Fn14 Expression is Lower in Female Compared to Male Skeletal Muscle, Independent of Adult Age

Abstract

BackgroundTweak, a pro‐inflammatory cytokine, and its receptor, Fn14, are recognized as important inflammatory mediators. In skeletal muscle (SkM), elevated TWEAK signaling is associated with reduced insulin sensitivity, reduced mitochondrial biogenesis and SkM atrophy. Further, TWEAK signaling is dysregulated with aging, underlying SkM deterioration and pathological remodeling. Several studies suggest a relationship between estrogen and TWEAK signaling, such that estrogen increases TWEAK expression while suppressing Fn14 expression. Sex‐specific differences in SkM TWEAK and Fn14 have not be identified, but are likely relevant for SkM health in aging.ObjectivesWe sought to evaluate whether SkM of young females has lower Fn14 and higher TWEAK expression compared to young males, and whether Fn14 and TWEAK changes with age in both sexes.MethodsSkM biopsies were collected from the vastus lateralis of young (n=5) and old females (n=6) and young (n=6) and old males (n=4). RNA was extracted from biopsy tissue using TRIzol reagent. Gene expression was measured using qPCR. In a follow‐up experiment human myogenic progenitor cells (MPCs) were isolated from young females (YF‐MPC, n=5) and young males (YM‐MPC, n=6). Cells were cultured in Ham's F12 supplemented with 20% FBS and 5ng/ml bFGF (GM). Once cells reached 75% confluence, all cultures were switched to differentiation media [(DM), Ham's F12 + 2% HI equine serum]. MPCs were cultured in DM for 72 hours and then continued in DM for an additional 48 hours with or without 10ng/ml TNFα. RNA was isolated and gene expression measured using qPCR.ResultsAt the whole tissue level, females had a 2.86 fold lower Fn14 expression (p<0.05) and 1.34 fold higher TWEAK expression (p<0.05) compared males, independent of age. In response to TNFα treatment, differentiated YF‐MPC experienced a 1.7 fold increase in Fn14 expression (p<0.05) while no change in Fn14 expression was observed in differentiated YM‐MPC. TNFα treatment did not affect TWEAK expression in either differentiated YF‐ or YM‐MPC (p>0.05).ConclusionAs hypothesized, females have lower expression of Fn14 and higher expression of TWEAK in skeletal muscle, likely driven, in part, through estrogen‐related mechanisms. However, differences in Fn14 and TWEAK were not differential with age in the female SkM. The young female SkM was collected when the participants had low levels of estrogen, which could partially explain why we observed no difference with age in the females. Intriguingly, TNFα induced Fn14 in differentiated YF‐MPCs, but had no effect in differentiated YM‐MPCs. The significance of this difference for pathological remodeling of skeletal muscle, especially during regeneration, requires further elucidation.Support or Funding InformationCornell University