| Summary: | The aim of this study was to. further elucidate the mechanisms whereby growth .. . hormone (GH) exerts its ~hronic effect on adipose tissue metabolism, and in particular the effects of GH on lipolysis. Previous studies had shown that turnout necrosis factor alpha (TNFa.) chronically increases basal lipolysis in rat epididymal cidipocytes: an effect that is similar to that of GH. Other research had implicated a protein with a halflife of less than 3 hours in the regulation of lipolysis and lipogenesis by GR. This led to the investigation if TNFa. might be this putative protein involved in mediating the chronic metabolic effects of GH. Initial studies used ovine adipose tissue explants. TNFa. caused a small increase in basal lipolysis and attenuated insulin effects on lipogenesis. However, the effects ofTNFa. were smaller than those ofGH and TNFa. did not appear to mimic the effects of GH on isoproterenol-stimulated lipolysis in this system. Therefore TNFa. was not the protein involved in GH regulation of lipolysis and lipogenesis. Previous studies in the laboratory on the mechanism of GH action had used inhibition of signal transduction components in an ex.plant system. More specific effects could be observed by using an antisense approach, but this required the use of a cell culture system rather than adipose tissue explants. The suitability of an ovine cell culture system was established for investigating the molecular basis of the lipolytic effects of GH; in particular the inhibitory effects of GH on adenosine inhibition of lipolysis. The lipolytic system partially developed in primary ovine adipocytes, but the antilipolytic system did not appear. to develop. However, by. manipulating the differentiation conditions, I significantly improved both cell differentiation and the lipolytic response and sensitivity to isoproterenol, but there· was no improvement in response to adenosine. As an alternative, the suitability of the murine cell line, 3T3-F442A, was .. . investigat.ed ·for determining the molecular basis of the lipolytic effects of GH. However, although the lipolytic system did· develop in differentiated 3T3-F442A adipocytes and response to isoproterenol was observed, the antilipolytic system did not appear io develop either. This line of investigation was not pursued further. Therefore, I decided to investigate the effects of GH and insulin on the lipogenic system in 3T3-F442A adipocytes i.nstead, with a view to extending previous observations by others (Millar, 1998) in the laboratory on the roles of specific isoforms of protein kinase C (PKC) on the modulation of lipogenesis by insulin and GH. The main objective was to determine the role of PKC isoforms in the modulation of the effect of insulin and GH on activation' and expression (mRNA) of the lipogenic enzyme . acetyl CoA carboxylase (ACC). However, the effect of the hormones on lipogenesis, and especially ACC, was considered to be too small to investigate the roles of specific PKC isoforms, despite trying many different ways of improving the hormone effects. A possible explanation for the poor response to insulin was that the lipogenic system was not "switching off' in the absence of insulin, so isoproterenol was added to the 3T3-F442A adipocytes to decrease lipogenesis. Isoproterenol did reduce the rate of lipogenesis, but the effect of insulin was still small. Therefore, modulation of the effect of specific phosphodiesterase (POE) isoforms on lipogenesis was explored as an alternative. The use of specific POE inhibitors showed that both POE3 and PDE4 enzymes were involved in the modulation of lipogenesis in 3T3-F442A adipocytes.
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