Leukocyte-type 12/15-lipoxygenase is essential for timely inflammation-resolution and effective tissue regeneration following skeletal muscle injury

OBJECTIVES: Unlike traditional anti-inflammatory therapies which may interfere with musculoskeletal tissue repair, pharmacological administration of specialized pro-resolving lipid mediators (SPMs) promotes timely resolution of inflammation while stimulating skeletal muscle regeneration. Despite this, the potential role of endogenous inflammation-resolution circuits in skeletal muscle injury and repair remains unknown. METHODS: We investigated the effect of whole-body knockout of leukocyte-type 12/15-lipoxygenase (12/15-LOX) on acute inflammation and regeneration in vivo following skeletal muscle injury in mice. We further tested the impact of 12/15-LOX deficiency on polarization of bone marrow-derived macrophages and differentiation of myogenic progenitor cells in vitro. RESULTS: Alox15(-/-) mice displayed lower intramuscular concentrations of 12/15-LOX-derived lipid mediators than wild type (WT) mice, and this was associated with chronic low-grade muscle inflammation. Alox15-/- mice mounted an exaggerated acute immune response to sterile skeletal muscle injury which was associated with a local imbalance of pro-inflammatory vs. pro-resolving lipid mediators. Alox15(-/-) mice also displayed defects in myogenic gene expression, myofiber size, and myonuclear accretion. Mechanistically, bone marrow-derived macrophages (MФ) obtained from Alox15(-/-) mice produced less 12/15-LOX-derived lipid mediators and this was associated with impaired M2 polarization. Isolated myogenic progenitor cells also produced many LOX metabolites in response to long chain polyunsaturated fatty acid (LC-PUFA) supplementation, including bioactive SPMs. Alox15(-/-) myoblasts were both impaired in their ability to produce SPMs and were insensitive to the stimulatory effect of LC-PUFAs on in vitro myogenesis. CONCLUSIONS: 12/15-LOX is essential for timely resolution of acute inflammation and direct determination of myogenic progenitor cell fate following skeletal muscle injury.