title: Quantitative expression of genes involved in the leptin receptor-mediated STAT signalling pathway in rodent models of obesity
Creators: name: Hanif, Shahid
Publisher: name: University of Glasgow
Year: 2001
Subjects: subject: Genetics.

description: In most cases of human obesity serum leptin levels are elevated, suggesting that reduced sensitivity to endogenous leptin may contribute towards the increase in body weight. There are several potential areas that may be involved in reduced leptin sensitivity, including the blood-brain-barrier (BBB) transport system, changes in expression of the functional leptin receptor, OB- Rb, and components of the leptin receptor-mediated STAT signalling pathway in which members of the SOCS family, SOCS-3 and CIS, act as negative regulators. The aim of this thesis was to examine how the mRNA expression of the leptin receptor and components of the leptin receptor-mediated signalling pathway alters in obesity models and thereby establish to what extent such genes are involved in leptin sensitivity. A novel fluorogenic method of quantitative RT-PCR, TaqMan, was utilised to examine the changes in mRNA expression. Gene expression was examined in central and peripheral tissues of obesity-prone AKR/J mice fed either a chow or a palatable diet for 14 weeks and compared to C57BL/6 lean and ob/ob mice fed a chow diet. The expression of OB-Ra mRNA was reduced in the hypothalamus and pituitary of AKR/J mice fed a palatable diet compared to those fed chow, which may implicate reduced OB-Ra-mediated signalling. In WAT of palatable diet-fed mice compared to those fed on chow, the expression of STATS and CIS mRNA was increased but SOCS-3 mRNA was reduced, which suggests this tissue may have increased sensitivity to leptin, but the increased CIS mRNA may compensate for the increased STATS mRNA. In contrast, in the small intestine, all of the components of leptin receptor-mediated signalling were reduced, except for CIS, which suggests this tissue has low leptin sensitivity and may have reduced leptin signalling. In the hypothalamus of ob/ob mice compared to leans, the expression of STATS and SOCS-3 mRNA were reduced. In the pituitary of ob/ob mice compared to leans, the expression of OB-Rb, STATS, STATS and SOCS-S mRNA was reduced, which may be due a consequence of the leptin deficiency. In the pancreas, the expression of STATS, STATS and SOCS-3 mRNA was increased perhaps reflecting an increase in leptin sensitivity or increased insulin signalling. In BAT, the increased expression of OB-Ra, OB-Rb and STATS mRNA and reduced CIS mRNA appears consistent with elevated leptin sensitivity in this tissue although SOCS-S mRNA was considerably elevated which may be due to obesity-related factors other than leptin. In order to further expand our understanding of the regulation of leptin sensitivity, the expression of components of the leptin receptor-mediated signalling pathway were analysed in the hypothalamus of ob/ob mice intraperitoneally treated with leptin for 24 hours compared to 2 weeks. In comparing ob/ob mice to leans in this study, the expression of OB-Rb, STATS, STATS was increased in contrast to the changes reported in the previous study. The treatment of ob/ob mice with leptin for 24 hours showed a significant reduction in the hypothalamus of SOCS-3 mRNA, but after 2 weeks of leptin treatment the expression of SOCS-S mRNA was increased not only in ob/ob mice but also in pair-fed controls. The involvement of SOCS-3 in hypothalamic leptin sensitivity has previously been reported but in this study the increase after 2 weeks of leptin treatment may be due to feeding rather than leptin. The expression of OB-Ra and OB-Rb mRNA was also examined in ZDF fa/fa rats to examine if the mutated leptin receptor has an effect on its expression. An increase of both OB-Ra and OB-Rb mRNA was observed in ZDF fa/fa rats compared to ZDF +/-- rats, which may be a compensatory change to the reduced leptin receptor-mediated signalling and leptin receptor cell surface expression in these animals. The gene expression changes in tissues of dietary obese and genetically obese mice followed no consistent pattern in relation to leptin sensitivity and can be interpreted in a number of different ways. The changes suggest, however, that some tissues may be more sensitive to the effects of leptin and changes in sensitivity compared to others but there appears to be no clear gene or set of gene changes predictive amongst the genes examined of leptin sensitivity. Since the expression of STATs and SOCs can be regulated by a number of signals it is difficult to definitively distinguish the effects of leptin, tissue-specific effects and other obesity-related factors in these tissues.