Allograft Inflammatory Factor-1 Mediates Macrophage-Induced Impairment of Insulin Signaling in Adipocytes

Allograft inflammatory factor-1 (AIF-1) is an inflammatory cytokine produced mainly by macrophages within human white adipose tissue. Its expression is increased in obese subjects and positively correlated with insulin resistance. The purpose of this study is to characterize the regulatory role of AIF-1 in insulin signaling of adipocyte.

AIF-1 was over-expressed via transfection of AIF-1 cDNA into murine RAW 264.7 macrophages, and the constitutive expression of AIF-1 was decreased via transfection of targeting siRNA. Murine 3T3L1 adipocytes were treated with macrophage-conditioned medium or AIF-1 protein. Intracellular lipid accumulation was assayed by oil red O stain. Reactive oxygen species production was determinated by a flow cytometer and adipokine secretion was measured with ELISA. Glucose uptake was detected using the glucose oxidase method and insulin-signal-transduction related molecules were analyzed by Western blot.

Short term (48 h) AIF-1 treatment slightly promoted intracellular lipid storage in differentiating 3T3L1 cells. The protein stimulated reactive oxygen species production, provoked TNFα, IL6, resistin, but suppressed adiponectin release and insulin-stimulated glucose uptake both under normal basal and insulin resistance conditions. Furthermore, AIF-1 induced NF-κB activation, inhibited PPARγ expression, GLUT4 translocation to plasma membrane and Akt phosphorylation.

Macrophage-derived AIF-1 up-regulated reactive oxygen species production, adipokine TNFα, IL6, resistin release, and inhibited adiponectin secretion. Moreover, it suppressed insulin-stimulated glucose uptake by down-regulating insulin signaling. Thus, AIF-1 could be related to obesity-related diseases.