ZINC DEFICIENCY AND MECHANISMS OF ENDOTHELIAL CELL DYSFUNCTION

Atherosclerosis is a chronic inflammatory disease thought to be initiated by endothelial cell dysfunction. Research described in this dissertation is focused on the role of zinc deficiency in endothelial cell activation with an emphasis on the function of the transcription factors nuclear factor-κB (NF-κB), peroxisome proliferator activated receptor (PPAR), and the aryl hydrocarbon receptor (AhR), which all play critical roles in the early pathology of atherosclerosis. Cultured porcine aortic vascular endothelial cells were deprived of zinc by the zinc chelator TPEN and/or treated with the NF-κB inhibitor CAPE or the PPARγ agonist rosiglitazone, followed by measurements of PPARα expression, cellular oxidative stress, NF-κB and PPAR DNA binding, COX-2 and E-selectin expression, and monocyte adhesion. Cellular labile zinc deficiency increased oxidative stress and NF-κB DNA binding activity, and induced COX-2 and Eselectin gene expression, as well as monocyte adhesion in endothelial cells. CAPE significantly reduced the zinc deficiency-induced COX-2 expression, suggesting regulation through NF-κB signaling. PPAR can inhibit NF-κB signaling. Zinc deficiency down-regulated PPARα expression and PPAR DNA binding activity in endothelial cells. Zinc deficiency compromised PPARγ transactivation activity in PPARγ and PPRE co-transfected rat aortic vascular smooth muscle cells. Furthermore, rosiglitazone was unable to inhibit the adhesion of monocytes to endothelial cells during zinc deficiency. Most of these effects of zinc deficiency could be reversed by zinc supplementation. An in vivo study utilizing the atherogenic LDL-R-/- mouse model generally supported the importance of PPAR dysregulation during zinc deficiency. LDLR-/- mice were maintained for four weeks on either zinc deficient or zinc adequate diets. Half of the mice within each zinc group were gavaged daily with rosiglitazone during the last stage of the study. Selected inflammation and lipid parameters were measured. The anti-inflammatory properties of rosiglitazone were compromised during zinc deficiency. Specifically, rosiglitazone induced inflammatory genes (MCP-1) in abdominal aorta only during zinc deficiency, and adequate zinc was required for rosiglitazone to down-regulate pro-inflammatory markers such as iNOS in abdominal aorta of the mice. Rosiglitazone significantly up-regulated liver IκBα protein expression only in zinc adequate mice. Plasma data also suggest an overall pro-inflammatory environment during zinc deficiency and support the concept that zinc is required for proper anti-inflammatory or protective functions of PPAR. Zinc deficiency also altered PPAR-regulated lipid metabolism in LDL-R-/- mice. Specifically, zinc deficiency increased plasma total cholesterol, and non- HDL (VLDL, IDL and LDL)-cholesterol. Plasma total fatty acids tended to be increased during zinc deficiency, and rosiglitazone treatment resulted in similar changes in fatty acid profile in zinc deficient mice. FAT/CD36 expression in abdominal aorta was upregulated by rosiglitazone only in zinc-deficient mice. In contrast, rosiglitazone treatment markedly increased LPL expression only in zinc-adequate mice. These data suggest that in this atherogenic mouse model treated with rosiglitazone, lipid metabolism can be compromised during zinc deficiency. AhR is another transcription factor involved in the development and homeostasis of the cardiovascular system. Cultured porcine aortic endothelial cells were exposed to the AhR ligands PCB77 or beta-naphthoflavone (β-NF) alone or in combination with the zinc chelator TPEN, followed by measurements of the AhR responsive cytochrome P450 enzymes CYP1A1 and 1B1. Zinc deficiency significantly reduced PCB77- induced CYP1A1 activity and mRNA expression, as well as PCB77 or β-NF-induced CYP1A1 protein expression, which could be restored by zinc supplementation. These data suggest that adequate zinc is required for the activation of the AhR-CYP1A1 pathway. Impairment of the AhR pathway presents an additional mechanism by which zinc deficiency negatively affects transcription factor function and homeostasis of the vascular system. Taken together, zinc nutrition can markedly modulate the pathogenesis of inflammatory diseases such as atherosclerosis

The Role of PPARs in Cancer

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that belong to the nuclear hormone receptor superfamily. PPARα is mainly expressed in the liver, where it activates fatty acid catabolism. PPARα activators have been used to treat dyslipidemia, causing a reduction in plasma triglyceride and elevation of high-density lipoprotein cholesterol. PPARδ is expressed ubiquitously and is implicated in fatty acid oxidation and keratinocyte differentiation. PPARδ activators have been proposed for the treatment of metabolic disease. PPARγ2 is expressed exclusively in adipose tissue and plays a pivotal role in adipocyte differentiation. PPARγ is involved in glucose metabolism through the improvement of insulin sensitivity and represents a potential therapeutic target of type 2 diabetes. Thus PPARs are molecular targets for the development of drugs treating metabolic syndrome. However, PPARs also play a role in the regulation of cancer cell growth. Here, we review the function of PPARs in tumor growth

Regulation of the Pregnane X Receptor Signaling Pathway

Liver-enriched nuclear receptors (NRs) collectively function as metabolic and toxicological `sensors' that mediate liver-specific gene-activation in mammals. NR-mediated gene-environment interaction regulates important steps in the hepatic uptake, metabolism and excretion of glucose, fatty acids, lipoproteins, cholesterol, bile acids, and xenobiotics. While it is well-recognized that ligand-binding is the primary mechanism behind activation of NRs, recent research is revealing that multiple signal transduction pathways modulate NR-function in liver. The interface between specific signal transduction pathways and NRs helps to determine their overall responsiveness to various environmental and physiological stimuli. The pregnane x receptor (PXR, NR1I2) was identified in 1998 as a member of the NR superfamily of ligand-activated transcription factors. PXR is activated by a broad range of lipophilic compounds in a species-specific manner. The primary function ascribed to PXR is the homeostatic control of steroids, bile acids, and xenobiotics. This function is mediated through PXR's ability to coordinately activate gene expression and regulate the subsequent activity of phase I and phase II metabolic enzymes, as well as several membrane transporter proteins. While PXR likely evolved primarily to protect the liver from toxic assault, its activation also represents the molecular basis for an important class of drug-drug, herb-drug, and food-drug interactions. While ligand binding is the primary mode of PXR activation, several signal transduction pathways interface with the PXR protein to determine its overall responsiveness to environmental stimuli. Multiple signaling pathways modulate the activity of PXR, likely through direct alteration of the phosphorylation status of the receptor and its protein cofactors. Therefore, specific combinations of ligand binding and cell signaling pathways affect PXR-mediated gene activation and determine the overall biological response. This dissertation contributes to the molecular understanding of the regulation of PXR by novel agonists, cAMP-dependent protein kinase (PKA) signaling, and phosphorylation. The results presented here were primarily obtained from mouse and tissue culture systems. This dissertation identifies Tian Xian, a traditional Chinese herbal anti-cancer remedy, as a novel PXR activator. This evidence suggests that Tian Xian should be used cautiously by cancer patients taking chemotherapy due to its potential to increase the metabolism of co-administered medications. In addition, data presented here show that activation of PKA signaling modulates PXR activity in a species-specific manner. It is further revealed that PXR exists as phospho-protein in vivo and that the activation of PKA signaling modulates the phospho-threonine status of PXR. Finally, the potential phosphorylation sites within the PXR protein are identified. These phosphorylation sites are characterized, using a phosphomimetic and phospho-deficient site-directed mutagenesis based approach, based on their ability to modulate PXR activity. Taken together, the work presented in this dissertation contributes to understanding the interface between ligands, signal transduction pathways and PXR activity, which is critical for the development of safe and effective therapeutic strategies

A Role for the PPARγ in Cancer Therapy

In 1997, the first published reports highlighted PPARγ as a novel cancer therapeutic target regulating differentiation of cancer cells. A subsequent flurry of papers described these activities more widely and fuelled further enthusiasm for differentiation therapy, as the ligands for the PPARγ were seen as well tolerated and in several cases well-established in other therapeutic contexts. This initial enthusiasm and promise was somewhat tempered by contradictory findings in several murine cancer models and equivocal trial findings. As more understanding has emerged in recent years, a renaissance has occurred in targeting PPARγ within the context of either chemoprevention or chemotherapy. This clarity has arisen in part through a clearer understanding of PPARγ biology, how the receptor interacts with other proteins and signaling events, and the mechanisms that modulate its transcriptional actions. Equally greater translational understanding of this target has arisen from a clearer understanding of in vivo murine cancer models. Clinical exploitation will most likely require precise and quantifiable description of PPARγ actions, and resolution of which targets are the most beneficial to target combined with an understanding of the mechanisms that limits its anticancer effectiveness

Nuclear receptors: Lipid and hormone sensors with essential roles in the control of cancer development

Nuclear receptors (NRs) are a superfamily of ligand-activated transcription factors that act as biological sensors and use a combination of mechanisms to modulate positively and negatively gene expression in a spatial and temporal manner. The highly orchestrated biological actions of several NRs influence the proliferation, differentiation, and apoptosis of many different cell types. Synthetic ligands for several NRs have been the focus of extensive drug discovery efforts for cancer intervention. This review summarizes the roles in tumour growth and metastasis of several relevant NR family members, namely androgen receptor (AR), estrogen receptor (ER), glucocorticoid receptor (GR), thyroid hormone receptor (TR), retinoic acid receptors (RARs), retinoid X receptors (RXRs), peroxisome proliferator-activated receptors (PPARs), and liver X receptors (LXRs). These studies are key to develop improved therapeutic agents based on novel modes of action with reduced side effects and overcoming resistance

UBIQUITINATION AND SUMOYLATION OF PREGNANE X RECEPTOR

Abstract Pregnane X receptor (PXR, NR1I2) is a ligand-activated nuclear receptor (NR) superfamily member expressed at high levels in the liver and intestine of mammals. PXR can be activated by a broad range of structurally diverse xenobiotics and endobiotics. As a key regulator of xenobiotic metabolism and clearance, activated PXR up-regulates the expression of genes encoding phase I (oxidation) and phase II (conjugation) metabolizing enzymes and phase III transporters to increase the metabolism and clearance of drugs and xenobiotics from the body, thus protecting the body from potential toxic insults. Besides xenobiotic metabolism and clearance, activation of PXR also involves in the regulation of many other important biochemical pathways, like inflammation and bile acid homeostasis. While ligand-binding is the primary mechanism for NRs activation, recent research indicates that post-translational modifications of NRs also help to determine their activities under different physiological conditions and represent new modes of regulation for NRs. Studies on post-translational modifications of PXR have just begun to emerge, how post-translational modifications regulate PXR activity is not well-understood. This dissertation focuses on ubiquitination and SUMOylation of PXR. These post-translational modifications of PXR were characterized and their effects on PXR activities were studied in both primary cultures of hepatocytes and immortalized cell lines. Data presented here indicate that PXR is a target of the ubiquitin proteasome system, and inhibition of proteasome activity decreases the transactivation of PXR. The E3s and SENPs (Sentrin-specific Protease) that regulate PXR SUMOylation and de-SUMOylation are identified. Utilizing the newly identified SENPs, SUMOylation is further confirmed to be indispensable for PXR to repress inflammatory response. Furthermore, the crosstalk between ubiquitination and SUMOylation at the level of PXR is explored. Our data indicate that SUMOylation increases the presence of ubiquitinated PXR, and many other substrates of ubiquitin. Taken together, this dissertation contributes to the understanding of post-translational modifications of PXR and their regulatory effects on drug metabolism and inflammation, which is expected to produce new opportunities for the development of novel and safe therapeutic strategies

Protective Effects of (E)-β-Caryophyllene (BCP) in Chronic Inflammation

(E)-β-caryophyllene (BCP) is a bicyclic sesquiterpene widely distributed in the plant kingdom, where it contributes a unique aroma to essential oils and has a pivotal role in the survival and evolution of higher plants. Recent studies provided evidence for protective roles of BCP in animal cells, highlighting its possible use as a novel therapeutic tool. Experimental results show the ability of BCP to reduce pro-inflammatory mediators such as tumor necrosis factor-alfa (TNF-α), interleukin-1β (IL-1β), interleukin-6 (IL-6), nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), thus ameliorating chronic pathologies characterized by inflammation and oxidative stress, in particular metabolic and neurological diseases. Through the binding to CB2 cannabinoid receptors and the interaction with members of the family of peroxisome proliferator-activated receptors (PPARs), BCP shows beneficial effects on obesity, non-alcoholic fatty liver disease/nonalcoholic steatohepatitis (NAFLD/NASH) liver diseases, diabetes, cardiovascular diseases, pain and other nervous system disorders. This review describes the current knowledge on the biosynthesis and natural sources of BCP, and reviews its role and mechanisms of action in different inflammation-related metabolic and neurologic disorders