Subcutaneous adipose tissue releases interleukin-6, but not tumor necrosis factor-α, in vivo

We measured arterio-venous differences in concentrations of tumor necrosis facter-α (TNFα) and interleukin-6 (IL-6) across a sc adipose tissue bed in the postabsorptive state in 39 subjects [22 women and 17 men; median age, 36 yr (interquartile range, 26 - 48 yr); body mass index, 31.8 kg/m² (range, 22.3- 38.7 kg/m²); percent body fat, 28.7% (range, 17.6- 60.7%)]. A subgroup of 8 subjects had arteriovenous differences measured across forearm muscle. Thirty subjects were studied from late morning to early evening; 19 ate a high carbohydrate meal around 1300 h, and 11 continued to fast. We found a greater than 2-fold increase in IL-6 concentrations across the adipose tissue bed [arterial, 2.27 pg/mL (range, 1.42-3.53 pg/mL); venous, 6.71 pg/mL (range, 3.36-9.62 pg/mL); P < 0.001], but not across forearm muscle. Arterial plasma concentrations of IL-6 correlated significantly with body mass index (Spearman's r = 0.48; P < 0.01) and percent body fat (Spearman's r = 0.49; P < 0.01). Subcutaneous adipose tissue IL-6 production increased by the early evening (1800-1900 h) in both subjects who had extended their fasting and those who had eaten. Neither deep forearm nor sc adipose tissue consistently released TNFα (across adipose tissue: arterial, 1.83 pg/mL (range, 1.36-2.34 pg/mL); venous, 1.85 pg/mL (range, 1.44-2.53 pg/mL); P = NS: across forearm muscle: arterial, 1.22 pg/mL (range, 0,74-2.76 pg/mL); venous, 0.99 pg/mL (range, 0.69-1.70 pg/mL); P = NS]. Al though beth IL-6 and TNFα are expressed by adipose tissue, our results show that there are important differences in their systemic release. TNFα is not released by this sc depot. In contrast, IL-6 is released from the depot and is thereby able to signal systemically.