Abstract
Energy homeostasis and metabolic processes have been shown to be under 24-hour control. Hormones secreted from adipocytes and about 20% of the genes in white adipose tissue exhibit daily variations. Changes in metabolism are able to influence the clock in adipose tissue and disruption of the clock mechanism has been shown to lead to metabolic consequences. The aims of this PhD are firstly to investigate links between peripheral clocks and metabolism by using in vivo human and in vitro experimental models and secondly to investigate a cell model, which might reflect adipocyte biology better than the currently available cell lines. The influence of overweight and type 2 diabetes mellitus on plasma hormones was investigated in men over a 24-hour time course under highly controlled conditions. Significantly higher nocturnal plasma melatonin concentrations were observed in obese compared to lean or diabetic subjects. All participants showed clear diurnal variation in plasma leptin, but there was surprisingly no significant difference between the groups, irrespective of whether the data were plotted relative to external clock time or endogenous melatonin phase. Rhythmicity and phase of the expression of core clock genes and clock controlled genes, in a novel adipocyte model derived from IMM0RTO mouse pre-adipocyte cells, support the presence of a molecular oscillator in those cells. Rhythms were phase advanced by about 5 hours in adipocytes, compared to the pre-adipocytes. Preliminary results revealed that lipolysis in those cells may be gated by the adipocyte clock. Our results suggest that the timing of the leptin secretion into the blood is not influenced by the metabolic state under well controlled conditions whilst the concentration of plasma melatonin is increased in overweight participants. The in vitro results show for the first time a molecular clock in primary-like murine adipose cells. The IMWAT cells appear to be a better model because of the robust rhythmicity of the clock genes. Moreover, the preliminary lipolysis data suggests circadian gating for lipolysis. The different phasing of gene expression in pre-adipocytes and adipocytes suggests possible differences in the response to entraining stimuli and that the molecular oscillator may be influenced by the metabolic or differentiation state of the cell.