Induction of the Ras activator Son of Sevenless 1 by environmental pollutants mediates their effects on cellular proliferation

Sensory nerves directly promote osteoclastogenesis by secreting peptidyl-prolyl cis-trans isomerase D (Cyp40)

Given afferent functions, sensory nerves have recently been found to exert efferent effects and directly alter organ physiology. Additionally, several studies have highlighted the indirect but crucial role of sensory nerves in the regulation of the physiological function of osteoclasts. Nonetheless, evidence regarding the direct sensory nerve efferent influence on osteoclasts is lacking. In the current study, we found that high levels of efferent signals were transported directly from the sensory nerves into osteoclasts. Furthermore, sensory hypersensitivity significantly increased osteoclastic bone resorption, and sensory neurons (SNs) directly promoted osteoclastogenesis in an in vitro coculture system. Moreover, we screened a novel neuropeptide, Cyp40, using an isobaric tag for relative and absolute quantitation (iTRAQ). We observed that Cyp40 is the efferent signal from sensory nerves, and it plays a critical role in osteoclastogenesis via the aryl hydrocarbon receptor (AhR)-Ras/Raf-p-Erk-NFATc1 pathway. These findings revealed a novel mechanism regarding the influence of sensory nerves on bone regulation, i.e., a direct promoting effect on osteoclastogenesis by the secretion of Cyp40. Therefore, inhibiting Cyp40 could serve as a strategy to improve bone quality in osteoporosis and promote bone repair after bone injury.

Role of RAS signaling in ovarian cancer

The RAS family of proteins is among the most frequently mutated genes in human malignancies. In ovarian cancer (OC), the most lethal gynecological malignancy, RAS, especially KRAS mutational status at codons 12, 13, and 61, ranges from 6-65% spanning different histo-types. Normally RAS regulates several signaling pathways involved in a myriad of cellular signaling cascades mediating numerous cellular processes like cell proliferation, differentiation, invasion, and death. Aberrant activation of RAS leads to uncontrolled induction of several downstream signaling pathways such as RAF-1/MAPK (mitogen-activated protein kinase), PI3K phosphoinositide-3 kinase (PI3K)/AKT, RalGEFs, Rac/Rho, BRAF (v-Raf murine sarcoma viral oncogene homolog B), MEK1 (mitogen-activated protein kinase kinase 1), ERK (extracellular signal-regulated kinase), PKB (protein kinase B) and PKC (protein kinase C) involved in cell proliferation as well as maintenance pathways thereby driving tumorigenesis and cancer cell propagation. KRAS mutation is also known to be a biomarker for poor outcome and chemoresistance in OC. As a malignancy with several histotypes showing varying histopathological characteristics, we focus on reviewing recent literature showcasing the involvement of oncogenic RAS in mediating carcinogenesis and chemoresistance in OC and its subtypes.

Sulfarotene, a synthetic retinoid, overcomes stemness and sorafenib resistance of hepatocellular carcinoma via suppressing SOS2-RAS pathway

BACKGROUND

Recurrent hepatocellular carcinoma (HCC) shows strong resistance to sorafenib, and the tumor-repopulating cells (TRCs) with cancer stem cell-like properties are considered a driver for its high recurrent rate and drug resistance.

METHODS

Suppression of TRCs may thus be an effective therapeutic strategy for treating this fatal disease. We evaluated the pharmacology and mechanism of sulfarotene, a new type of synthetic retinoid, on the cancer stem cell-like properties of HCC TRCs, and assessed its preclinical efficacy in models of HCC patient-derived xenografts (PDXs).

RESULTS

Sulfarotene selectively inhibited the growth of HCC TRCs in vitro and significantly deterred TRC-mediated tumor formation and lung metastasis in vivo without apparent toxicity, with an IC₅₀ superior to that of acyclic retinoid and sorafenib, to which the recurrent HCC exhibits significant resistance at advanced stage. Sulfarotene promoted the expression and activation of RARα, which down-regulated SOS2, a key signal mediator associated with RAS activation and signal transduction involved in multiple downstream pathways. Moreover, sulfarotene selectively inhibited tumorigenesis of HCC PDXs with high expression for SOS2.

CONCLUSIONS

Our study identified sulfarotene as a selective inhibitor for the TRCs of HCC, which targets a novel RARα-SOS2-RAS signal nexus, shedding light on a new, promising strategy of target therapy for advanced liver cancer.

A New Insight into the Potential Role of Tryptophan-Derived AhR Ligands in Skin Physiological and Pathological Processes

The aryl hydrocarbon receptor (AhR) plays a crucial role in environmental responses and xenobiotic metabolism, as it controls the transcription profiles of several genes in a ligand-specific and cell-type-specific manner. Various barrier tissues, including skin, display the expression of AhR. Recent studies revealed multiple roles of AhR in skin physiology and disease, including melanogenesis, inflammation and cancer. Tryptophan metabolites are distinguished among the groups of natural and synthetic AhR ligands, and these include kynurenine, kynurenic acid and 6-formylindolo[3,2-b]carbazole (FICZ). Tryptophan derivatives can affect and regulate a variety of signaling pathways. Thus, the interest in how these substances influence physiological and pathological processes in the skin is expanding rapidly. The widespread presence of these substances and potential continuous exposure of the skin to their biological effects indicate the important role of AhR and its ligands in the prevention, pathogenesis and progression of skin diseases. In this review, we summarize the current knowledge of AhR in skin physiology. Moreover, we discuss the role of AhR in skin pathological processes, including inflammatory skin diseases, pigmentation disorders and cancer. Finally, the impact of FICZ, kynurenic acid, and kynurenine on physiological and pathological processes in the skin is considered. However, the mechanisms of how AhR regulates skin function require further investigation.

SOS GEFs in health and disease

SOS1 and SOS2 are the most universal and widely expressed family of guanine exchange factors (GEFs) capable or activating RAS or RAC1 proteins in metazoan cells. SOS proteins contain a sequence of modular domains that are responsible for different intramolecular and intermolecular interactions modulating mechanisms of self-inhibition, allosteric activation and intracellular homeostasis. Despite their homology, analyses of SOS1/2-KO mice demonstrate functional prevalence of SOS1 over SOS2 in cellular processes including proliferation, migration, inflammation or maintenance of intracellular redox homeostasis, although some functional redundancy cannot be excluded, particularly at the organismal level. Specific SOS1 gain-of-function mutations have been identified in inherited RASopathies and various sporadic human cancers. SOS1 depletion reduces tumorigenesis mediated by RAS or RAC1 in mouse models and is associated with increased intracellular oxidative stress and mitochondrial dysfunction. Since WT RAS is essential for development of RAS-mutant tumors, the SOS GEFs may be considered as relevant biomarkers or therapy targets in RAS-dependent cancers. Inhibitors blocking SOS expression, intrinsic GEF activity, or productive SOS protein-protein interactions with cellular regulators and/or RAS/RAC targets have been recently developed and shown preclinical and clinical effectiveness blocking aberrant RAS signaling in RAS-driven and RTK-driven tumors.

2,3,7,8-Tetrachlorodibenzo-p-dioxin promotes injury-induced vascular neointima formation in mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an environmental pollutant that causes cardiovascular toxicity. The phenotypic transformation of vascular smooth muscle cells (VSMCs) from the contractile to the synthetic phenotype is a hallmark of vascular response to injury. However, the precise role and molecular mechanism of TCDD in vascular remodeling remains unknown. In the present study, we found that TCDD treatment promoted VSMC phenotypic transition from contractile to synthetic phenotype and exaggerated vascular neointimal hyperplasia after wire injury in mice. TCDD treatment enhanced VSMC entry into cell cycle from G0/G1 phase to S and G2/M phase. The expression of cyclin D1, cyclin-dependent kinase 4 (CDK4), and its phosphorylation were coordinately increased in response to TCDD treatment. Knocking down of aryl hydrocarbon receptor (AHR) inhibited VSMC phenotypic transition induced by TCDD and promoted S/G2 phase cell cycle arrest. TCDD treatment markedly increased oncogenic c-Jun gene expression in VSMCs. ChIP assay revealed the direct binding of AHR on the promoter of c-Jun to up-regulate the mRNA expression of c-Jun. Silencing of c-Jun gene enhanced the expression of p53 and p21, whereas attenuated the expression of CDK4 and cyclin D1 leading to the decrease in the TCDD-stimulated VSMC proliferation and synthetic phenotype transition in vitro. In vivo study showed that genetic ablation of c-Jun in VSMCs restricted injury-induced neointimal hyperplasia in TCDD-treated mice. Thus, TCDD exposure exaggerated injury-induced vascular remodeling by the activation of AHR and up-regulation of the expression of its target gene c-Jun, indicating that inhibition of AHR may be a promising prevention strategy for TCDD-associated cardiovascular diseases.-Guo, S., Zhang, R., Liu, Q., Wan, Q., Wang, Y., Yu, Y., Liu, G., Shen, Y., Yu, Y., Zhang, J. 2,3,7,8-Tetrachlorodibenzo-p-dioxin promotes injury-induced vascular neointima formation in mice.

The Aryl Hydrocarbon Receptor and the Nervous System

The aryl hydrocarbon receptor (or AhR) is a cytoplasmic receptor of pollutants. It translocates into the nucleus upon binding to its ligands, and forms a heterodimer with ARNT (AhR nuclear translocator). The heterodimer is a transcription factor, which regulates the transcription of xenobiotic metabolizing enzymes. Expressed in many cells in vertebrates, it is mostly present in neuronal cell types in invertebrates, where it regulates dendritic morphology or feeding behavior. Surprisingly, few investigations have been conducted to unravel the function of the AhR in the central or peripheral nervous systems of vertebrates. In this review, we will present how the AhR regulates neural functions in both invertebrates and vertebrates as deduced mainly from the effects of xenobiotics. We will introduce some of the molecular mechanisms triggered by the well-known AhR ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which impact on neuronal proliferation, differentiation, and survival. Finally, we will point out the common features found in mice that are exposed to pollutants, and in AhR knockout mice.

Differential Role of the RasGEFs Sos1 and Sos2 in Mouse Skin Homeostasis and Carcinogenesis

Using Sos1 knockout (Sos1-KO), Sos2-KO, and Sos1/2 double-knockout (Sos1/2-DKO) mice, we assessed the functional role of Sos1 and Sos2 in skin homeostasis under physiological and/or pathological conditions. Sos1 depletion resulted in significant alterations of skin homeostasis, including reduced keratinocyte proliferation, altered hair follicle and blood vessel integrity in dermis, and reduced adipose tissue in hypodermis. These defects worsened significantly when both Sos1 and Sos2 were absent. Simultaneous Sos1/2 disruption led to severe impairment of the ability to repair skin wounds, as well as to almost complete ablation of the neutrophil-mediated inflammatory response in the injury site. Furthermore, Sos1 disruption delayed the onset of tumor initiation, decreased tumor growth, and prevented malignant progression of papillomas in a DMBA (7,12-dimethylbenz[α]anthracene)/TPA (12-O-tetradecanoylphorbol-13-acetate)-induced skin carcinogenesis model. Finally, Sos1 depletion in preexisting chemically induced papillomas resulted also in decreased tumor growth, probably linked to significantly reduced underlying keratinocyte proliferation. Our data unveil novel, distinctive mechanistic roles of Sos 1 and Sos2 in physiological control of skin homeostasis and wound repair, as well as in pathological development of chemically induced skin tumors. These observations underscore the essential role of Sos proteins in cellular proliferation and migration and support the consideration of these RasGEFs as potential biomarkers/therapy targets in Ras-driven epidermal tumors.

Sos1 disruption impairs cellular proliferation and viability through an increase in mitochondrial oxidative stress in primary MEFs

Using a 4-hydroxytamoxifen (4OHT)-inducible, conditional Sos1-null mutation, we analyzed wild-type (WT), single Sos1-KO, Sos2-KO and double Sos1/2 KO primary mouse embryonic fibroblasts (MEF) with an aim at evaluating the functional specificity or redundancy of the Sos1 and Sos2 alleles at the cellular level. The 4OHT-induced Sos1-KO and Sos1/2-DKO MEFs exhibited distinct flat morphology, enlarged cell perimeter and altered cytoskeletal organization that were not observed in the WT and Sos2-KO counterparts. The Sos1-KO and Sos1/2-DKO MEFs also displayed significant accumulation, in comparison with WT and Sos2-KO MEFs, of cytoplasmic vesicular bodies identified as autophagosomes containing degraded mitochondria by means of electron microscopy and specific markers. Cellular proliferation and migration were impaired in Sos1-KO and Sos1/2-DKO MEFs in comparison with WT and Sos2-KO MEFs, whereas cell adhesion was only impaired upon depletion of both Sos isoforms. RasGTP formation was practically absent in Sos1/2-DKO MEFs as compared with the other genotypes and extracellular signal-regulated kinase phosphorylation showed only significant reduction after combined Sos1/2 depletion. Consistent with a mitophagic phenotype, in vivo labeling with specific fluorophores uncovered increased levels of oxidative stress (elevated intracellular reactive oxygen species and mitochondrial superoxide and loss of mitochondrial membrane potential) in the Sos1-KO and the Sos1/2-DKO cells as compared with Sos2-KO and WT MEFs. Interestingly, treatment of the MEF cultures with antioxidants corrected the altered phenotypes of Sos1-KO and Sos1/2-DKO MEFs by restoring their altered perimeter size and proliferative rate to levels similar to those of WT and Sos2-KO MEFs. Our data uncover a direct mechanistic link between Sos1 and control of intracellular oxidative stress, and demonstrate functional prevalence of Sos1 over Sos2 with regards to cellular proliferation and viability.

Functional and molecular characterization of mouse Gata2-independent hematopoietic progenitors

The Gata2 transcription factor is a pivotal regulator of hematopoietic cell development and maintenance, highlighted by the fact that Gata2 haploinsufficiency has been identified as the cause of some familial cases of acute myelogenous leukemia/myelodysplastic syndrome and in MonoMac syndrome. Genetic deletion in mice has shown that Gata2 is pivotal to the embryonic generation of hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs). It functions in the embryo during endothelial cell to hematopoietic cell transition to affect hematopoietic cluster, HPC, and HSC formation. Gata2 conditional deletion and overexpression studies show the importance of Gata2 levels in hematopoiesis, during all developmental stages. Although previous studies of cell populations phenotypically enriched in HPCs and HSCs show expression of Gata2, there has been no direct study of Gata2 expressing cells during normal hematopoiesis. In this study, we generate a Gata2Venus reporter mouse model with unperturbed Gata2 expression to examine the hematopoietic function and transcriptome of Gata2 expressing and nonexpressing cells. We show that all the HSCs are Gata2 expressing. However, not all HPCs in the aorta, vitelline and umbilical arteries, and fetal liver require or express Gata2. These Gata2-independent HPCs exhibit a different functional output and genetic program, including Ras and cyclic AMP response element-binding protein pathways and other Gata factors, compared with Gata2-dependent HPCs. Our results, indicating that Gata2 is of major importance in programming toward HSC fate but not in all cells with HPC fate, have implications for current reprogramming strategies.

TCDD promotes lung tumors via attenuation of apoptosis through activation of the Akt and ERK1/2 signaling pathways

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a multiple-site, multiple-species carcinogen that induces cancer in multiple organs. The molecular mechanisms underlying TCDD-induced lung tumorigenesis remain unclear. In the present study, a two-stage lung tumorigenesis model was established by administrating a single low dose of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) combined with TCDD to female A/J mice. The results indicated that TCDD combined with low-dose NNK has a significant tumor-promoting effect compared with TCDD or low-dose NNK alone. Resistance to apoptosis is a hallmark of cancer and is thought to be one of the tumor-promoting mechanisms regulated by TCDD. We performed an additional series of experiments in the normal human bronchial epithelial cell line Beas2B cells, in which TCDD was combined with the apoptosis inducer staurosporine. Our in vitro results confirmed that TCDD could rescue cells from apoptosis induced by staurosporine. The inhibition of apoptosis is likely mediated by the activation of the Akt and ERK1/2 pathways, as determined by the effectiveness of pathway-specific inhibitors in abrogating the anti-apoptotic activity of TCDD. In conclusion, we demonstrated that TCDD promoted NNK-induced lung tumorigenesis and revealed that TCDD inhibits staurosporine-induced apoptosis, at least in part, through the Akt and ERK1/2 signaling pathways.

3-Methylcholanthrene, an AhR agonist, caused cell-cycle arrest by histone deacetylation through a RhoA-dependent recruitment of HDAC1 and pRb2 to E2F1 complex

We previously showed that treating vascular endothelial cells with 3-methylcholanthrene (3MC) caused cell-cycle arrest in the Go/G1 phase; this resulted from the induction of p21 and p27 and a decreased level and activity of the cyclin-dependent kinase, Cdk2. We further investigated the molecular mechanisms that modulate cell-cycle regulatory proteins through the aryl-hydrocarbon receptor (AhR)/Ras homolog gene family, member A (RhoA) dependent epigenetic modification of histone. AhR/RhoA activation mediated by 3MC was essential for the upregulation of retinoblastoma 2 (pRb2) and histone deacetylase 1 (HDAC1), whereas their nuclear translocation was primarily modulated by RhoA activation. The combination of increased phosphatase and tensin homolog (PTEN) activity and decreased phosphatidylinositide 3-kinase (PI3K) activation by 3MC led to the inactivation of the Ras-cRaf pathway, which contributed to pRb2 hypophosphorylation. Increased HDAC1/pRb2 recruitment to the E2F1 complex decreased E2F1-transactivational activity and H3/H4 deacetylation, resulting in the downregulation of cell-cycle regulatory proteins (Cdk2/4 and Cyclin D3/E). Co-immunoprecipitation and electrophoretic mobility shift assay (EMSA) results showed that simvastatin prevented the 3MC-increased binding activities of E2F1 proteins in their promoter regions. Additionally, RhoA inhibitors (statins) reversed the effect of 3MC in inhibiting DNA synthesis by decreasing the nuclear translocation of pRb2/HDAC1, leading to a recovery of the levels of cell-cycle regulatory proteins. In summary, 3MC decreased cell proliferation by the epigenetic modification of histone through an AhR/RhoA-dependent mechanism that can be rescued by statins.

2,3,7,8-Tetrachlorodibenzo-p-dioxin stimulates proliferation of HAPI microglia by affecting the Akt/GSK-3β/cyclin D1 signaling pathway

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an environmental toxin that induces apoptosis of neurons and a pro-inflammatory response in microglial cells. First, we found that TCDD induced proliferation of HAPI microglial cells in a dose- and time-dependent manner. Flow cytometry analysis showed that this proliferation by TCDD was due to mainly enhancing the G1 to S phase transition. Next, it was found that TCDD treatment led to up-regulation of cyclin D1, which induces cell cycle progression from G1 to S phase, in a time-dependent manner. As for molecular mechanism, we revealed that TCDD was capable of inducing Akt phosphorylation and activation, resulting in phosphorylation and inactivation of glycogen synthase kinase-3β (GSK-3β). Inactivated GSK-3β attenuated proteasomal degradation of cyclin D1 by reducing Thr(286)-phosphorylated cyclin D1 levels. Moreover, inactivated GSK-3β increased cyclin D1 gene transcription by increasing its transcription factor β-catenin in the nucleus. Further, blockage of phosphoinositide 3-kinase/Akt kinase with their specific inhibitors, LY294002 and Akt 1/2 kinase inhibitor, significantly reduced TCDD-enhanced proliferation of HAPI microglial cells. In conclusion, TCDD stimulates proliferation of HAPI microglial cells by affecting the Akt/GSK-3β/cyclin D1 signaling pathway.

DOK3 negatively regulates LPS responses and endotoxin tolerance

Innate immune activation via Toll-like receptors (TLRs), although critical for host defense against infection, must be regulated to prevent sustained cell activation that can lead to cell death. Cells repeatedly stimulated with lipopolysaccharide (LPS) develop endotoxin tolerance making the cells hypo-responsive to additional TLR stimulation. We show here that DOK3 is a negative regulator of TLR signaling by limiting LPS-induced ERK activation and cytokine responses in macrophages. LPS induces ubiquitin-mediated degradation of DOK3 leading to SOS1 degradation and inhibition of ERK activation. DOK3 mice are hypersensitive to sublethal doses of LPS and have altered cytokine responses in vivo. During endotoxin tolerance, DOK3 expression remains stable, and it negatively regulates the expression of SHIP1, IRAK-M, SOCS1, and SOS1. As such, DOK3-deficient macrophages are more sensitive to LPS-induced tolerance becoming tolerant at lower levels of LPS than wild type cells. Taken together, the absence of DOK3 increases LPS signaling, contributing to LPS-induced tolerance. Thus, DOK3 plays a role in TLR signaling during both naïve and endotoxin-induced tolerant conditions.

The aryl hydrocarbon receptor regulates focal adhesion sites through a non-genomic FAK/Src pathway

The aryl hydrocarbon receptor (AhR) is commonly described as a transcription factor, which regulates xenobiotic-metabolizing enzymes. Recent studies have suggested that the binding of ligands to the AhR also activates the Src kinase. In this manuscript, we show that the AhR, through the activation of Src, activates focal adhesion kinase (FAK) and promotes integrin clustering. These effects contribute to cell migration. Further, we show that the activation of the AhR increases the interaction of FAK with the metastatic marker, HEF1/NEDD9/CAS-L, and the expression of several integrins. Xenobiotic exposure, thus, may contribute to novel cell-migratory programs.

Identification of a new stilbene-derived inducer of paraoxonase 1 and ligand of the Aryl hydrocarbon Receptor

Paraoxonase 1 (PON1) is a high-density lipoprotein-associated enzyme, synthesized in the liver and secreted into the blood. PON1 displays antioxidant properties and is involved in organophosphorous compounds and oxidized lipids degradation. Because of these beneficial effects, pharmacological regulation of PON1 appears to be highly relevant in toxicology and cardiology. Recent studies undertaken on the regulation of the PON1 promoter in our laboratory have identified resveratrol, through its activation of the Aryl hydrocarbon Receptor (AhR), as a putative inducer of PON1. We have tested a new modulator of AhR, (Z)-2,3-bis (4-nitrophenyl)-acrylonitrile, and established that it is a more potent inducer of PON1 at the mRNA, protein and enzymatic activity as compared to resveratrol. It also acts by activating the AhR. However, in contrast with traditional AhR agonists, it does not induce cyp1A1 transcription. (Z)-2,3-bis (4-nitrophenyl)-acrylonitrile is therefore a specific AhR modulator targeting PON1.

Mammalian son of sevenless Guanine nucleotide exchange factors: old concepts and new perspectives

The Son of Sevenless (Sos) factors were originally discovered 2 decades ago as specialized Ras activators in signaling pathways controlling the process of R7 cell development in the eye of Drosophila melanogaster. The 2 known members of the mammalian Sos family (Sos1 and Sos2) code for ubiquitously expressed, highly homologous (69% overall) proteins involved in coupling signals originated by cell surface receptor tyrosine kinases (RTKs) to downstream, Ras-dependent mitogenic signaling pathways. Mechanistically, the Sos proteins function as enzymatic factors interacting with Ras proteins in response to upstream stimuli to promote guanine nucleotide exchange (GDP/GTP) and subsequent formation of the active Ras-GTP complex. In this review, we summarize current knowledge on structural, regulatory, and functional aspects of the Sos family, focusing on specific aspects of Sos biology such as structure-function relationship, crosstalk with different signaling pathways, and in vivo functional significance as deduced from phenotypic characterization of Sos knockout mice and human genetic syndromes caused by germline hSos1 mutations.

Exactly the same but different: promiscuity and diversity in the molecular mechanisms of action of the aryl hydrocarbon (dioxin) receptor

The Ah receptor (AhR) is a ligand-dependent transcription factor that mediates a wide range of biological and toxicological effects that result from exposure to a structurally diverse variety of synthetic and naturally occurring chemicals. Although the overall mechanism of action of the AhR has been extensively studied and involves a classical nuclear receptor mechanism of action (i.e., ligand-dependent nuclear localization, protein heterodimerization, binding of liganded receptor as a protein complex to its specific DNA recognition sequence and activation of gene expression), details of the exact molecular events that result in most AhR-dependent biochemical, physiological, and toxicological effects are generally lacking. Ongoing research efforts continue to describe an ever-expanding list of ligand-, species-, and tissue-specific spectrum of AhR-dependent biological and toxicological effects that seemingly add even more complexity to the mechanism. However, at the same time, these studies are also identifying and characterizing new pathways and molecular mechanisms by which the AhR exerts its actions and plays key modulatory roles in both endogenous developmental and physiological pathways and response to exogenous chemicals. Here we provide an overview of the classical and nonclassical mechanisms that can contribute to the differential sensitivity and diversity in responses observed in humans and other species following ligand-dependent activation of the AhR signal transduction pathway.

Integrating transcriptomics and metabonomics to unravel modes-of-action of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in HepG2 cells

BACKGROUND

The integration of different 'omics' technologies has already been shown in several in vivo studies to offer a complementary insight into cellular responses to toxic challenges. Being interested in developing in vitro cellular models as alternative to animal-based toxicity assays, we hypothesize that combining transcriptomics and metabonomics data improves the understanding of molecular mechanisms underlying the effects caused by a toxic compound also in vitro in human cells. To test this hypothesis, and with the focus on non-genotoxic carcinogenesis as an endpoint of toxicity, in the present study, the human hepatocarcinoma cell line HepG2 was exposed to the well-known environmental carcinogen 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD).

RESULTS

Transcriptomics as well as metabonomics analyses demonstrated changes in TCDD-exposed HepG2 in common metabolic processes, e.g. amino acid metabolism, of which some of the changes only being confirmed if both 'omics' were integrated. In particular, this integrated analysis identified unique pathway maps involved in receptor-mediated mechanisms, such as the G-protein coupled receptor protein (GPCR) signaling pathway maps, in which the significantly up-regulated gene son of sevenless 1 (SOS1) seems to play an important role. SOS1 is an activator of several members of the RAS superfamily, a group of small GTPases known for their role in carcinogenesis.

CONCLUSIONS

The results presented here were not only comparable with other in vitro studies but also with in vivo studies. Moreover, new insights on the molecular responses caused by TCDD exposure were gained by the cross-omics analysis.

Understanding SOS (Son of Sevenless)

Son of Sevenless (SOS) was discovered in Drosophila melanogaster. Essential for normal eye development in Drosophila, SOS has two human homologues, SOS1 and SOS2. The SOS1 gene encodes the Son of Sevenless 1 protein, a Ras and Rac guanine nucleotide exchange factor. This protein is composed of several important domains. The CDC25 and REM domains provide the catalytic activity of SOS1 towards Ras and the histone fold DH/PH (Dbl homology and Pleckstrin homology) domains function, in tandem, to stimulate GTP/GDP exchange for Rac. In contrast to Ras, there have been few studies that implicate SOS1 in human disease and, initially, less attention was given to this gene. However, mutations in SOS1 have been reported recently in Noonan syndrome and in type 1 hereditary gingival fibromatosis. Although, there have been very few studies that focus on the regulation of this important gene by physiological or exogenous factors, we recently found that the SOS1 gene was induced by the environmental toxin, dioxin, and that this effect was mediated by the aryl hydrocarbon receptor (AhR). These recent observations raise the possibility that alterations in the expression of the SOS1 gene and, consequently, in the activity of the SOS1 protein may affect toxicological endpoints and lead to clinical disease. These possibilities, thus, have stimulated much interest in SOS1 recently. In this article, we review the functions of SOS1 and the evidence for its roles in physiology and pathology across species.