Aggravation by prostaglandin E2 of interleukin-6-dependent insulin resistance in hepatocytes

Abstract

Hepatic insulin resistance is a major contributor to fasting hyperglycemia in patients with metabolic syndrome and type 2 diabetes. Circumstantial evidence suggests that cyclooxygenase products in addition to cytokines might contribute to insulin resistance. However, direct evidence for a role of prostaglandins in the development of hepatic insulin resistance is lacking. Therefore, the impact of prostaglandin E 2 (PGE 2 ) alone and in combination with interleukin-6 (IL-6) on insulin signaling was studied in primary hepatocyte cultures. Rat hepatocytes were incubated with IL-6 and/or PGE 2 and subsequently with insulin. Glycogen synthesis was monitored by radiochemical analysis; the activation state of proteins of the insulin receptor signal chain was analyzed by western blot with phosphospecific antibodies. In hepatocytes, insulin-stimulated glycogen synthesis and insulin-dependent phosphorylation of Akt-kinase were attenuated synergistically by prior incubation with IL-6 and/or PGE 2 while insulin receptor autophosphorylation was barely affected. IL-6 but not PGE 2 induced suppressors of cytokine signaling (SOCS3). PGE 2 but not IL-6 activated extracellular signal-regulated kinase 1/2 (ERK1/2) persistently. Inhibition of ERK1/2 activation by PD98059 abolished the PGE 2 -dependent but not the IL-6-dependent attenuation of insulin signaling. In HepG2 cells expressing a recombinant EP3-receptor, PGE 2 pre-incubation activated ERK1/2, caused a serine phosphorylation of insulin receptor substrate 1 (IRS1), and reduced the insulin-dependent Akt-phosphorylation. Conclusion: PGE 2 might contribute to hepatic insulin resistance via an EP3-receptor-dependent ERK1/2 activation resulting in a serine phosphorylation of insulin receptor substrate, thereby preventing an insulindependent activation of Akt and glycogen synthesis. Since different molecular mechanisms appear to be employed, PGE 2 may synergize with IL-6, which interrupted the insulin receptor signal chain, principally by an induction of SOCS, namely SOCS3. (HEPATOLOGY 2009;50: 781-790.)

Interleukin-6-deficient mice develop hepatic inflammation and systemic insulin resistance

“…While the role of IL-6 in whole-body insulin resistance is unclear, it is generally accepted that excessive circulating IL-6 causes hepatic insulin resistance [30]. This conclusion is based on several observations demonstrating that exogenous administration of IL-6 impairs insulin signal transduction in the liver in vitro [36,37] and in vivo [13]. In addition, IL-6-neutralising antibodies have been shown to reverse hepatic insulin resistance in mice [38,39], while transient overproduction of IL-6 in skeletal muscle by in vivo electroporation resulted in liver inflammation [40].…”

Involvement of sphingosine 1-phosphate in palmitate-induced insulin resistance of hepatocytes via the S1P2 receptor subtype

“…The cells were plated on 35 mm diameter culture plates (1×10 6 cells/plate), and experimental treatments were performed after 24 or 44 h [17].…”

“…The primers used in this study are shown in electronic supplementary material (ESM) Table 1. 14 C]glucose incorporation into glycogen Glycogen synthesis was assessed by measuring D-[U-14 C]glucose incorporation into glycogen as recently described [17]. Briefly, cultured hepatocytes were preincubated for 5 h with 1 μmol/l S1P in M199 medium containing 1% penicillin/streptomycin and dexamethasone (100 nmol/l).…”

Stimulation of fat accumulation in hepatocytes by PGE2-dependent repression of hepatic lipolysis, β-oxidation and VLDL-synthesis

“…PGE 2 biosynthesis is upregulated by the induction of cyclooxygenase 2 (COX2) and microsomal prostaglandin E synthases 1 and 2 (mPGES1, mPGES2). 5,6 Prostaglandins may affect hepatocyte metabolism directly 7 or they can modulate the regulation of hepatocyte metabolism by hormones 8,9 or cytokines. 10,11 There are conflicting reports about the impact of prostaglandins on hepatic lipid metabolism.…”

“…This is of importance because hyperinsulinaemia contributes to the pathogenesis of hepatic steatosis. 2,3 As we have previously shown that PGE 2 can attenuate insulin signaling, 8 we addressed the question whether PGE 2 is capable of attenuating insulin-dependent changes in hepatic lipid metabolism.…”