title: Contraction and AICAR stimulate IL-6 vesicle depletion from skeletal muscle fibers in vivo.
abstract: Recent studies suggest that interleukin 6 (IL-6) is released from contracting skeletal muscles; however, the cellular origin, secretion kinetics, and signaling mechanisms regulating IL-6 secretion are unknown. To address these questions, we developed imaging methodology to study IL-6 in fixed mouse muscle fibers and in live animals in vivo. Using confocal imaging to visualize endogenous IL-6 protein in fixed muscle fibers, we found IL-6 in small vesicle structures distributed throughout the fibers under basal (resting) conditions. To determine the kinetics of IL-6 secretion, intact quadriceps muscles were transfected with enhanced green fluorescent protein (EGFP)-tagged IL-6 (IL-6-EGFP), and 5 days later anesthetized mice were imaged before and after muscle contractions in situ. Contractions decreased IL-6-EGFP-containing vesicles and protein by 62% (P < 0.05), occurring rapidly and progressively over 25 min of contraction. However, contraction-mediated IL-6-EGFP reduction was normal in muscle-specific AMP-activated protein kinase (AMPK) α2-inactive transgenic mice. In contrast, the AMPK activator AICAR decreased IL-6-EGFP vesicles, an effect that was inhibited in the transgenic mice. In conclusion, resting skeletal muscles contain IL-6-positive vesicles that are expressed throughout myofibers. Contractions stimulate the rapid reduction of IL-6 in myofibers, occurring through an AMPKα2-independent mechanism. This novel imaging methodology clearly establishes IL-6 as a contraction-stimulated myokine and can be used to characterize the secretion kinetics of other putative myokines.
authors.affiliation: Research Division, Joslin Diabetes Center and Harvard Medical School, Boston, Massachusetts, USA.
authors.forename: Hans P M M
authors.initials: HP
authors.lastname: Lauritzen
authors.forename: Josef
authors.initials: J
authors.lastname: Brandauer
authors.forename: Peter
authors.initials: P
authors.lastname: Schjerling
authors.forename: Ho-Jin
authors.initials: HJ
authors.lastname: Koh
authors.forename: Jonas T
authors.initials: JT
authors.lastname: Treebak
authors.forename: Michael F
authors.initials: MF
authors.lastname: Hirshman
authors.forename: Henrik
authors.initials: H
authors.lastname: Galbo
authors.forename: Laurie J
authors.initials: LJ
authors.lastname: Goodyear
mesh_terms: D000620:Aminoimidazole Carboxamide / Q000031:analogs & derivatives* / Q000494:pharmacology; D000818:Animals; D049452:Green Fluorescent Proteins / Q000378:metabolism; D015850:Interleukin-6 / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D018613:Microscopy, Confocal; D009119:Muscle Contraction / Q000187:drug effects; D018485:Muscle Fibers, Skeletal / Q000187:drug effects* / Q000378:metabolism*; D012265:Ribonucleotides / Q000494:pharmacology*
chemical_list: D015850:Interleukin-6; D012265:Ribonucleotides; C485184:enhanced green fluorescent protein; D049452:Green Fluorescent Proteins; D000620:Aminoimidazole Carboxamide; C031143:AICA ribonucleotide