Browsing by Subject "Cell Hypoxia"
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Item Analysis of Cervical Tumor Metabolism and Design of Magnetic Resonance Imaging Sequences for Abdominal and Pelvic Tissue/Tumor Hypoxia Studies(2012-11-19) Ding, Yao 1971-; Lewis, Matthew Allen; McColl, Roderick W.; Mason, Ralph P.; Zhao, Dawen; Lenkinski, RobertTumor oxygenation is increasingly recognized as an important factor to enhance the efficacy of chemo- and radiotherapy. MRI is becoming a widely accepted diagnostic imaging modality for investigation of tumor oxygenation. Research to improve the effectiveness of MR techniques for detection of oxygenation biomarkers (T1 and T2*) in the tissue/tumor hypoxia studies mainly includes efforts to improve sensitivity, efficiency and accuracy of measurements and to minimize scan duration. This dissertation mainly focuses on the development of novel combined multi-parametric techniques to obtain both BOLD and TOLD images simultaneously. A novel approach (ms-mGEPI-T1,2*) is developed to simultaneously measure both T1- and T2*-weighted signal changes, as well as and T1- and T2*-maps serially during a single dynamic MRI scan. This method has also been validated in both phantom and human abdominal tissue experiments and both in vitro and in vivo results are in good agreement with those obtained using conventional methods and the literature. The ms-mGEPI-T1,2* has been found to provide sensitive BOLD and TOLD responses under an oxygen challenge. Two enhanced versions of the ms-mGEPI-T1,2* technique with higher temporal resolution (SR-based sequence) or more accurate relaxation time estimation (IR-based sequence) are developed and validated in phantom and 3 volunteer studies. Relaxation times measured by these novel methods were in good agreement with those obtained using conventional pulse sequences. A pulse sequence which combines all three methods was developed for use on Philips MR user interface and has great potential in clinical MR examinations.Item Noninvasive Assessment of Tumor Hypoxia Using MRI in Clinical and Preclinical Tumor Models(2013-01-16) Hallac, Rami R.; Mason, Ralph P.Tumor oxygenation influences response to radiation and plays important roles in malignant progression, angiogenesis and metastasis. While methods are available to quantitatively map pO2 dynamics in preclinical studies, new techniques are needed to noninvasively characterize tumor hypoxia and response to interventions in patients. Blood Oxygen Level Dependent (BOLD) MRI based on T2* contrast induced by deoxyhemoglobin concentration [dHb] is sensitive to tumor vascular oxygenation and blood flow. Meanwhile TOLD (Tissue Oxygen Level Dependent) MRI is sensitive to tissue oxygenation based on the shortening of the tissue water T1 due to molecular oxygen [O2]. In this study, I investigate the utility of BOLD and TOLD to evaluate tumor hypoxia in response to breathing hyperoxic gas in rats and test the feasibility of such measurements in patients. All MRI experiments were performed on either a 4.7T small animal Varian or a 3T clinical Philips scanner. Variation in BOLD and TOLD signal response observed in two syngeneic prostate tumor models: Dunning R3327-AT1 and -HI, with respect to oxygen and carbogen breathing were compared with quantitative change in pO2 measured using Fluorocarbon Relaxometry using Echo Planar Imaging for Dynamic Oxygen Mapping (FREDOM). In addition, BOLD and TOLD MRI measurements were used to predict radiation treatment outcome following a single dose of 30 Gy. BOLD MRI was also assessed in cervical cancer patients in response to breathing oxygen (15dm3/min). Two sequences were tested, multiple-shot EPI and multi echo gradient echo, to allow comparison. Significant correlations were found between BOLD and TOLD MRI and quantitative pO2 measurements for both oxygen and carbogen breathing. However, both gases had similar effect on modulating tumor hypoxia with no significant difference observed. The AT1 tumors showed a correlation between tumor growth delay for the animals breathing O2 during radiation and pre-irradiation TOLD responses to oxygen challenge. Finally, BOLD MRI at 3T was feasible for examining the potentially valuable biomarker of oxygenation seen in cervical cancer. Further parameters such as vascular perfusion and permeability based on DCE, cellularity based on diffusion, and TOLD response to oxygen challenge may also be readily incorporated into a dynamic evaluation.Item Oxygen-Mediated Regulation of Cholesterol Synthesis Through Accelerated Degradation of HMG COA Reductase(2009-09-04) Nguyen, Andrew Tuan Duc; DeBose-Boyd, Russell A.Endoplasmic reticulum-associated degradation of the enzyme 3-hydroxy-3-methylglutaryl CoA reductase represents one mechanism by which cholesterol synthesis is controlled in mammalian cells. The key reaction in this degradation is binding of reductase to Insig proteins in the endoplasmic reticulum, which is stimulated by the methylated cholesterol precursors lanosterol and 24,25-dihydrolanosterol. Conversion of these sterols to cholesterol requires the removal of three methyl groups, which consumes nine molecules of oxygen. Here, we report that oxygen deprivation (hypoxia) slows the rate of demethylation of lanosterol and its reduced metabolite 24,25-dihydrolanosterol, causing both sterols to accumulate in cells. These methylated sterols serve as one signal to stimulate rapid Insig-mediated degradation of reductase. In addition, hypoxia increases the expression of Insig-2 in a response mediated by hypoxia-inducible factor. Our analysis of the mouse Insig-2 gene revealed the presence of a functional hypoxia response element in the first intron. Importantly, hepatic Insig-2a expression is upregulated in three independent mouse models of hypoxia. These studies establish that Insig-2 is a target gene of hypoxia-inducible factor. The hypoxia-dependent increase in Insig levels confers cells with enhanced sensitivity to sterol-induced degradation of reductase. In this way, hypoxia-inducible factor-mediated induction of Insig-2 provides a second signal for stimulating reductase degradation. To address the specificity of methylated sterols in promoting reductase degradation, we reconstituted Insig-dependent, sterol-accelerated degradation of the membrane domain of mammalian reductase in Drosophila S2 cells. Studies in this system revealed that 24,25-dihydrolanosterol, and lanosterol, is active in accelerating degradation of reductase. These results were confirmed by examining ubiquitination of reductase in vitro using permeabilized mammalian cells. Collectively, these studies show that under hypoxic conditions reductase undergoes accelerated Insig-dependent degradation as the combined result of two events: 1) accumulation of 24,25-dihydrolanosterol and 2) hypoxia-inducible factor-mediated upregulation of Insig-2. Degradation of reductase ultimately slows a rate-determining step in cholesterol synthesis. These results highlight the importance of 24,25-dihydrolanosterol as a physiologic regulator of reductase degradation and define a novel oxygen-sensing mechanism in the mammalian cholesterol biosynthetic pathway.