I have investigated the potential role of Toll-Like Receptors (TLRs) in mediating
adipose inflammation in obesity. TLRs are a family of pattern recognition
receptors that play a key role in host defence and are also implicated in chronic
inflammatory disorders. The finding that TLR4-deficient mice are protected
against obesity-induced diabetes led me to hypothesise that TLR expression
on adipocytes could play a role in obesity-induced adipose inflammation and
metabolic dysfunction.
The first chapter investigates the expression and function of TLRs in in vitro
differentiated human subcutaneous adipocytes. I found that stimulation with
ligands for TLR2, TLR3 and TLR4 but not the other TLRs, induces the
expression of pro-inflammatory cytokines. I also explored the use of the TLR
adapter molecules MyD88, Mal and TRIF by different TLRs.
The second chapter examines whether TLR activation in adipocytes has an
effect on glucose uptake. I established a 3H-2-deoxy-D-glucose (2DOG) uptake
assay which led to an interesting yet unexpected observation: Stimulation with
TLR3 and TLR4 ligands led to a decrease in insulin-stimulated glucose uptake
but at the same time, insulin-independent glucose uptake was increased. I
showed that these observations are at least partly due to altered expression of
different glucose transporter (GLUT) isoforms. As the effects were seen only
after prolonged TLR stimulation, I speculated that this was mediated via a
secondary secreted factor.
The third chapter is based on a cytokine and adipokine array, which I performed
to identify cytokines that may be responsible for the effects described
in Chapter 2. The secretion of several cytokines/chemokines with diverse pro-inflammatory functions was observed following stimulation with TLR3 and
TLR4 ligands. The contribution of some of these factors to altered glucose
handling was investigated. Whilst a contribution for ENA-78 was ruled out, I
present evidence that IL-1 can contribute to this.