Direct and indirect modulation of LPS-induced cytokine production by insulin in human macrophages.

Overweight and obesity are accompanied by insulin resistance, impaired intestinal barrier function resulting in increased lipopolysaccharide (LPS) levels, and a low-grade chronic inflammation that results in macrophage activation. Macrophages produce a range of interleukins as well as prostaglandin E (PGE). To cope with insulin resistance, hyperinsulinemia develops. The purpose of the study was to elucidate how LPS, insulin and PGE might interact to modulate the inflammatory response in macrophages. Human macrophages were either derived by differentiation from U937 cells or isolated from blood mononuclear cells. The macrophages were stimulated with LPS, insulin and PGE. Insulin significantly enhanced the LPS-dependent expression of interleukin-1β and interleukin-8 on both the mRNA and protein levels. Additionally, insulin increased the LPS-dependent induction of enzymes involved in the PGE-synthesis and the production of PGE by macrophages. PGE in turn further enhanced the LPS-dependent expression of cytokines via its G-coupled receptors EP2 and EP4, the latter of which appeared to be more relevant. The combination of all three stimuli resulted in an even higher induction than the combination of LPS plus insulin or LPS plus PGE. Thus, the compensatory hyperinsulinemia might directly and indirectly enhance the LPS-dependent cytokine production in obese individuals.