Insulin enhances dendritic cell maturation and scavenger receptor-mediated uptake of oxidised low-density lipoprotein

Abstract

Objectives: The prevalence of atherosclerotic cardiovascular disease is increased in patients with type 2 diabetes. The role of hyperinsulinaemia as an independent participant in the atherogenic process is controversial. Therefore, we examined whether insulin regulates the expression of scavenger receptors responsible for oxidised low-density lipoprotein (oxLDL) uptake in dendritic cells (DCs). In addition, we investigated the impact of insulin on DC maturation with regard to changes in phenotype and cytokine secretion.

Methods: Immature DCs were cultured with different concentrations of insulin (1nmol/L, 10nmol/L, 50nmol/L, and 100nmol/L) in the absence or presence of LY294002 or PD98059 for 24h. The expression of the scavenger receptors SR-A and CD36 was determined by real-time PCR and Western blot analysis. Furthermore, DCs were incubated with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI)-labelled oxLDL. The DiI-oxLDL-incorporated fraction was investigated by flow cytometry. Finally, flow cytometry was used to investigate immunophenotypic protein expression (CD83, CD86, and CD11a). Supernatant cytokine measurements were used as indicators of immune function.

Results: The incubation of DCs with insulin enhanced SR-A and CD36 gene and protein expression in a dose-dependent manner. This effect was partially abolished by PD98059, which is an extracellular signal-regulated kinase (ERK) inhibitor. However, LY294002 did not inhibit the effect of insulin on scavenger receptor expression. A high concentration of insulin increased the oxLDL-uptake capacity of DCs. Inhibition of the scavenger receptors SR-A and CD36 significantly reduced oxLDL uptake. Furthermore, a high concentration of insulin induced DC maturation. The pro-atherosclerotic chemokines IL-6 and IL-12 were induced by a high concentration of insulin, whereas the release of anti-atherosclerotic IL-10 was reduced.

Conclusion: This study suggests that hyperinsulinaemia can promote DC activation and up-regulate the expression of the scavenger receptors SR-A and CD36, which can increase the oxLDL-uptake capacity of DCs. The results of the present study indicate that one of the mechanisms by which insulin promotes atherogenesis is mediated by its effects on DCs.

Keywords: Atherosclerosis; Dendritic cell; Hyperinsulinaemia; Scavenger receptors; Type 2 diabetes.