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Biochemical Investigation of Anti-Cancer Activity of Tulbaghia Violacea

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Biochemical Investigation of Anti-Cancer Activity of Tulbaghia Violacea Biochemical investigation of anti-cancer activity of Tulbaghia violacea Gbemisola Morounke Saibu A thesis submitted in fulfilment of the requirements for the degree of Doctor Philosophiae in the Faculty of Sciences, University of the Western Cape Supervisors: Dr Mervin Meyer Co-supervisor: Prof David Katerere November 2012 ABSTRACT Natural products have been a source of many pharmaceutical drugs and a number of drugs that are currently used in the treatment of cancer are derivatives of compounds originally isolated from natural products. There is evidence that extracts of Tulbaghia violacea can be used to treat cancer. The activation of apoptosis in cancer cells is a target for the development of novel anti-cancer drugs since one of the characteristics of cancer cells is resistance to apoptosis due to the deregulation of biochemical pathways leading to apoptosis. In fact, many current anti-cancer drugs exert their effects through the activation of apoptosis. Previous studies showed that extracts of T.violacea induce apoptosis in cancer cells and one study reported on the isolation of a compound (methyl-- D-glucopyranoside), which is responsible for the pro-apoptotic activity of the T.violacea extract. Therefore the aim of this study was to investigate the anti-cancer activity of methyl--D- glucopyranoside and extracts prepared from T.violacea. In this study the pro-apoptotic activity of methyl--D-glucopyranoside and extracts prepared from T.violacea were investigated on a panel of human cancer cell lines, which included HepG2, MCF7, H157, HT29 and the non-cancerous cell line, KMST6. The induction of apoptosis was evaluated by flow cytometry using several bioassays which measures biochemical events (caspase activation, phosphatidylserine externalisation and reactive oxygen species (ROS) production that is associated with the induction of apoptosis. The results demonstrated that the effects of methyl--D-glucopyranoside on cultured cells are transient and that the cells recover from the effects of methyl--D-glucopyranoside. This suggested that methyl--D-glucopyranoside is not the compound responsible for the pro-apoptotic bioactivity in the T.violacea extract. This study also showed that cytotoxic and pro-apoptotic bioactivity of the leaf-extract was significantly higher in comparison to the tuber-extract. The bioactivity of the organic solvent extracts (dichloromethane, hexane, methanol and 50% methanol/water) of ii T.violacea leaves was also significantly higher than water extracts of T.violacea leaves. A comparison of the different organic extracts prepared from the T.violacea leaves showed that the highest activity was observed for the dichloromethane and hexane extracts. In an effort to identify the bioactive compound(s) the dichloromethane extract was subjected to Versaflash® column chromatography. However, due to problems experienced with the solubility of the dichloromethane sub-fractions, these compounds could not be tested for their bioactivity. Palmitone (16- hentriacontanone) was identified as one of the major compounds present in the dichloromethane sub-fractions. This compound was previously shown to have anticonvulsant bioactivity but there is no evidence in the literature that it has anti-cancer or pro-apoptotic activities. Fingerprinting of the methanol extract showed the presence of long chain fatty acid derivatives, flavonoids and allicin derivatives in the methanol extract. Although, this study failed to isolate the pro-apoptotic bioactive compound(s) present in the extracts of T.violacea, it confirmed that extracts of this plant induce apoptosis in cultured human cancer cell lines. Key words: Tulbaghia violacea, Apoptosis, Cancer, Cytotoxicity, Bio-assay guided fractionation, Flow cytometry, Natural products, Palmitone, Methyl--D-glucopyranoside iii DECLARATION I declare that “Biochemical investigation of anti-cancer activity of Tulbaghia violacea” is my own work, that it has not been submitted for any degree or examination in any other university, and that all the sources I have used or quoted have been indicated and acknowledged by complete references. Saibu Gbemisola Morounke November 2012 iv DEDICATION To God almighty, the author and finisher of my faith, to the memory of my father- Late Michael Adegboyega Osituga, to the jewel of my inestimable value (My mum) - Mrs Beatrice Adefoluke Osituga and to my wonderful family, Mr Olusola Saibu, Ayomide and Tanitoluwa The best academies were your love, arms and knees! v Table of contents ABSTRACT ........................................................................................................................................ ii DECLARATION ................................................................................................................................ iv DEDICATION .................................................................................................................................... v Table of contents ................................................................................................................................ vi LIST OF FIGURES .......................................................................................................................... xiii ABBREVIATIONS ........................................................................................................................... xx ACKNOWLEDGEMENTS ........................................................................................................... xxiv CHAPTER ONE: INTRODUCTION AND LITERATURE REVIEW ............................................ 1 1.0 Cancer Overview ...................................................................................................................... 1 1.1 Carcinogenesis .......................................................................................................................... 1 1.2 Apoptosis Overview ............................................................................................................. 1 1.2.1 Molecular mechanisms of apoptosis ............................................................................. 1 1.5 Treatment of Cancer .......................................................................................................... 2 1.6 Drug discovery from natural products .............................................................................. 2 1.6.1 Ethnobotanical bioprospecting for anti-cancer drugs .................................................... 2 1.0 Cancer Overview ....................................................................................................................... 3 1.1 Carcinogenesis ........................................................................................................................... 5 1.2 Apoptosis Overview .................................................................................................................. 6 1.2.1 Molecular mechanisms of apoptosis ........................................................................... 10 1.2.2 Regulation of apoptosis .................................................................................................... 16 1.3 Anti-cancer therapies targeting the extrinsic and intrinsic apoptosis pathways ...................... 20 1.3.1 Death receptor ligands ...................................................................................................... 22 vi 1.3.2 Nuclear Factor κB (NF- κB) ................................ ............................................................. 23 1.3.3 Bcl-2 Proteins ................................................................ ................................................... 23 1.3.4 Mitochondria permeabilization in intrinsic (MMP) pathway ........................................... 25 1.3.5 P53 proteins ....................................................................................................................... 25 1.3.6 Caspases ........................................................................................................................... 26 1.3.7 Inhibitors of apoptosis proteins (IAPs) ............................................................................ 27 1.4 Reactive Oxygen Species ........................................................................................................ 28 1.5 Treatment of Cancer ................................................................................................................ 30 1.6 Drug discovery from natural products ..................................................................................... 32 1.6.1 Ethnobotanical bioprospecting for anti-cancer drugs ....................................................... 33 1.6.1.1 Vinca alkaloids .............................................................................................................. 34 1.6.1.2 Epipodophyllotoxins ..................................................................................................... 35 1.6.1.3 Taxanes .......................................................................................................................... 36 1.6.1.4 Camptothecin ................................................................................................................. 37 1.6.1.5 Homoharringtonine ....................................................................................................... 38 1.6.1.6 Elliptinium ....................................................................................................................
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