Year

2017

Degree Name

Doctor of Philosophy

Department

School of Medicine

Abstract

Obesity is major health problem today due to its associated complications such as type 2 diabetes, cardiovascular disease and colon cancer. Among these complications there is low grade systemic inflammation, altered gut microbiota as well as an altered autonomic regulation of body metabolism. Previous studies have shown that bardoxolone methyl (BARD), a compound derived from oleanolic acid, is capable of reducing inflammation. This study systematically examined the preventative effects of BARD on inflammation in various tissues, including visceral white and brown adipose tissues as well as colon tissues, in a chronic high-fat diet induced obesity mouse model. Furthermore, this project investigated the effect of BARD on brainstem autonomic regulatory centers.

Obesity is characterized by increased fat deposition, enlarged adipocytes and fat masses. An overload of energy intake induces an inflammatory response, cell stress and altered autonomic function. This study showed that BARD treatment prevents high-fat diet induced inflammation. This is evidenced by a prevention of macrophage invasion as well as increased pro-inflammatory tumor necrosis factor alpha (TNF-α) and IL-1β in visceral fat tissues such as epidydimal and mesenteric fat masses. BARD also prevented the high-fat diet induced increase in intracellular signaling molecules associated with the inflammatory response, including nuclear factor-kappa light-chain enhancer of activated B cells (NF-қB), Akt and ERK, in visceral white adipose tissue. The nervous system, including dopamine- peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), UCP2, and AMP-activated protein kinase (AMPK) neuronal signaling, regulates visceral fat mass size. Tyrosine hydroxylase (TH) is a biomarker for dopamine production. This study showed that BARD stimulates this signaling pathway, from which it prevented excessive deposition of white visceral fat induced by a high-fat diet. Interestingly, this study showed that there was virtually no desensitisation effect following a chronic BARD treatment. Differing from white adipocytes, brown adipocytes promote energy expenditure, which is beneficial for obesity. This study showed that BARD stimulated sympathetic regulation of brainstem autonomic regulatory centers by increasing TH expression and brown adipose tissue ii beta 3-adrenergic receptor and PGC-1α signaling. In addition, BARD has antiinflammatory effects in brown adipose tissue by promoting the conversion of M1 (CD11c) to M2 (CD260) macrophages and reducing lipid droplet deposition.

It is known that obesity associated inflammation may be largely due to high-fat diet induced alterations in gut microbiota and leaking gut. The colon is extremely important in this regard. This study showed that BARD prevented macrophage invasion to the colon tissue, which was induced by a high-fat diet. The macrophage biomarker F4/80 was significantly less in the colon of high-fat diet mice treated with BARD compared with vehicle controls. Also, this study found that BARD reduced M1 (CD11) but increased M2 (CD260) cells in the colon. Furthermore, we found that BARD prevented the high-fat diet induced increase in colonic IL-6, IL-1β and TNF-α. Furthermore, this study suggests that BARD may have anti-colon cancer properties as the mice treated with BARD had reduced cyclooxygenase 2 protein, increased IL-10, reduced Ki67 as well as reduced colon thickness, increased colon crypt depth and number of goblet cells per crypt.

In summary, this mechanistic study examining the effects of BARD in chronic high-fat diet induced obesity has shown that BARD has a number of beneficial effects to visceral white adipose tissues, brown adipose tissues, brainstem and colon tissues and may be a good candidate drug for combating obesity. This research, particularly, is leading to new insights of BARD in prevention of inflammation and obesity, as inflammation is one of the culprits causing obesity complications. However, an animal study will never be able to replace a human study. Even then, a thorough toxicity study must be carried out before a large human clinical trial should be performed.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.