Pharmaceutical and nutraceutical potential of the halophytic plant Carpobrotus rossii

"Metabolic syndrome" refers to the triumvirate of obesity-related, cardiovascular diseases such as hyperlipidaemia, type 2 diabetes, atherosclerosis and hypertension. The worldwide prevalence of these diseases have increased to such an extent that they are now the leading cause of human morbidity and mortality. Metabolic syndrome is characterised by elevated levels of plasma lipids, hyperglycaemia, compromised insulin signalling, excessive production of reactive oxygen species (ROS) and a vasculature that is in a persistently inflamed state. Because of the increasing prevalence of these diseases, considerable research effort has gone into understanding the disease processes and developing appropriate therapies. Two metabolic syndrome targets which have been identified and for which therapeutics have been successfully developed are hyperlipidaemia and hyperglycaemia. A common target of the lipid-lowering therapies is the HMG-CoA reductase enzyme which catalyses the rate limiting step in the cholesterol synthesis pathway namely the conversion of 3-hydroxy-3-methylglutaric acid coenzyme-A (HMG-CoA) to mevalonate. Statins are the primary class of drugs with this HMG-CoA inhibiting ability. Polyphenolic compounds produced by plants have also been shown to have hypolipidaemic activity by inhibiting HMG-CoA as well as other enzymes involved in the processes of lipid manufacture and delivery to cells. Polyphenolic compounds have also been shown to improve the glucose status of diseased subjects by improving vascular health, improving insulin signalling and glucose uptake into muscle. Of these plant-derived polyphenolic compounds, members of the flavonoid sub-family been shown to be particularly successful in treating both hyperlipidaemia and hyperglycaemia. Carpobrotus rossii (CR) is a succulent halophyte native to Australia and commonly found growing along the coastal margins of southern Australia. The plant has a history of use by both the indigenous aboriginal population and early Tasmanian settlers. CR was reportedly consumed as a food, to treat gastrointestinal upsets, and applied topically for the treatment of bites and scratches. Preliminary investigations conducted at the University of Tasmania have shown that crude extracts from its leaves inhibit platelet aggregation, inflammatory cytokine release (interleukin-1-beta, tumour necrosis factor-alpha) and lipid oxidation in vitro (Geraghty et al., 2011). This activity is believed to be due to the flavonoid compounds that the plant produces in its leaves. Several of these flavonoids have a known HMG-CoA inhibitor 3-hydroxy-3methylglutaric acid (HMG) present as a substituent (Jager, 2009).The presence of this moiety, in conjunction with the known hypoglycaemic and hypolipidaemic activities of other flavonoids, mean that the consumption of CR flavonoids could potentially improve endothelial health, cardiovascular function and health via a combination of effects related to both their flavonoid and statin properties. In planta, the primary function of flavonoids appears to be as antioxidants, and their production has been shown to be induced under a suite of conditions which cause the plant to experience oxidative stress. The ROS generation and signalling process in planta are complex, and the effect of environmental conditions on a plant's redox status, and hence flavonoid production, is likely to vary between species. The effects of environment on flavonoid product has not been previously investigated for CR. The flavonoid structures described in chapter 4 are extremely complex, and based on informal discussions with an organic chemist familiar with similar compounds, not easily amenable to synthesis. As such, the ability to produce sufficient material and improve the efficiency of their production e.g. increasing biomass or increasing flavonoid concentration by the modification of environmental parameters is a key component of overall CR pharmaceutical and nutraceutical investigations. This thesis has involved using techniques relevant to the disciplines of pharmacology, organic chemistry and plant physiology. The primary aims were to investigate the pharmaceutical and nutraceutical potential of the flavonoids derived from CR leaves. To do this, several basic questions were addressed, namely: 1. What effect do environmental conditions have on metabolite production, 2. What is the structure of the CR flavonoids, 3. Is the consumption of CR leaf derived extracts safe, and, 4. Do the CR leaf flavonoids possess pharmacological activity in metabolic syndrome, specifically an improvement in either glycaemic or lipid profile. A suite of novel findings which pave the way for further study of this plant are the result of this research. The body of the thesis is presented as a series of articles for publication, of which three are published at the time of thesis submission