Introduction: Obesity is associated with chronic low-grade inflammation and hyperinsulinemia. Despite its beneficial effects, insulin has also pro-inflammatory activities. This study analyzes the influence of insulin on interleukin(IL)-6 and IL-1β in cultivated human and murine adipocytes and isolated primary human adipocytes. The role of the PI3Kinase/Akt-pathway in insulin meditated cytokine release was analyzed using PI3Kinase- and Akt- inhibitors. Cytokine gene expression in human subcutaneous adipose tissue biopsies was investigated during an euglycemic-hyperinsulinemic clamp.
Methods: Human Simpson-Golabi-Behmel syndrome (SGBS), murine 3T3-L1 and primary human adipocytes were stimulated with 100nM insulin alone or in combination with inhibitors of the insulin signaling pathway for PI3Kinase (LY294002) and Akt (MK-2206) for 4 hours. Adipose tissue biopsies were taken from overweight humans during an euglycemic-hyperinsulinemic clamp. RT-PCR was used for gene expression analysis of IL-1β and IL-6.
Results: After 4h of incubation with insulin, gene expression increased 2.50-fold for IL-6 in 3T3-L1 adipocytes and 1.90-fold for IL-1β in SGBS cells (both p < 0.05). Stimulation with insulin in combination with the inhibitors LY or MK neutralized the increase of IL-1β and IL-6 seen after insulin stimulation. In primary human adipocytes a 2.60-fold increase of IL-1β gene expression was observed after insulin stimulation. During the euglycemic-hyperinsulinemic clamp IL-6 and IL-1β gene expression was not elevated in human adipose tissue.
Conclusion: Insulin increases gene expression of the proinflammatory cytokines IL-6 and IL-1β in mature adipocytes via the PI3Kinase/Akt-pathway indicating that hyperinsulinemia, seen in obesity and type 2 diabetes, may contribute to low-grade inflammation in adipose tissue. Human euglycemic-hyperinsulinemic clamp studies suggest regulatory mechanisms reducing the pro-inflammatory potential of insulin under physiological conditions.