SB203580-and the abovementioned experiment was repeated and analyzed 6 hours after the SAP induction.

RESULTS

The phospho-p38MAPK reached a peak value at 6 hours after the SAP induction with obvious pathological injury to the pancreas and intestine. Treatment with SB203580 led to a less damage to the pancreatic and intestinal tissues.

CONCLUSIONS

These results suggest that SAP activates the p38MAPK/p53 signaling pathway and induces injury to the intestinal mucosal barrier, which can be alleviated by inhibiting the p38MAPK/p53 pathway.

C/EBPβ and C/EBPδ transcription factors: Basic biology and roles in the CNS

CCAAT/enhancer binding protein (C/EBP) β and C/EBPδ are transcription factors of the basic-leucine zipper class which share phylogenetic, structural and functional features. In this review we first describe in depth their basic molecular biology which includes fascinating aspects such as the regulated use of alternative initiation codons in the C/EBPβ mRNA. The physical interactions with multiple transcription factors which greatly opens the number of potentially regulated genes or the presence of at least five different types of post-translational modifications are also remarkable molecular mechanisms that modulate C/EBPβ and C/EBPδ function. In the second part, we review the present knowledge on the localization, expression changes and physiological roles of C/EBPβ and C/EBPδ in neurons, astrocytes and microglia. We conclude that C/EBPβ and C/EBPδ share two unique features related to their role in the CNS: whereas in neurons they participate in memory formation and synaptic plasticity, in glial cells they regulate the pro-inflammatory program. Because of their role in neuroinflammation, C/EBPβ and C/EBPδ in microglia are potential targets for treatment of neurodegenerative disorders. Any strategy to reduce C/EBPβ and C/EBPδ activity in neuroinflammation needs to take into account its potential side-effects in neurons. Therefore, cell-specific treatments will be required for the successful application of this strategy.

The many faces of C/EBPδ and their relevance for inflammation and cancer

The CCAAT/enhancer binding protein delta (CEBPD, C/EBPδ) is a transcription factor that modulates many biological processes including cell differentiation, motility, growth arrest, proliferation, and cell death. The diversity of C/EBPδ's functions depends in part on the cell type and cellular context and can have opposing outcomes. For example, C/EBPδ promotes inflammatory signaling, but it can also inhibit pro-inflammatory pathways, and in a mouse model of mammary tumorigenesis, C/EBPδ reduces tumor incidence but promotes tumor metastasis. This review highlights the multifaceted nature of C/EBPδ's functions, with an emphasis on pathways that are relevant for cancer and inflammation, and illustrates how C/EBPδ emerged from the shadow of its family members as a fascinating “jack of all trades.” Our current knowledge on C/EBPδ indicates that, rather than being essential for a specific cellular process, C/EBPδ helps to interpret a variety of cues in a cell-type and context-dependent manner, to adjust cellular functions to specific situations. Therefore, insights into the roles and mechanisms of C/EBPδ signaling can lead to a better understanding of how the integration of different signaling pathways dictates normal and pathological cell functions and physiology.

Multiple functions of the RNA-binding protein HuR in cancer progression, treatment responses and prognosis

The human embryonic lethal abnormal vision-like protein, HuR, is a member of the Hu family of RNA-binding proteins. Over the past decade, this ubiquitously expressed protein has been extensively investigated in cancer research because it is involved in the regulation of mRNA stability and translation in many cell types. HuR activity and function is associated with its subcellular distribution, transcriptional regulation, translational and post-translational modifications. HuR regulation of target mRNAs is based on the interaction between the three specific domains of HuR protein and one or several U- or AU-rich elements (AREs) in the untranslated region of target mRNAs. A number of cancer-related transcripts containing AREs, including mRNAs for proto-oncogenes, cytokines, growth factors, and invasion factors, have been characterized as HuR targets. It has been proposed that HuR has a central tumorigenic activity by enabling multiple cancer phenotypes. In this review, we comprehensively survey the existing evidence with regard to the diverse functions of HuR in caner development and progression. The current data also suggest that HuR might be a novel and promising therapeutic target and a marker for treatment response and prognostic evaluation.

Sox9 modulates cell survival and adipogenic differentiation of multipotent adult rat mesenchymal stem cells

Sox9 is a key transcription factor in early chondrogenesis with distinct roles in differentiation processes and during embryonic development. Here, we report that Sox9 modulates cell survival and contributes to the commitment of mesenchymal stem cells (MSC) to adipogenic or osteogenic differentiation lineages. We found that the Sox9 activity level affects the expression of the key transcription factor in adipogenic differentiation, C/EBPβ, and that cyclin D1 mediates the expression of the osteogenic marker osteocalcin in undifferentiated adult bone-marrow-derived rat MSC. Introducing a stable Sox9 knockdown into undifferentiated rat MSC resulted in a marked decrease in proliferation rate and an increase in apoptotic activity. This was linked to a profound upregulation of p21 and cyclin D1 gene and protein expression accompanied by an induction of caspase 3/7 activity and an inhibition of Bcl-2. We observed that Sox9 silencing provoked a delayed S-phase progression and an increased nuclear localization of p21. The protein stability of cyclin D1 was induced in the absence of Sox9 presumably as a function of altered p38 signalling. In addition, the major transcription factor for adipogenic differentiation, C/EBPβ, was repressed after silencing Sox9. The nearly complete absence of C/EBPβ protein as a result of increased destabilization of the C/EBPβ mRNA and the impact on osteocalcin gene expression and protein synthesis, suggests that a delicate balance of Sox9 level is not only imperative for proper chondrogenic differentiation of progenitor cells, but also affects the adipogenic and probably osteogenic differentiation pathways of MSC. Our results identified Sox9 as an important link between differentiation, proliferation and apoptosis in undifferentiated adult rat mesenchymal stem cells, emphasizing the importance of the delicate balance of a precisely regulated Sox9 activity in MSC not only for proper skeletal development during embryogenesis but probably also for successful repair and regeneration of tissues and organs in adults.

Genomic analysis of the role of transcription factor C/EBPδ in the regulation of cell behaviour on nanometric grooves

C/EBPδ is a tumour suppressor transcription factor that induces gene expression involved in suppressing cell migration. Here we investigate whether C/EBPδ-dependent gene expression also affects cell responses to nanometric topology. We found that ablation of the C/EBPδ gene in mouse embryonal fibroblasts (MEFs) decreased cell size, adhesion and cytoskeleton spreading on 240 nm and 540 nm nanometric grooves. ChIP-SEQ and cDNA microarray analyses demonstrated that many binding sites for C/EBPδ, and the closely related C/EBPβ, exist throughout the mouse genome and control the upregulation or downregulation of many adjacent genes. We also identified a group of C/EBPδ-dependent, trans-regulated genes, whose promoters contained no C/EBPδ binding sites and yet their activity was regulated in a C/EBPδ-dependent manner. These genes include signalling molecules (e.g. SOCS3), cytoskeletal components (Tubb2, Krt16 and Krt20) and cytoskeletal regulators (ArhGEF33 and Rnd3) and are possibly regulated by cis-regulated diffusible mediators, such as IL6. Of particular note, SOCS3 was shown to be absolutely required for efficient cell spreading and contact guidance on 240 nm and 540 nm nanometric grooves. C/EBPδ is therefore involved in the complex regulation of multiple genes, including cytoskeletal components and signalling mediators, which influence the nature of cell interactions with nanometric topology.

Transcriptional repression of ATF4 gene by CCAAT/enhancer-binding protein β (C/EBPβ) differentially regulates integrated stress response

Different environmental stresses induce the phosphorylation of eIF2 (eIF2∼P), repressing global protein synthesis coincident with preferential translation of ATF4. ATF4 is a transcriptional activator of genes involved in metabolism and nutrient uptake, antioxidation, and regulation of apoptosis. Because ATF4 is a common downstream target that integrates signaling from different eIF2 kinases and their respective stress signals, the eIF2∼P/ATF4 pathway is collectively referred to as the integrated stress response. Although eIF2∼P elicits translational control in response to many different stresses, there are selected stresses, such as exposure to UV irradiation, that do not increase ATF4 expression despite robust eIF2∼P. The rationale for this discordant induction of ATF4 expression and eIF2∼P in response to UV irradiation is that transcription of ATF4 is repressed, and therefore ATF4 mRNA is not available for preferential translation. In this study, we show that C/EBPβ is a transcriptional repressor of ATF4 during UV stress. C/EBPβ binds to critical elements in the ATF4 promoter, resulting in its transcriptional repression. Expression of C/EBPβ increases in response to UV stress, and the liver-enriched inhibitory protein (LIP) isoform of C/EBPβ, but not the liver-enriched activating protein (LAP) version, represses ATF4 transcription. Loss of the liver-enriched inhibitory protein isoform results in increased ATF4 mRNA levels in response to UV irradiation and subsequent recovery of ATF4 translation, leading to enhanced expression of its target genes. Together these results illustrate how eIF2∼P and translational control combined with transcription factors regulated by alternative signaling pathways can direct programs of gene expression that are specifically tailored to each environmental stress.

HITS-CLIP: panoramic views of protein-RNA regulation in living cells

The study of gene regulation in cells has recently begun to shift from a period dominated by the study of transcription factor-DNA interactions to a new focus on RNA regulation. This was sparked by the still-emerging recognition of the central role for RNA in cellular complexity emanating from the RNA World hypothesis, and has been facilitated by technologic advances, in particular high throughput RNA sequencing and crosslinking methods (RNA-Seq, CLIP, and HITS-CLIP). This study will place these advances in context, and, focusing on CLIP, will explain the method, what it can be used for, and how to approach using it. Examples of the successes, limitations, and future of the technique will be discussed.

Function-specific intracellular signaling pathways downstream of heparin-binding EGF-like growth factor utilized by human trophoblasts

Heparin-binding EGF-like growth factor (HBEGF) is expressed by trophoblast cells throughout gestation. First-trimester cytotrophoblast cells are protected from hypoxia-induced apoptosis because of the accumulation of HBEGF through a posttranscriptional autocrine mechanism. Exogenous application of HBEGF is cytoprotective in a hypoxia/reoxygenation (H/R) injury model and initiates trophoblast extravillous differentiation to an invasive phenotype. The downstream signaling pathways induced by HBEGF that mediate these various cellular activities were identified using two human first-trimester cytotrophoblast cell lines, HTR-8/SVneo and SW.71, with similar results. Recombinant HBEGF (1 nM) induced transient phosphorylation of MAPK3/1 (ERK), MAPK14 (p38), and AKT within 15 min and JNK after 1-2 h. To determine which downstream pathways regulate the various functions of HBEGF, cells were treated with specific inhibitors of the ERK upstream regulator MEK (U0126), the AKT upstream regulator phosphoinositide-3 (PI3)-kinase (LY294002), MAPK14 (SB203580), and JNK (SP600125), as well as with inactive structural analogues. Only SB203580 specifically prevented HBEGF-mediated rescue during H/R, while each inhibitor attenuated HBEGF-stimulated cell migration. Accumulation of HBEGF at reduced oxygen was blocked only by a combination of U0126, SB203580, and SP600125. We conclude that HBEGF advances trophoblast extravillous differentiation through coordinate activation of PI3 kinase, ERK, MAPK14, and JNK, while only MAPK14 is required for its antiapoptotic activity. Additionally, hypoxia induces an autocrine increase in HBEGF protein levels through MAPK14, JNK or ERK. These experiments reveal a complexity of the intracellular signaling circuitry that regulates trophoblast functions critical for implantation and placentation.

Interacting with HBsAg compromises resistance of jumping translocation breakpoint protein to ultraviolet radiation-induced apoptosis in 293FT cells

Jumping translocation breakpoint protein (JTB) is suppressed in many cancers, implying it plays a role in the neoplastic transformation of cells. In order to explore the role of JTB in the carcinogenesis of liver, we used mammalian two-hybrid, co-immunoprecipitation, GST pull-down and laser scanning confocal to verify the interaction between HBs and JTB. According to the results, HBs interacts with JTB. In addition, we further determined that S region within HBs is sufficient for binding JTB. Overexpression of JTB conferred resistance to apoptosis induced by ultraviolet radiation, whereas this effect was compromised by the co-overexpression of HBs.

Identification and characterization of CCAAT/Enhancer Binding proteindelta (C/EBPdelta) target genes in G0 growth arrested mammary epithelial cells

BACKGROUND

CCAAT/Enhancer Binding Proteindelta (C/EBPdelta) is a member of the highly conserved C/EBP family of leucine zipper (bZIP) proteins. C/EBPdelta is highly expressed in G0 growth arrested mammary epithelial cells (MECs) and "loss of function" alterations in C/EBPdelta have been associated with impaired contact inhibition, increased genomic instability and increased cell migration. Reduced C/EBPdelta expression has also been reported in breast cancer and acute myeloid leukemia (AML). C/EBPdelta functions as a transcriptional activator, however, only a limited number of C/EBPdelta target genes have been reported. As a result, the role of C/EBPdelta in growth control and the potential mechanisms by which "loss of function" alterations in C/EBPdelta contribute to tumorigenesis are poorly understood. The goals of the present study were to identify C/EBPdelta target genes using Chromatin Immunoprecipitation coupled with a CpG Island (HCG12K) Array gene chip ("ChIP-chip") assay and to assess the expression and potential functional roles of C/EBPdelta target genes in growth control.

RESULTS

ChIP-chip assays identified approximately 100 C/EBPdelta target gene loci which were classified by gene ontology (GO) into cell adhesion, cell cycle regulation, apoptosis, signal transduction, intermediary metabolism, gene transcription, DNA repair and solute transport categories. Conventional ChIP assays validated the ChIP-chip results and demonstrated that 14/14 C/EBPdelta target loci were bound by C/EBPdelta in G0 growth arrested MCF-12A MECs. Gene-specific RT-PCR analysis also demonstrated C/EBPdelta-inducible expression of 14/14 C/EBPdelta target genes in G0 growth arrested MCF-12A MECs. Finally, expression of endogenous C/EBPdelta and selected C/EBPdelta target genes was also demonstrated in contact-inhibited G0 growth arrested nontransformed human MCF-10A MECs and in mouse HC11 MECs. The results demonstrate consistent activation and downstream function of C/EBPdelta in growth arrested human and murine MECs.

CONCLUSION

C/EBPdelta target genes were identified by a global gene array approach and classified into functional categories that are consistent with biological contexts in which C/EBPdelta is induced, such as contact-mediated G0 growth arrest, apoptosis, metabolism and inflammation. The identification and validation of C/EBPdelta target genes provides new insights into the mechanistic role of C/EBPdelta in mammary epithelial cell biology and sheds new light on the potential impact of "loss of function" alterations in C/EBPdelta in tumorigenesis.

P38MAPK regulates caspase-3 by binding to caspase-3 in nucleus of human hepatoma Bel-7402 cells during anti-Fas antibody- and actinomycin D-induced apoptosis

Anti-Fas antibody- and actinomycin D (FA/AD) has been shown to have anti-tumor activity in some tumor cells. However, many of the molecular mechanism of FA/AD-induced apoptosis of human hepatoma Bel-7402 cells have not been fully clarified. In the present study, therefore, the effect of FA/AD in presence or absence of p38MAPK inhibitor SB203580 on the proliferation, apoptosis, p38MAPK, caspase-3, location of p38MAPK and caspase-3, and interaction between p38MAPK and caspase-3 in Bel-7402 cell was investigated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT), annexin V-FITC/propidium iodide (PI) double staining, electron microscopy, immunoblot, immunofluorescence and immunoprecipitation/immunoblot assay, respectively. We found that FA/AD significantly resulted in the inhibition of proliferation, induction of apoptosis, activation and up-regulation of p38MAPK, activation and up-regulation of caspase-3, translocation of p38MAPK and caspase-3 from cytosol to nucleus, and formation of p38MAPK/caspase-3 complex in Bel-7402 cells. In contrast, SB203580, a p38MAPK-specific inhibitor, apparently blocked induction of apoptosis, activation and up-regulation of p38MAPK, activation and up-regulation of caspase-3, and translocation of p38MAPK and caspase-3 from cytosol to nucleus in FA/AD-treated Bel-7402 cells. Taken together, we conclude that p38MAPK regulates caspase-3 by binding to caspase-3 in nucleus of Bel-7402 cells during FA/AD-induced apoptosis.

Role of mitogen-activated protein kinases in aryl hydrocarbon receptor signaling

Human populations are increasingly exposed to a number of environmental pollutants such as polycyclic aromatic hydrocarbons, polychlorinated biphenyls and dioxins. These compounds are activators of the aryl hydrocarbon receptor (AhR) that controls the expression of many genes including those for detoxification enzymes. The regulatory mechanisms of AhR are multi-factorial and include phosphorylation by various protein kinases. Significant progress in the research of mitogen-activated protein kinases (MAPKs) has been achieved in the last decade. Isolated reports have been published on the role of MAPKs in AhR functions and vice versa, with activation of MAPKs by AhR ligands. This mini-review summarizes current knowledge on the mutual interactions between MAPKs and AhR. The majority of studies has been done on cancer-derived cell lines that have impaired cell cycle regulation and lacks the complete detoxification apparatus. We emphasize the importance of the future studies that should be done on non-transformed cells to distinguish the role of MAPKs in cancer and normal cells. Primary cultures of human or rodent hepatocytes that are equipped with a fully functional biotransformation battery or xenobiotics-metabolizing extra-hepatic tissues should be the models of choice, as the results in our experiments confirm.