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Transcript
Abstract
Hyperglycaemia secondary to sepsis is a common feature in critically ill patients and
represents one of the leading causes of death in intensive care. Sepsis-induced
hyperglycaemia is a complex condition involving altered insulin sensitivity and beta cell
function. However little is known about the potential roles in sepsis-induced
hyperglycaemia of other major modulators of glucose homeostasis the incretins, glucosedependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1).
The overall aim of this study was to investigate the role of receptors for incretins and
other glucose-regulating hormones in sepsis-induced hyperglycaemia. In order to achieve
these aims two in vitro models was established, one of inflammation employing LPS in
U937 monocytes and one of glucose metabolism employing the HUH7 hepatocyte cell
line. The specific hypotheses to be tested were:
• Hyperglycaemia in sepsis results from the effects of endotoxin on glucose-modulating
hormone receptors, in addition to insulin resistance
• The effects of sepsis on glucose-modulating hormone receptors are augmented by high
glucose concentration
• Incretins are able to modulate cytokine secretion
• Effects of sepsis on glucose-modulating hormone receptors are altered by incretins
The hepatocyte cells HUH7 and leukocyte cells U937 were used in this study as
representative cell lines. Different sets of experiments were performed by culturing these
cells with lipopolysaccharide (LPS) to induce an acute proinflammatory state. Analyses
of receptor expression mainly by transcriptional analysis with some confirmation by
immunocytochemistry and Western blot were performed.
The major findings were that transcripts for receptors for GIP, GLP-1, GLP-2, insulin and
adiponectin (R1 &R2) were detected in HUH7 cells. Expression of GIPR protein was
also confirmed. Transcripts for receptors GIP, insulin and adiponectin (R1 &R2) were
also detected in U937, but much less abundantly than for HUH7.
Incubation of HUH7 and U937 with LPS for 24 hr resulted in a dose dependent decrease
in expression of receptors for GIP, GLP-1, GLP-2, insulin and adiponectin (R1 &R2) in
both cell lines. This inhibition was accompanied by an induction of cytokine IL-1β and
IL6 but there was no effect on morphology and viability of the cells.
Incubating HUH7 and U937 cells with a range of glucose concentrations alone for 24 hrs
induced a dose dependent down-regulation in the expression of receptors for GIP, GLP-1,
insulin and adiponectin (R1&R2) in HUH7 cells and GIP and adiponectin R2 in U937
cells. Adding LPS to the culture media potentiated these effects markedly.
Incubation of HUH7 and U937 with LPS and GLP-1 showed that GLP-1 decreased the
LPS-induced IL-1β, IL-6 up-regulations, and reversed the LPS effects on the INSR
expression, which had been inhibited by LPS in both cell lines. Similarly, incubation of
these cells with LPS and GIP partially suppressed LPS-induced IL-1β up-regulation.
Exendin-4 suppressed the LPS-induced GLP-1R, GLP-2R down-regulation in HUH7
cells, and GIPR, INSR in U937 cells and blocked the LPS-induced IL-6 up-regulation in
the both cell lines,
Taken collectively, the data presented in this thesis indicate that hepatocytes and
monocytes express glucose-modulating hormone receptors. Furthermore, exposure of
these cells to sepsis significantly inhibits the expression of these receptors whilst
inducing cytokine expression. Suppressed expression of the glucose-modulating hormone
receptors in sepsis-induced hyperglycaemia may provide a possible explanation for the
impairment of the incretin effects and insulin resistance in critically ill patients or in
diabetic individuals.
This research suggests that prevention of the down-regulation of glucose-modulating
hormone receptors in sepsis could result in improvement in the action of insulin and
incretins and thereby improve glucose control during hyperglycaemia or sepsis-induced
hyperglycaemia. Furthermore, this thesis presents evidence to suggest that incretins may
protect against pathological inflammatory consequences during sepsis-induced
hyperglycaemia.