Essays on Corporate Cash holdings and Business Groups

This dissertation studies corporate cash holdings in Korea, with its own business groups("chaebols"), and the impact of Asian financial crisis in 1997-1998. The first essay examines the effects of cash on performance from 1994 to 2006. It first shows that cash-rich firms are more likely to survive and the exit rate of chaebol firms with large cash holdings during the crisis is much smaller. Moreover, large cash holdings in non-chaebol firms increase profitability compared to industry rivals after the crisis when external capital becomes more costly. Efficiently allocated internal cash holdings in chaebols, rather than affiliates' own cash, are related to better performance in pre-crisis period. My results suggest that business group affiliation is a key to explain the effect of cash on market performance, and that the precautionary motive plays a central role. The second essay studies the determinants of cash in Korean listed firms from 1993 to 2006. I find that the amount of cash in business groups is significantly smaller than that in stand-alones. Chaebol firms with financial subsidiaries, which can provide alternative financial resources, can reduce cash holdings. I also find that diversification contributes to lower the level of cash in chaebols. Both sources became more important after the crisis.The results also suggest that the smaller amount of cash in business groups is driven by a smaller need for precautionary cash

Symmetry Breaking Phase Transitions in ABJM Theory with a Finite U(1) Chemical Potential

We consider the U(1) charged sector of ABJM theory at finite temperature, which corresponds to the Reissner-Nordstrom AdS black hole in the dual type IIA supergravity description. Including back-reaction to the bulk geometry, we show that phase transitions occur to a broken phase where SU(4) R-symmetry of the field theory is broken spontaneously by the condensation of dimension one or two operators. We show both numerically and analytically that the relevant critical exponents for the dimension one operator agree precisely with those of mean field theory in the strongly coupled regime of the large N planar limit.Comment: 22 pages, 6 figures, typos corrected, references added, improved figures, minor changes, accepted for publication in Phys. Rev.

The Morphologic Assessment of Rectal Neuroendocrine Tumors

AbstractBackground and aimsThe histopathologic features of rectal neuroendocrine tumors (NETs), including size, lymphovascular invasion, invasion of proper muscle, and mitotic rate, have a limited role to play in determining a treatment plan preoperatively. We aimed to investigate the morphologic parameters associated with metastasis, and to evaluate their predictive value.MethodsBetween January 2000 and May 2011, the medical records and endoscopic findings of 468 patients presenting with rectal NETs at the Samsung Medical Center were analyzed retrospectively. All tumors were classified according to size and endoscopic features such as color, shape, contour, and surface change.ResultsTwenty-one of the 468 patients (4.5%) with rectal NETs had lymph node (LN) metastasis and 11 patients (2.4%) had distant metastasis. Risk factors for metastasis included tumor size (≥10mm in diameter), hyperemic change, polypoid lesions, irregular contours, and surface ulceration (p=0.000). Independent risk factors that were predictive of metastasis on multivariate analysis included tumor size (≥10mm in diameter), hyperemic change, and surface ulceration. As the number of independent risk factors for metastasis increased, the risk of metastasis rose.ConclusionsEndoscopic features such as hyperemic change, polypoid lesions, irregular contours, and surface ulcers with tumor size ≥10mm in diameter are associated with metastasis in rectal NETs. In particular, atypical endoscopic features including hyperemic change, and surface ulcer with tumor size ≥10mm in diameter may help to predict the risk of metastasis of rectal NETs

Detective quantum efficiency of photon-counting x-ray detectors.

PURPOSE: Single-photon-counting (SPC) x-ray imaging has the potential to improve image quality and enable novel energy-dependent imaging methods. Similar to conventional detectors, optimizing image SPC quality will require systems that produce the highest possible detective quantum efficiency (DQE). This paper builds on the cascaded-systems analysis (CSA) framework to develop a comprehensive description of the DQE of SPC detectors that implement adaptive binning. METHODS: The DQE of SPC systems can be described using the CSA approach by propagating the probability density function (PDF) of the number of image-forming quanta through simple quantum processes. New relationships are developed to describe PDF transfer through serial and parallel cascades to accommodate scatter reabsorption. Results are applied to hypothetical silicon and selenium-based flat-panel SPC detectors including the effects of reabsorption of characteristic/scatter photons from photoelectric and Compton interactions, stochastic conversion of x-ray energy to secondary quanta, depth-dependent charge collection, and electronic noise. Results are compared with a Monte Carlo study. RESULTS: Depth-dependent collection efficiency can result in substantial broadening of photopeaks that in turn may result in reduced DQE at lower x-ray energies (20-45 keV). Double-counting interaction events caused by reabsorption of characteristic/scatter photons may result in falsely inflated image signal-to-noise ratio and potential overestimation of the DQE. CONCLUSIONS: The CSA approach is extended to describe signal and noise propagation through photoelectric and Compton interactions in SPC detectors, including the effects of escape and reabsorption of emission/scatter photons. High-performance SPC systems can be achieved but only for certain combinations of secondary conversion gain, depth-dependent collection efficiency, electronic noise, and reabsorption characteristics

The genome-scale metabolic network analysis of Zymomonas mobilis ZM4 explains physiological features and suggests ethanol and succinic acid production strategies

<p>Abstract</p> <p>Background</p> <p><it>Zymomonas mobilis </it>ZM4 is a Gram-negative bacterium that can efficiently produce ethanol from various carbon substrates, including glucose, fructose, and sucrose, <it>via </it>the Entner-Doudoroff pathway. However, systems metabolic engineering is required to further enhance its metabolic performance for industrial application. As an important step towards this goal, the genome-scale metabolic model of <it>Z. mobilis </it>is required to systematically analyze <it>in silico </it>the metabolic characteristics of this bacterium under a wide range of genotypic and environmental conditions.</p> <p>Results</p> <p>The genome-scale metabolic model of <it>Z. mobilis </it>ZM4, ZmoMBEL601, was reconstructed based on its annotated genes, literature, physiological and biochemical databases. The metabolic model comprises 579 metabolites and 601 metabolic reactions (571 biochemical conversion and 30 transport reactions), built upon extensive search of existing knowledge. Physiological features of <it>Z. mobilis </it>were then examined using constraints-based flux analysis in detail as follows. First, the physiological changes of <it>Z. mobilis </it>as it shifts from anaerobic to aerobic environments (i.e. aerobic shift) were investigated. Then the intensities of flux-sum, which is the cluster of either all ingoing or outgoing fluxes through a metabolite, and the maximum <it>in silico </it>yields of ethanol for <it>Z. mobilis </it>and <it>Escherichia coli </it>were compared and analyzed. Furthermore, the substrate utilization range of <it>Z. mobilis </it>was expanded to include pentose sugar metabolism by introducing metabolic pathways to allow <it>Z. mobilis </it>to utilize pentose sugars. Finally, double gene knock-out simulations were performed to design a strategy for efficiently producing succinic acid as another example of application of the genome-scale metabolic model of <it>Z. mobilis</it>.</p> <p>Conclusion</p> <p>The genome-scale metabolic model reconstructed in this study was able to successfully represent the metabolic characteristics of <it>Z. mobilis </it>under various conditions as validated by experiments and literature information. This reconstructed metabolic model will allow better understanding of <it>Z. mobilis </it>metabolism and consequently designing metabolic engineering strategies for various biotechnological applications.</p

Synergistic Effects of Hyaluronate - Epidermal Growth Factor Conjugate Patch on Chronic Wound Healing

The proteolytic microenvironment in the wound area reduces the stability and the half-life of growth factors in vivo, making difficult the topical delivery of growth factors. Here, epidermal growth factor (EGF) was conjugated to hyaluronate (HA) to improve the long-term stability against enzymatic degradation and the therapeutic effect by enhancing the biological interaction with HA receptors on skin cells. After the synthesis of HA-EGF conjugates, they were incorporated into a patch-type formulation for the facile topical application and sustained release of EGF. According to ELISA, the HA-EGF conjugates showed a long-term stability compared with native EGF. Furthermore, HA-EGF conjugates appeared to interact with skin cells through two types of HA and EGF receptors, resulting in a synergistically improved healing effect. Taken together, we could confirm the feasibility of HA-EGF conjugates for the transdermal treatment of chronic wounds.11Ysciescopu

Anoikis Resistance: An Essential Prerequisite for Tumor Metastasis

Metastasis is a multistep process including dissociation of cancer cells from primary sites, survival in the vascular system, and proliferation in distant target organs. As a barrier to metastasis, cells normally undergo an apoptotic process known as “anoikis,” a form of cell death due to loss of contact with the extracellular matrix or neighboring cells. Cancer cells acquire anoikis resistance to survive after detachment from the primary sites and travel through the circulatory and lymphatic systems to disseminate throughout the body. Because recent technological advances enable us to detect rare circulating tumor cells, which are anoikis resistant, currently, anoikis resistance becomes a hot topic in cancer research. Detailed molecular and functional analyses of anoikis resistant cells may provide insight into the biology of cancer metastasis and identify novel therapeutic targets for prevention of cancer dissemination. This paper comprehensively describes recent investigations of the molecular and cellular mechanisms underlying anoikis and anoikis resistance in relation to intrinsic and extrinsic death signaling, epithelial-mesenchymal transition, growth factor receptors, energy metabolism, reactive oxygen species, membrane microdomains, and lipid rafts