Martin O. Leonard

Coordinated adenine nucleotide phosphohydrolysis and nucleoside signaling in posthypoxic endothelium: role of ectonucleotidases and adenosine A2B receptors.

Limited oxygen delivery to tissues (hypoxia) is common in a variety of disease states. A number of parallels exist between hypoxia and acute inflammation, including the observation that both influence vascular permeability. As such, we compared the functional influence of activated polymorphonuclear leukocytes (PMN) on normoxic and posthypoxic endothelial cells. Initial studies indicated that activated PMN preferentially promote endothelial barrier function in posthypoxic endothelial cells (>60% increase over normoxia). Extension of these findings identified at least one soluble mediator as extracellular adenosine triphosphate (ATP). Subsequent studies revealed that ATP is coordinately hydrolyzed to adenosine at the endothelial cell surface by hypoxia-induced CD39 and CD73 (>20-and >12-fold increase in mRNA, respectively). Studies in vitro and in cd39-null mice identified these surface ecto-enzymes as critical control points for posthypoxia-associated protection of vascular permeability. Furthermore, insight gained through microarray analysis revealed that the adenosine A2B receptor (AdoRA2B) is selectively up-regulated by hypoxia (>5-fold increase in mRNA), and that AdoRA2B antagonists effectively neutralize ATP-mediated changes in posthypoxic endothelial permeability. Taken together, these results demonstrate transcription coordination of adenine nucleotide and nucleoside signaling at the vascular interface during hypoxia.

MicroRNA-155 promotes resolution of hypoxia-inducible factor 1α activity during prolonged hypoxia

The hypoxia-inducible factor (HIF) is a key regulator of the transcriptional response to hypoxia. While the mechanism underpinning HIF activation is well understood, little is known about its resolution. Both the protein and the mRNA levels of HIF-1α (but not HIF-2α) were decreased in intestinal epithelial cells exposed to prolonged hypoxia. Coincident with this, microRNA (miRNA) array analysis revealed multiple hypoxiainducible miRNAs. Among these was miRNA-155 (miR-155), which is predicted to target HIF-1α mRNA. We confirmed the hypoxic upregulation of miR-155 in cultured cells and intestinal tissue from mice exposed to hypoxia. Furthermore, a role for HIF-1α in the induction of miR-155 in hypoxia was suggested by the identification of hypoxia response elements in the miR-155 promoter and confirmed experimentally. Application of miR-155 decreased the HIF-1α mRNA, protein, and transcriptional activity in hypoxia, and neutralization of endogenous miR-155 reversed the resolution of HIF-1α stabilization and activity. Based on these data and a mathematical model of HIF-1α suppression by miR-155, we propose that miR-155 induction contributes to an isoform-specific negative-feedback loop for the resolution of HIF-1α activity in cells exposed to prolonged hypoxia, leading to oscillatory behavior of HIF-1α-dependent transcription.ABSTRACT The hypoxia-inducible factor (HIF) is a key regulator of the transcriptional response to hypoxia. While the mechanism underpinning HIF activation is well understood, little is known about its resolution. Both the protein and the mRNA levels of HIF-1α (but not HIF-2α) were decreased in intestinal epithelial cells exposed to prolonged hypoxia. Coincident with this, microRNA (miRNA) array analysis revealed multiple hypoxia-inducible miRNAs. Among these was miRNA-155 (miR-155), which is predicted to target HIF-1α mRNA. We confirmed the hypoxic upregulation of miR-155 in cultured cells and intestinal tissue from mice exposed to hypoxia. Furthermore, a role for HIF-1α in the induction of miR-155 in hypoxia was suggested by the identification of hypoxia response elements in the miR-155 promoter and confirmed experimentally. Application of miR-155 decreased the HIF-1α mRNA, protein, and transcriptional activity in hypoxia, and neutralization of endogenous miR-155 reversed the resolution of HIF-1α stabilization and activity. Based on these data and a mathematical model of HIF-1α suppression by miR-155, we propose that miR-155 induction contributes to an isoform-specific negative-feedback loop for the resolution of HIF-1α activity in cells exposed to prolonged hypoxia, leading to oscillatory behavior of HIF-1α-dependent transcription.

The role of HIF-1α in transcriptional regulation of the proximal tubular epithelial cell response to hypoxia

Epithelial cells of the kidney represent a primary target for hypoxic injury in ischemic acute renal failure (ARF); however, the underlying transcriptional mechanism(s) remain undefined. In this study, human proximal tubular epithelial cells (HK-2) exposed to hypoxia in vitro demonstrated a non-lethal but dysfunctional phenotype, closely reflective of the epithelial pathobiology of ARF. HK-2 cells exposed to hypoxia demonstrated increased paracellular permeability, decreased proliferation, loss of tight junctional integrity, and significant actin disassembly in the absence of cell death. Microarray analysis of transcriptomic changes underlying this response identified a distinct cohort of 48 genes with a closely shared hypoxia-dependent expression profile. Within this hypoxia-sensitive cluster were genes identified previously as hypoxia-inducible factor-1 (HIF-1)-dependent (e.g. vascular endothelial growth factor and adrenomedullin) as well as genes not previously known to be hypoxia-responsive (e.g. stanniocalcin 2). In hypoxia, HIF-1 bound to evolutionarily conserved hypoxia-response elements (HRE) in the promoters of these genes as well as to the HRE consensus motif. A further subset of these genes, not associated with transcriptional regulation by HIF-1, was also present, suggesting alternative HIF-1-independent pathways. Overexpression of HIF-1α in normoxia induced the expression of a significant number of the hypoxia-dependent genes; however, it did not induce the pathophysiologic epithelial response. In summary, hypoxia-elicited alterations in renal proximal tubular epithelial cells in vitro closely resemble the epithelial pathophysiology of ARF. Our data indicate that although this event may rely heavily on HIF-1-dependent gene transcription, it is likely that separate hypoxia-dependent transcriptional regulators also play a role.

Small ubiquitin-related modifier-1 modification mediates resolution of CREB-dependent responses to hypoxia

Phosphorylation-dependent ubiquitination combined with proteasomal degradation of transcriptional regulators is a recently appreciated mechanism for control of a number of inflammatory genes. Far less is known about the counterregulatory mechanisms that repress transcriptional activity in these pathways during resolution. Here, we investigated the transient nature of hypoxia-induced tumor necrosis factor (TNF)α in T84 cells, a process we have previously shown to involve phosphorylation-dependent degradation of the cAMP-response element-binding protein (CREB). Initial studies indicate hypoxia-induced TNFα to be a transient event, the resolution of which is associated with the appearance of a higher molecular weight modified form of CREB. Gene array analysis of mRNA derived from hypoxic cells identified a time-dependent induction of small ubiquitin-related modifier (SUMO)-1 mRNA. In prolonged hypoxia, CREB is posttranslationally modified by SUMO-1. Furthermore, SUMO-1 overexpression stabilizes CREB in hypoxia and enhances CREB-dependent reporter gene activity. Site-directed mutagenesis of lysine residues K285 and K304 identifies them as SUMO acceptors in vivo and in vitro. Mutation of K304 also results in loss of CREB nuclear localization, implying a role for SUMO-1 modification at this site in the subcellular localization of CREB. Thus, in prolonged hypoxia, CREB is modified by association with SUMO-1. Furthermore, we hypothesize that such an event stabilizes and promotes nuclear localization of CREB and thus complements an endogenous resolution phase for hypoxia-induced inflammatory processes.

15-Epi-16-(Para-Fluorophenoxy)-Lipoxin A4-Methyl Ester, a Synthetic Analogue of 15-epi-Lipoxin A4, Is Protective in Experimental Ischemic Acute Renal Failure

Lipoxins are endogenous lipoxygenase-derived eicosanoids, generated during inflammatory, hypersensitivity, and vascular events, that display vasodilatory, antiinflammatory, and pro-resolution activity. Here, we evaluated the efficacy of 15-epi-16-(para-fluorophenoxy)-lipoxin A(4)-methyl ester (15-epi-16-(FPhO)-LXA(4)-Me), a stable synthetic analogue of aspirin-triggered 15-epi-lipoxin A(4) in ischemic acute renal failure (ARF) in NIH Swiss mice. ARF was induced by 30-min crossclamping of renal pedicles and was associated with elevated serum creatinine, morphologic injury, polymorphonuclear leukocyte (PMN) recruitment, and increased mRNA levels for adhesion molecules (intercellular adhesion molecule-1 [ICAM-1] and vascular cell adhesion molecule-1 [VCAM-1]), chemokines (growth regulated oncogene-1 [GRO1]), and cytokines (interleukin-1beta [IL-1beta] and IL-6) after 24-h reperfusion. A single bolus of 15-epi-16-(FPhO)-LXA(4)-Me afforded striking functional (mean +/- SEM creatinine in mg/dl: sham-operated, 0.77 +/- 0.04; ARF + vehicle, 2.49 +/- 0.19; ARF + 15-epi-16-(FPhO)-LXA(4)-Me, 0.75 +/- 0.12; P < 0.001) and morphologic protection and reduced PMN infiltration. Treatment with 15-epi-16-(FPhO)-LXA(4)-Me was also associated with lower IL-1beta, IL-6, and GRO1 mRNA levels, whereas ICAM-1 and VCAM-1 mRNA levels were unchanged. Compatible with these results, LXA(4) blunted chemoattractant-stimulated PMN migration across HK-2 renal epithelial cell monolayers in vitro, but it did not inhibit cytokine-induced HK-2 ICAM-1 expression or adhesiveness for PMN. Interestingly 15-epi-16-(FPhO)-LXA(4)-Me-treated animals also displayed increased renal mRNA levels for suppressors of cytokine signaling-1 (SOCS-1) and SOCS-2, but not CIS-1, endogenous inhibitors of cytokine-elicited Jak/Stat-signaling pathways. These results indicate that 15-epi-16-(FPhO)-LXA(4)-Me is protective in renal ischemia reperfusion injury in vivo, at least partially by modulating cytokine and chemokine expression and PMN recruitment, and provides a rationale for further exploration of the efficacy of LXA(4) structural analogues in ischemic ARF and other renal diseases.

Application of integrated transcriptomic, proteomic and metabolomic profiling for the delineation of mechanisms of drug induced cell stress.

High content omic techniques in combination with stable human in vitro cell culture systems have the potential to improve on current pre-clinical safety regimes by providing detailed mechanistic information of altered cellular processes. Here we investigated the added benefit of integrating transcriptomics, proteomics and metabolomics together with pharmacokinetics for drug testing regimes. Cultured human renal epithelial cells (RPTEC/TERT1) were exposed to the nephrotoxin Cyclosporine A (CsA) at therapeutic and supratherapeutic concentrations for 14days. CsA was quantified in supernatants and cellular lysates by LC-MS/MS for kinetic modeling. There was a rapid cellular uptake and accumulation of CsA, with a non-linear relationship between intracellular and applied concentrations. CsA at 15μM induced mitochondrial disturbances and activation of the Nrf2-oxidative-damage and the unfolded protein-response pathways. All three omic streams provided complementary information, especially pertaining to Nrf2 and ATF4 activation. No stress induction was detected with 5μM CsA; however, both concentrations resulted in a maximal secretion of cyclophilin B. The study demonstrates for the first time that CsA-induced stress is not directly linked to its primary pharmacology. In addition we demonstrate the power of integrated omics for the elucidation of signaling cascades brought about by compound induced cell stress.

An overview of transcriptional regulation in response to toxicological insult

The completion of the human genome project and the subsequent advent of DNA microarray and high-throughput sequencing technologies have led to a renaissance in molecular toxicology. Toxicogenomic data sets, from both in vivo and in vitro studies, are growing exponentially, providing a wealth of information on regulation of stress pathways at the transcriptome level. Through such studies, we are now beginning to appreciate the diversity and complexity of biological responses to xenobiotics. In this review, we aim to consolidate and summarise the major toxicologically relevant transcription factor-governed molecular pathways. It is becoming clear that different chemical entities can cause oxidative, genotoxic and proteotoxic stress, which induce cellular responses in an effort to restore homoeostasis. Primary among the response pathways involved are NFE2L2 (Nrf2), NFE2L1 (Nrf1), p53, heat shock factor and the unfolded protein response. Additionally, more specific mechanisms exist where xenobiotics act as ligands, including the aryl hydrocarbon receptor, metal-responsive transcription factor-1 and the nuclear receptor family of transcription factors. Other pathways including the immunomodulatory transcription factors NF-κB and STAT together with the hypoxia-inducible transcription factor HIF are also implicated in cellular responses to xenobiotic exposure. A less specific but equally important aspect to cellular injury controlled by transcriptional activity is loss of tissue-specific gene expression, resulting in dedifferentiation of target cells and compromise of tissue function. Here, we review these pathways and the genes they regulate in order to provide an overview of this growing field of molecular toxicology.

Potentiation of Glucocorticoid Activity in Hypoxia through Induction of the Glucocorticoid Receptor

Tissue hypoxia is intimately associated with chronic inflammatory disease and may signal to the resolution of inflammatory processes. Glucocorticoid signaling through the glucocorticoid receptor (GR) represents a clinically important endogenous anti-inflammatory pathway. Microarray analysis reveals that the GR is transcriptionally up-regulated by hypoxia in human renal proximal tubular epithelial cells. Hypoxic up-regulation of the GR was confirmed at the level of promoter activity, mRNA, and protein expression. Furthermore, functional potentiation of glucocorticoid activity in hypoxia was observed as an enhancement of dexamethasone-induced glucocorticoid response element promoter activity and enhanced dexamethasone-mediated inhibition of IL-1β-stimulated IL-8 expression and hypoxia-induced vascular endothelial growth factor expression. Knockdown of enhanced GR gene expression in hypoxia using specific GR small inhibitory RNA (siRNA) resulted in an attenuation of the enhanced glucocorticoid sensitivity. A role for the hypoxia-inducible transcription factor, HIF-1α, in the regulation of GR expression and the associated potentiation of glucocorticoid activity in hypoxia was also demonstrated. These results reveal a novel signaling aspect responsible for the incorporation of hypoxic and glucocorticoid stimuli, which we hypothesize to be an important co-operative pathway for the control of gene expression observed in complex tissue microenvironments in inflamed states.

Gremlin Plays a Key Role in the Pathogenesis of Pulmonary Hypertension

Background— Pulmonary hypertension occurs in chronic hypoxic lung diseases, significantly worsening morbidity and mortality. The important role of altered bone morphogenetic protein (BMP) signaling in pulmonary hypertension was first suspected after the identification of heterozygous BMP receptor mutations as the underlying defect in the rare heritable form of pulmonary arterial hypertension. Subsequently, it was demonstrated that BMP signaling was also reduced in common forms of pulmonary hypertension, including hypoxic pulmonary hypertension; however, the mechanism of this reduction has not previously been elucidated. Methods and Results— Expression of 2 BMP antagonists, gremlin 1 and gremlin 2, was higher in the lung than in other organs, and gremlin 1 was further increased in the walls of small intrapulmonary vessels of mice during the development of hypoxic pulmonary hypertension. Hypoxia stimulated gremlin secretion from human pulmonary microvascular endothelial cells in vitro, which inhibited endothelial BMP signaling and BMP-stimulated endothelial repair. Haplodeficiency of gremlin 1 augmented BMP signaling in the hypoxic mouse lung and reduced pulmonary vascular resistance by attenuating vascular remodeling. Furthermore, gremlin was increased in the walls of small intrapulmonary vessels in idiopathic pulmonary arterial hypertension and the rare heritable form of pulmonary arterial hypertension in a distribution suggesting endothelial localization. Conclusions— These findings demonstrate a central role for increased gremlin in hypoxia-induced pulmonary vascular remodeling and the increased pulmonary vascular resistance in hypoxic pulmonary hypertension. High levels of basal gremlin expression in the lung may account for the unique vulnerability of the pulmonary circulation to heterozygous mutations of BMP type 2 receptor in pulmonary arterial hypertension.

Transforming growth factor‐β‐regulated gene transcription and protein expression in human GFAP‐negative lamina cribrosa cells

Primary open‐angle glaucoma (POAG) is a progressive optic neuropathy, which is a major cause of worldwide visual impairment and blindness. Pathological hallmarks of the glaucomatous optic nerve head (ONH) include retinal ganglion cell axon loss and extracellular matrix (ECM) remodeling of the lamina cribrosa layer. Transforming growth factor‐β (TGF‐β) is an important pro‐fibrotic modulator of ECM metabolism, whose levels are elevated in human POAG lamina cribrosa tissue compared with non‐glaucomatous controls. We hypothesize that in POAG, lamina cribrosa (LC) glial cells respond to elevated TGF‐β, producing a remodeled ONH ECM. Using Affymetrix microarrays, we report the first study examining the effect of TGF‐β1 on global gene expression profiles in glial fibrillary acidic acid (GFAP)‐negative LC glial cells in vitro. Prominent among the differentially expressed genes were those with established fibrogenic potential, including CTGF, collagen I, elastin, thrombospondin, decorin, biglycan, and fibromodulin. Independent TaqMan and Sybr Green quantitative PCR analysis significantly validated genes involved in regulation of cell proliferation (platelet‐derived growth factor [PDGF‐α]), angiogenesis (vascular endothelial growth factor [VEGF]), ECM accumulation and degradation (CTGF, IL‐11, and ADAMT‐S5), and growth factor binding (ESM‐1). Bioinformatic analysis of the ESM‐1 promoter identified putative Smad and Runx transcription factor binding sites, and luciferase assays confirmed that TGF‐β1 drives transcription of the ESM‐1 gene. TGF‐β1 induces expression and release of ECM components in LC cells, which may be important in regulating matrix remodeling in the lamina cribrosa. In disease states such as POAG, the LC cell may represent an important pro‐fibrotic cell type and an attractive target for novel therapeutic strategies.