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Background  Chronic inflammation with aging contributes to sarcopenia. Previous studies have suggested that the accumulation of adipose tissue in skeletal muscle, referred to as intermuscular adipose tissue (IMAT), increases with age and is associated with inflammation. However, the mechanism governing ectopic inflammation in skeletal muscle due to aging is not fully understood. Leptin, an adipocytokine derived from adipose tissue, is an important mediator of inflammatory processes. We examined changes in leptin levels with age and whether leptin contributes to ectopic inflammation.    Methods  To evaluate ectopic inflammation in skeletal muscle, we measured alterations to the expression of inflammatory cytokine genes ( Il1b, Il6 , and  Tnfa ) and muscle break down-related gene ( MuRF1  and  Atrogin1 ) in the quadricep muscles of young (10 weeks) and aged (48 weeks) female rats using quantitative reverse-transcription polymerase chain reaction (Q-RT-PCR). Histological examination was performed to identify the extent of IMAT.  Leptin  mRNA and leptin protein expression were examined using Q-RT-PCR and enzyme-linked immunosorbent assay, respectively. The effect of leptin on the mRNA expression of  Il1b ,  Il6 , and  Tnfa  in quadricep muscle-derived cells was also examined by stimulating the cells with 0 (control), 1, or 10 μg/mL rat recombinant leptin using Q-RT-PCR.    Results  Aged rats had significantly higher  Il6, MuRF1, and Atrogin1  but not  Il1b  and  Tnfa,  expression and greater levels of IMAT in their quadricep muscles than young rats. Aged rats also had significantly higher  leptin  expression and leptin protein concentration in their quadricep muscles than young rats. The addition of exogenous leptin to quadricep muscle-derived cells significantly increased the gene expression of  Il1b  and  Il6  but not  Tnfa .    Conclusions  Our results suggest that elevated leptin levels due to aging cause ectopic inflammation through IL-6 in the skeletal muscle of aged rats.    Electronic supplementary material  The online version of this article (10.1186/s12891-019-2581-5) contains supplementary material, which is available to authorized users.

Elevated leptin levels induce inflammation through IL-6 in skeletal muscle of aged female rats