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The production of cytokine-like molecules IL-6, TNF-α and leptin and their soluble receptors, from an abdominal adipose tissue bed

Goodrick, Steven J.; (2001) The production of cytokine-like molecules IL-6, TNF-α and leptin and their soluble receptors, from an abdominal adipose tissue bed. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

Adipose tissue occupies a unique role in body physiology by providing a dynamic long term energy store. The purpose of this thesis is to determine whether endocrine signals are secreted from an abdominal subcutaneous adipose tissue bed. In particular, the release of the cytokine-like molecules leptin, IL-6 and TNF-α will be investigated. This triad of molecules are have the potential to alter lipid mechanism at the autocrine/paracrine level in addition to potentially acting as endocrine signals. As no assay for leptin was available at the commencement of this project, the first step was the development of one with sufficient specificity and sensitivity. Due to the high degree of conservation between vertebrate leptin, the production of high affinity antibodies to leptin was anticipated to be problematic. Therefore, different immunisation strategies were investigated to optimise the immune response; variation of host (rabbit/mouse), variation of immunogen (peptides/whole leptin) and variation of immunisation site (subcutaneous/footpad) with varying success. The resulting panel of polyclonal antiserum and monoclonal antibodies were subsequently used in the development of a one-site RIA and two-site ELISA. The former was validated against a commercially available assay and used in a number of clinical studies. The latter was proported to differentiate between bound and free leptin, although the sensitivity of the assay was too low to be used with serum samples. Using the in-house RIA and commercially available high sensitivity assays for cytokines, and their soluble receptors, and a sensitive arterial-venous sampling technique which allows the output of the target organ to be evaluated, release of the target molecules was evaluated. IL-6 and leptin were seen to be released by the adipose tissue studied in the baseline (median arterial IL-6 2.33pg/ml [interquartile range, 1.36-3.77 pg/ml] vs. median venous IL-6 6.06 pg/ml [interquartile range, 3.28-9.43 pg/ml], p<0.001; median arterial leptin 28.45 ng/ml [interquartile range, 9.64-50.64 ng/ml] vs. median venous leptin 35.28 ng/ml [interquartile range, 12.65-61.44 ng/ml], p<0.0001. No net release of TNF-α was observed; median arterial TNF-α 1.79 pg/ml [interquartile range, 1.20-2.26 pg/ml] vs. median venous TNF-α 1.67 pg/ml [interquartile range , 1.33-2.22 pg/ml, p=0.446]. Both leptin and IL-6 were higher in the obese subjects (venous IL-6 vs. % body fat r=0.652, p<0.005; leptin vs. % body fat r=0.890, p<0.0001) and were regulated in a nutritionally independent manner. No net release of IL-6 soluble receptor (IL-6sR) or leptin soluble receptor/binding protein (as determined using a novel semi-quantitative assay) were observed. Significant release of TNF soluble receptor type I was observed (p=0.002) and levels of this correlated with obesity (vs. % body fat r=0.72, p<0.0001). TNF soluble receptor II was also released by this depot, but the levels were not significantly higher than circulating levels. In summary, IL-6 and leptin, but not TNF-α, are released by abdominal subcutaneous adipose tissue. IL-6 and TNF-α are released at levels which are far below those observed in response to pathogenic stimuli. The soluble forms of the IL-6 and leptin receptors are not released by this depot, but both types of TNF soluble receptors are. All these molecules are released at levels which reflect adiposity. Soluble forms of the cellular cytokine receptors have profound effects on the bioactivity of cytokine-like molecules. Therefore, the reciprocal release of the cytokine or the soluble receptor isoforms from the abdominal adipose tissue may constitute a post-secretion regulatory mechanism. Based on these data presented here, the activity of the IL-6 produced by the adipose tissue will be augmented by the presence of the soluble form of the receptor in the circulation. With increasing obesity, levels of IL-6 released from the adipose tissue increase, accompanied by a decrease in the circulating soluble receptor levels - possibly to limit the effects of increasing levels of IL-6. Conversely, the effects of TNF will be negated by the soluble receptor isoforms released at this site. This may work on the membrane bound forms of TNF, or in an autocrine/paracrine fashion. No clear role for the soluble form of the leptin receptor has been determined although it may increase the half-life of leptin by preventing renal clearance. Whether the regulatory mechanisms underlying leptin and IL-6 production are the same as those governing their respective soluble receptor production also remains unknown. A clear link can be established between the products of the adipose tissue and the immune system; IL-6 and TNF-α are potent proinflammatory cytokines and leptin is thought to be an important permissive signal in immunological processes. Other adipose tissue products are also analogous to components of the immune system such as adipsin, acylation stimulating protein (ASP) and adiponectin. This is suggestive of a strong link between energy balance and immune function. The release of these molecules from this depot and their potential to act distally, as well as in an autocrine/paracrine fashion strengthen the argument for an endocrine-like role of adipose tissue.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: The production of cytokine-like molecules IL-6, TNF-α and leptin and their soluble receptors, from an abdominal adipose tissue bed
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest.
Keywords: Biological sciences
URI: https://discovery.ucl.ac.uk/id/eprint/10102899
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