Felix Grün

Endocrine disrupters as obesogens.

The recent dramatic rise in obesity rates is an alarming global health trend that consumes an ever increasing portion of health care budgets in Western countries. The root cause of obesity is thought to be a prolonged positive energy balance. Hence, the major focus of preventative programs for obesity has been to target overeating and inadequate physical exercise. Recent research implicates environmental risk factors, including nutrient quality, stress, fetal environment and pharmaceutical or chemical exposure as relevant contributing influences. Evidence points to endocrine disrupting chemicals that interfere with the bodys adipose tissue biology, endocrine hormone systems or central hypothalamic-pituitary-adrenal axis as suspects in derailing the homeostatic mechanisms important to weight control. This review highlights recent advances in our understanding of the molecular targets and mechanisms of action for these compounds and areas of future research needed to evaluate the significance of their contribution to obesity.

Mutual repression between steroid and xenobiotic receptor and NF-κB signaling pathways links xenobiotic metabolism and inflammation

While it has long been known that inflammation and infection reduce expression of hepatic cytochrome P450 (CYP) genes involved in xenobiotic metabolism and that exposure to xenobiotic chemicals can impair immune function, the molecular mechanisms underlying both of these phenomena have remained largely unknown. Here we show that activation of the nuclear steroid and xenobiotic receptor (SXR) by commonly used drugs in humans inhibits the activity of NF-kappaB, a key regulator of inflammation and the immune response. NF-kappaB target genes are upregulated and small bowel inflammation is significantly increased in mice lacking the SXR ortholog pregnane X receptor (PXR), thereby demonstrating a direct link between SXR and drug-mediated antagonism of NF-kappaB. Interestingly, NF-kappaB activation reciprocally inhibits SXR and its target genes whereas inhibition of NF-kappaB enhances SXR activity. This SXR/PXR-NF-kappaB axis provides a molecular explanation for the suppression of hepatic CYP mRNAs by inflammatory stimuli as well as the immunosuppressant effects of xenobiotics and SXR-responsive drugs. This mechanistic relationship has clinical consequences for individuals undergoing therapeutic exposure to the wide variety of drugs that are also SXR agonists.

Vitamin K2 Regulation of Bone Homeostasis Is Mediated by the Steroid and Xenobiotic Receptor SXR

Vitamin K2 is a critical nutrient required for blood clotting that also plays an important role in bone formation. Vitamin K2 supplementation up-regulates the expression of bone markers, increases bone density in vivo, and is used clinically in the management of osteoporosis. The mechanism of vitamin K2 action in bone formation was thought to involve its normal role as an essential cofactor for γ-carboxylation of bone matrix proteins. However, there is evidence that suggests vitamin K2 also has a transcriptional regulatory function. Vitamin K2 bound to and activated the orphan nuclear receptor SXR and induced expression of the SXR target gene, CYP3A4, identifying it as a bona fide SXR ligand. Vitamin K2 treatment of osteosarcoma cells increased mRNA levels for the osteoblast markers bone alkaline phosphatase, osteoprotegerin, osteopontin, and matrix Gla protein. The known SXR activators rifampicin and hyperforin induced this panel of bone markers to an extent similar to vitamin K2. Vitamin K2 was able to induce bone markers in primary osteocytes isolated from wild-type murine calvaria but not in cells isolated from mice deficient in the SXR ortholog PXR. We infer that vitamin K2 is a transcriptional regulator of bone-specific genes that acts through SXR to favor the expression of osteoblastic markers. Thus, SXR has a novel role as a mediator of bone homeostasis in addition to its role as a xenobiotic sensor. An important implication of this work is that a subset of SXR activators may function as effective therapeutic agents for the management of osteoporosis.

Perturbed nuclear receptor signaling by environmental obesogens as emerging factors in the obesity crisis

The modern world is plagued with expanding epidemics of diseases related to metabolic dysfunction. The factors that are driving obesity, diabetes, cardiovascular disease, hypertension, and dyslipidemias (collectively termed metabolic syndrome) are usually ascribed to a mismatch between the body’s homeostatic nutrient requirements and dietary excess, coupled with insufficient exercise. The environmental obesogen hypothesis proposes that exposure to a toxic chemical burden is superimposed on these conditions to initiate or exacerbate the development of obesity and its associated health consequences. Recent studies have proposed a first set of candidate obesogens (diethylstilbestrol, bisphenol A, phthalates and organotins among others) that target nuclear hormone receptor signaling pathways (sex steroid, RXR–PPARγ and GR) with relevance to adipocyte biology and the developmental origins of health and disease (DOHaD). Perturbed nuclear receptor signaling can alter adipocyte proliferation, differentiation or modulate systemic homeostatic controls, leading to long-term consequences that may be magnified if disruption occurs during sensitive periods during fetal or early childhood development.

The Dietary Isothiocyanate Sulforaphane Is an Antagonist of the Human Steroid and Xenobiotic Nuclear Receptor

Sulforaphane (SFN) is a biologically active phytochemical found abundantly in broccoli. SFN has been promoted as a putative chemopreventive agent to reduce cancer, and most studies have associated its anti-cancer effects with the induction of phase II xenobiotic metabolism enzymes via activation of the Keap1/Nrf2 antioxidant response pathway. Interestingly, SFN can significantly down-regulate cytochrome P450 3A4 (CYP3A4) expression in human primary hepatocytes. CYP3A4 is responsible for the hepatic and intestinal metabolism of numerous protoxicants, pharmaceutical compounds, and endogenous sterols. Among the most important mediators of CYP3A4 expression is the nuclear hormone receptor, steroid and xenobiotic receptor (SXR; also called “hPXR”). SXR functions as a xenobiotic sensor to coordinately regulate xenobiotic metabolism via transcriptional regulation of xenobiotic-detoxifying enzymes and transporters. Here, we report that SFN is a specific antagonist of human SXR and that it inhibits SXR-mediated induction of drug clearance. SFN can bind directly to SXR, inhibit SXR coactivator recruitment, and efficiently repress SXR activities. Furthermore, SFN inhibited SXR-mediated CYP3A4 expression and CYP3A4-catalyzed midazolam clearance in human primary hepatocytes. Thus, SFN is the first identified naturally occurring antagonist for SXR (hPXR). Because induction of CYP3A4 can result in adverse drug responses (e.g., lack of efficacy), which are a major public health problem, this discovery could lead to the development of important new therapeutic and dietary approaches to reduce the frequency of undesirable inducer-drug interactions.

Minireview: The Case for Obesogens

Obesity and obesity-related disorders, such as type 2 diabetes, hypertension, and cardiovascular disease, are epidemic in Western countries, particularly the United States. The conventional wisdom holds that obesity is primarily the result of a positive energy balance, i.e. too many calories in and too few calories burned. Although it is self-evident that fat cannot be accumulated without a higher caloric intake than expenditure, recent research in a number of laboratories suggests the existence of chemicals that alter regulation of energy balance to favor weight gain and obesity. These obesogens derail the homeostatic mechanisms important for weight control, such that exposed individuals are predisposed to weight gain, despite normal diet and exercise. This review considers the evidence for obesogens, how they might act, and where future research is needed to clarify their relative contribution to the obesity epidemic.

TOCOTRIENOLS ACTIVATE THE STEROID AND XENOBIOTIC RECEPTOR, SXR, AND SELECTIVELY REGULATE EXPRESSION OF ITS TARGET GENES

Vitamin E is an essential nutrient with antioxidant activity. Vitamin E is comprised of eight members, α-, β-, γ-, and δ-tocopherols and α-, β-, γ-, and δ-tocotrienols. All forms of vitamin E are initially metabolized by ω-oxidation, which is catalyzed by cytochrome P450 enzymes. The steroid and xenobiotic receptor (SXR) is a nuclear receptor that regulates drug clearance in the liver and intestine via induction of genes involved in drug and xenobiotic metabolism. We show here that all four tocotrienols specifically bind to and activate SXR, whereas tocopherols neither bind nor activate. Surprisingly, tocotrienols show tissue-specific induction of SXR target genes, particularly CYP3A4. Tocotrienols up-regulate expression of CYP3A4 but not UDP-glucuronosyltransferase 1A1 (UGT1A1) or multidrug resistance protein-1 (MDR1) in primary hepatocytes. In contrast, tocotrienols induce MDR1 and UGT1A1 but not CYP3A4 expression in intestinal LS180 cells. We found that nuclear receptor corepressor (NCoR) is expressed at relatively high levels in intestinal LS180 cells compared with primary hepatocytes. The unliganded SXR interacts with NCoR, and this interaction is only partially disrupted by tocotrienols. Expression of a dominant-negative NCoR enhanced the ability of tocotrienols to induce CYP3A4 in LS180 cells, suggesting that NCoR plays an important role in tissue-specific gene regulation by SXR. Our findings provide a molecular mechanism explaining how vitamin supplements affect the absorption and effectiveness of drugs. Knowledge of drug-nutrient interactions may help reduce the incidence of decreased drug efficacy.

Retinoic acid signaling in the brain marks formation of optic projections, maturation of the dorsal telencephalon, and function of limbic sites.

As retinoic acid (RA) is known to regulate the expression of many neuronal proteins, it is likely to influence overall development and function of the brain; few particulars, however, are available about its role in neurobiological contexts due mainly to problems in RA detection. To ask whether the function of RA in the rostral brain is concentrated in particular neurobiological systems, we compared sites of RA synthesis and actions, as detected by RA signaling in reporter mice, for embryonic and adult ages. We found that most sites of RA actions in the forebrain do not colocalize with RA synthesis, consistent with a dominant RA supply by diffusion and the circulation. The changing RA patterns distinguish preferentially two complex functional schemes. (1) Within the visual system when the first optic axons grow toward their targets, RA signaling delineates the topographical adjustment of the retinal map, which is encoded in the coordinates of the visual world, to central visual maps, which are formed in the segmental brain coordinates. (2) The second scheme begins early in forebrain morphogenesis as a distinction of the dorsal telencephalon. With progressing development, and in the adult, the RA patterns then focus on widely distributed structures, most of which belong to the limbic system. These are sites in which emotional perception is combined with higher cognitive processes and in which normal function requires ongoing remodeling of synaptic connections, indicating that the developmental role of RA in promotion of neuronal differentiation programs continues in the adult brain for highly flexible neural circuits. J. Comp. Neurol. 470:297–316, 2004.

Deformed frogs and environmental retinoids

Since the early 1990s, a substantial number of deformed frogs have been observed in North America, particularly in the upper Midwest and Canada. Attempts to understand the etiology of the deformed frog problem have met with limited success to date with nearly as many proposed explanations as research groups working on the problem. Models for the mechanism underlying the development of deformed frogs include parasite/predation, ultraviolet radiation, and chemical exposure. Each model has its strengths and weaknesses. Despite contentious debate among researchers, there is an overall consensus that the increasing prevalence of deformed frogs is the result of a water-borne contaminant that has recently appeared, or reached a critical concentration. Our detailed analysis of malformed frogs collected in Minnesota ponds and lakes suggested that limb patterning was being modified by the disruption of a retinoid-sensitive developmental signaling pathway. Accordingly, we focused in the identification and characterization of bioactive retinoids from lake water and showed that retinoid treatment of frog embryos at sensitive times of development could recapitulate the full spectrum of limb abnormalities observed in field specimens in the laboratory. These data have led to the conclusion that inappropriate modulation of retinoid signaling by environmental contaminants is the mechanism underlying the increased incidence of frog malformations.

Activation of Steroid and Xenobiotic Receptor (SXR, NR1I2) and Its Orthologs in Laboratory Toxicologic, and Genome Model Species

Background Nuclear receptor subfamily 1, group I, member 2 (NR1I2), commonly known as steroid and xenobiotic receptor (SXR) in humans, is a key ligand-dependent transcription factor responsible for the regulation of xenobiotic, steroid, and bile acid metabolism. The ligand-binding domain is principally responsible for species-specific activation of NR1I2 in response to xenobiotic exposure. Objectives Our objective in this study was to create a common framework for screening NR1I2 orthologs from a variety of model species against environmentally relevant xenobiotics and to evaluate the results in light of using these species as predictors of xenobiotic disposition and for assessment of environmental health risk. Methods Sixteen chimeric fusion plasmid vectors expressing the Gal4 DNA-binding domain and species-specific NR1I2 ligand-binding domain were screened for activation against a spectrum of 27 xenobiotic compounds using a standardized cotransfection receptor activation assay. Results NR1I2 orthologs were activated by various ligands in a dose-dependent manner. Closely related species show broadly similar patterns of activation; however, considerable variation to individual compounds exists, even among species varying in only a few amino acid residues. Conclusions Interspecies variation in NR1I2 activation by various ligands can be screened through the use of in vitro NR1I2 activation assays and should be taken into account when choosing appropriate animal models for assessing environmental health risk.