Impairment of insulin signaling in hepatocytes by supernatants of insulin-activated macrophages

Background: Hepatic insulin resistance and hyperinsulinemia are hallmarks in pre-diabetic obese patients. It is generally assumed that hyperinsulinemia is a consequence of the attempt of the body to compensate insulin resistance by enhanced release of insulin from β-cells. Here, evidence is provided that hyperinsulinemia itself might contribute to the development of hepatic insulin resistance by enhancing cytokine production in macrophages.

Methods: U937 cells were differentiated into macrophages by PMA treatment. These macrophages were stimulated with 100 nM insulin for 24h and cytokine gene expression was determined by qPCR. Supernatants of control and insulin-stimulated macrophages were added simultaneously with insulin to primary cultures of hepatocytes. Activation of insulin and cytokine signaling was quantified by westernblot and RT-qPCR.

Results: Incubation of macrophages with insulin for 24h significantly increased the mRNA level of IL-1β, TNFα, IL-6 and Oncostatin M and enhanced the LPS-dependent induction of these cytokines. It also enhanced the prostaglandin E2-dependent induction of IL-1β whereas it attenuated the PGE2-dependent repression of TNFα. In contrast to supernatants of untreated macrophages, supernatants of insulin-treated macrophages reduced the insulin-dependent induction of glucokinase in primary cultures of rat hepatocytes, activated ERK1/2 kinase and provoked an inhibitory serine phosphorylation of the insulin receptor substrate.

Discussion: The current data provide evidence that hyperinsulinemia in a vicious cycle feed forward loop may both elicit cytokine release and enhance LPS-stimulated cytokine release from macrophages and thereby contribute to the development of hepatic insulin resistance.

Corresponding author: Henkel, Janin

E-Mail: [e-mail]