Effects of aging and life-long moderate calorie restriction on IL-15 signaling in the rat white adipose tissue

OBJECTIVE: Phosphorylation of insulin receptor substrate (IRS) 1 by tumor necrosis factor alpha (TNF-α) has been implicated as a factor contributing to insulin resistance. Administration of IL-15 reduces adipose tissue deposition in young rats and stimulates secretion of adiponectin, an insulin sensitizing hormone that inhibits the production and activity of TNF-a. We aimed at investigating the effects of age life-long moderate calorie restriction (CR) on IL-15 and TNF-α signaling in rat white adipose tissue (WAT). MATERIALS AND METHODS: Thirty-six 8-month-old, 18-month-old, and 29-month-old male Fischer344 Brown Norway F1 rats (6 per group) were either fed ad libitum (AL) or calorie restricted by 40%. The serum levels of IL-15 and IL-15 receptor a-chain (IL-15Ra) were increased by CR controls regardless of age. An opposite pattern was detected in WAT. In addition, CR reduced gene expression of TNF-a and cytosolic IRS1 serine phosphorylation in WAT, independently from age. RESULTS: IL-15 signaling in WAT is increased over the course of aging in AL rats compared with CR rodents. Protein levels of IL-15Ra are greater in WAT of AL than in CR rats independently from age. This adaptation was paralleled by increased IRS1 phosphorylation through TNF-a-mediated insulin resistance. Adiponectin decreased at old age in AL rats, while no changes were evident in CR rats across age groups. CONCLUSIONS: IL-15 signaling could therefore represent a potential target for interventions to counteract metabolic alterations and the deterioration of body composition during aging.