The role of visfatin in adipose tissue metabolism and metabolic disease

It is clear that sub-clinical inflammation is a key factor that triggers type 2 diabetes mellitus (T2DM) and is directly influenced by weight gain. Current studies highlight that obesity, particularly central obesity, heightens the pathogenesis of T2DM. Additionally, factors produced by adipose tissue (AT), referred to as adipocytokines, can influence the degree of insulin resistance as well as inflammation, due to their duality of function. A recently implicated adipocytokine, visfatin, has been identified as a potential insulin mimetic, an enzyme associated with mitochondrial biogenesis and an inflammatory factor. However, current studies lack a clear understanding as to the role and influence of visfatin in human AT and insulin resistant states. Therefore, this thesis examined visfatin expression within specific human AT depots and the influence of adiposity - with specific consideration given to insulin resistant states including T2DM, non-alcoholic fatty liver disease (NAFLD) and human immunodeficiency virus (HIV). Further, serum studies addressed how different insulin resistant states influenced circulating visfatin levels, whilst mechanistic studies explored how the role of visfatin was altered by insulin, an insulin sensitiser, inflammation and/or disease. This current thesis identified that visfatin was abundant in both abdominal depots, with highest expression in the omental (Om) AT, and isolated adipocytes, with an apparent relationship with insulin resistance. Subsequent in vivo and in vitro analysis further identified that, whilst insulin appeared to increase visfatin protein expression in isolated abdominal subcutaneous (Abd Sc) adipocytes, the use of an insulin sensitiser - either used in cultured Abd Sc adipocytes or as part of an oral therapeutic treatment in subjects with T2DM - decreased circulating visfatin levels. In addition, intracellular signalling studies highlighted that visfatin regulation within AT appeared to be dependent upon both nuclear factor (NF)-B and c-Jun N-terminal kinase (JNK) activation, influencing interleukin (IL)-6 as part of a visfatin regulatory feedback mechanism. Following the potential influence of visfatin in T2DM, this thesis explored its’ potential role in disease associated with other insulin resistant phenotypes, such as liver disease and HIV. From serum assessment of visfatin in subjects with NAFLD, it was identified that progression of liver disease was accompanied by a reduction in circulating visfatin levels – a finding that occurred independently of diabetic status. However, circulating visfatin still remained significantly higher in NAFLD with T2DM than those without. Finally, this thesis examined a potentially severe insulin resistant phenotype noted in HIV patients, due to an apparent lipodystrophy - a condition which alters fat oxidation and mitochondrial activation or regulation. In such a condition, circulating visfatin levels and visfatin mRNA AT expression remained unaltered by HIV status or drug therapy. Due to the capacity of visfatin to act as an enzyme involved in nicotinamide adenine dinucleotide (NAD) biosynthesis, essential for mitochondrial function and oxidative phosphorylation (OXPHOS), its role in AT remained unchanged by disease status, whilst other mitochondrial and fat metabolism factors were altered by both disease state and drug treatment. Taken together, these current data suggest a duality of function of which visfatin appears to be regulated by insulin, in addition to inflammation, in different disease states and therefore expands our current understanding of this multi-functional adipocytokine.EThOS - Electronic Theses Online ServiceGreat Britain. Dept. of Health (DoH)GBUnited Kingdo